EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: elima on September 23, 2017, 12:19:33 am
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As per the title, I am looking into designing a circuit that will be able to detect the status of a contactor and give an isolated digital output
for example. I know that a contactor with auxiliary circuit would do this job easily but unfortunately I can't use such.
The current can flow in both directions if that matters.
The contactor is used in 400-800Vdc applications and the currents can peak at 170A.
Any ideas?
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As per the title, I am looking into designing a circuit that will be able to detect the status of a contactor and give an isolated digital output
for example. I know that a contactor with auxiliary circuit would do this job easily but unfortunately I can't use such.
The current can flow in both directions if that matters.
The contactor is used in 400-800Vdc applications and the currents can peak at 170A.
Any ideas?
Could you be more clear about exactly what it is that you are looking for:
If you are looking into detecting if the contactor's coil is energized, then you have several options, including an optoisolator powered from the same source as the coil. Or a magnetic sensor picking up the coil's flux (depending on the design of the contactor, etc).
If you are actually looking for the contact closure itself, you probably will need to do something to measure the voltage on both sides of the contactor and make sure they are the same within some reasonable limits. This is assuming that there is always a voltage potential across the contactor.
Another thought would be to sense the current through the contactor using a hall effect current sensor. This only works if there is always a current flowing.
I have many more similar thoughts, but without knowing the exact goal of the sensing, additional ideas would also be similarly 'a shot in the dark'.
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A simple magnetic reed switch sandwiched between two output wires with some heat shrink might work but would depend on current flowing. Veris Industries makes some split core Hall effect current transducers for DC that might be worth looking into. Veris is a small company and you can actually call and talk to an actual engineer ( not a "sales" engineer).
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Another option may be a neon opto-isolator.
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current sense transformer/loop
isolated & ...
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Hall-effect sensor?
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Thank you all for the replies.
Let me make this more clear. I have a DC source (700Vdc battery for example) and there are 2 separate contactors connected to each pole, one on the positive terminal and one on the negative. I need to be able to detect if any of these contactors stays closed for some reason when it shouldn't (welded contactor). I need to detect the contact closure itself and not the coil.
Hall effect sensor/current sensing methods are simple and easy but they are not really suitable here due to the fact that there isn't current flowing always(or it is very very small).
Haven't found much about neon opto-isolators but I would guess they have a polarity so perhaps it won't be suitable as well. I should check further though, so I would appreciate if you can point to a good reading source.
Voltage drop across the contactor terminals might be perhaps a good idea but it seems to be quite complex due to reference voltages and so on.
Any more ideas?
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700V battery ? contactors ? |O You are kidding ? There are serious safety concerns here....a commun contactor is not able to interrupt safely 700V dc.... |O
And using an electronic circuit to detect the opening of the contactor is not safe at all....
If you would be qualified to work with such dangerous voltages, you should not need to ask help on this forum.... :scared:
Ask for help to a qualified engineer....On this forum, you will have a lot of answers of hobbyist who does not know nothing about high power, high dc voltages nor about safety.
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700VDC then you better be using a contactor rated for that kind of use, and preferably one that has already built in multiple contact poles and arc separation that will break the current of a shorted battery pack. Then back it up with appropriate DC bus fuses right by the battery ( preferably a few between the individual cell units as well, so that you stand a chance of them actually breaking the fault irrespective of how the battery pack is failed) and then use an auxillary NO contact block on the contactor to show that the contacts have closed, and a NC block to show again that they are open. The NO and NC blocks typically do have a bit of travel on them so they do not overlap while the relay is operating.
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First of all, please don't do any assumptions regarding violating any safety rules. I am qualified to work with high voltage systems and the systems are following all safety rules.
As I mentioned earlier, there is one contactor on each pole of the battery. The contactors are rated to work with the voltage/current specifications of the system which are taken with very high safety margins of the capabilities of the battery pack. Moreover, there is a fuse that is appropriately rated protecting the whole pack. Precharge and dishcharge circuits exist as well. So everything is done safely with no risks. The idea about using a contactor without auxiliary contacts as I used to do thus far, is mainly for weight reduction and mainly because there doesn't exist a light contactor for very small currents with auxiliary contacts. I challenge you to find me a light relay/contactor(light weight wise) that is rated for 600V and has an auxiliary contacts that won't be an overkill for a precharge current which in my case is max. 2.5A. The contactors/relays that u will find will be rated over 100A. (overkill, waste of money, extra weight)
The chances that both contactors will weld at the same time and the fuse won't burn are close to ZERO. The detection of welded contactor is just another safety measure taken to make the system more fault proof.
Now that I hope things are more clear, does anyone have a good idea on implementing such circuit?
Thanks.
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elima, I fully understand what you are trying to accomplish as I have worked on DC machines like huge lathes and milling machines. If one of your contactors sticks closed you have a potential and real safety hazard.
You could possibly make your own neon opto-isolator but keep in mind that the small neon bulbs only light on one of the two internal electrodes with DC depending on the polarity and the neon bulb would have to be ballasted with a proper resistor or better yet several resistors in series and potted to make it less likely for an arc to jump. I have only seen neon opto-isolators on one large 480 volt AC transfer switch and thought it was innovative.
A picture of your contactors and setup may help. Perhaps a limit switch could be rigged with insulating materials to provide an isolated aux contact. Industrial limit switches are readily available, some have plastic housings; you could fabricate an insulated actuating arm out of fiberglass or polycarbonate.
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If you would be qualified to work with such dangerous voltages, you should not need to ask help on this forum.... :scared:
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Bullshit and gatekeeping. There could be a whole host of legitimate reasons that he can't / doesn't want to ask a fellow (local to him) engineer about this. Time constraints. Weekend. He's a solo operator at a small company. Just moved. In any case, asking a fellow engineer is precisely what he's doing here right now!
The way health and safety advocates overwhelm discussion on this forum sometimes is very annoying. We all know there are risks. We know that the risks increase with voltage and current.
If you have a safety concern and an answer to the question, then voice your concern, answer the question, and move on.
If you have a safety concern and no answer to the question, then don't bother.
As for the original question, my first attempt would be a pair of hall effect sensors. Or, because presumably the current through the contacts is so much higher than the current through the solenoid that it would saturate the hall sensor on that side, use a current shunt instead on that side. So when there is no current through the shunt (through the solenoid), but the hall effect sensor is still high, then you have welded contact situation.
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There are 2 separate contactors connected to each pole, one on the positive terminal and one on the negative.
Hall effect sensor/current sensing methods are simple and easy but they are not really suitable here due to the fact that there isn't current flowing always(or it is very very small).
So we cannot rely on a current or voltage measurement at one contactor by itself.
Do we care if we detect a failure in one of the contactors without knowing which one? That might make things easier.
Is there any amount of low current which may be bypassed around the contactors? For instance is the detection circuit allowed to leak 1 milliamp around each contactor?
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You're looking to detect in particular a mechanical failure where the contacts fail to open due to, say, welded contacts or a broken spring? A whisker switch set to trip when the armature is released is what I'd look at if it's an open frame.
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Thanks for the replies. Let me point out that at this stage I do care about knowing which contactor is faulty. If it seems that the complexity of such circuit is really high, then I will consider
settling down with a solution that will just detect the failure and not the location of the failure.
Bypass current or what I was thinking of as a test current is allowed. I think we might have the same thinking direction at this point.
My idea is to inject a test current through the contacts once in a while (at some frequency) but I can't figure out the whole idea of the circuit.
I am quite positive that there should be a simple solution for this problem.
Just to give an example of a contactor that I currently use for precharge(2.5A max): EV200HAANA http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=EV200_R_TBD_KILOVAC_EV200_Ser_Contactors&DocType=CS&DocLang=English (http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=EV200_R_TBD_KILOVAC_EV200_Ser_Contactors&DocType=CS&DocLang=English)
and what I would like to use for example: https://www.cynergy3.com/sites/default/files/D%20series%202016_5.pdf (https://www.cynergy3.com/sites/default/files/D%20series%202016_5.pdf)
I am using this contactor only because I have the aux. contacts that can help me to detect failures. It is a total overkill for my application and I am trying to cut expenses and weight by designing such circuit.
To make things even more clear and to avoid solutions that will the kill the idea of this whole "upgrade":
1. The system has to be cheaper. Just with using different relay I can save more than 75% (that's a lot!)
2. The system has to be lighter. With the given examples the weight reduction is more than 90% (not taking into account the needed pcb though)
3. The system has to be reliable.
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If your safety case demands a contactor with auxiliary contact status indication, then that's what it needs.. "Cheap" or "light" doesn't come into it. The reason such things aren't cheap is that they have been rated, tested and validated to do what they claim to do. If you are going to engineer a DIY solution then to make it as robust as the specific parts designed to do that task then it's also going to be expensive and time consuming.
There are numerous isometery systems that can detect welded contactors, most use some sort of small AC signal imposed on top of the DC voltage across the contactor in order to detect the real time impedance of the system.
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Do you realize that 700V is as high as the voltage of the third rail of a subway?
I do not know what is the short circuit current of the batteries, but it is in all cases much higher than the 30mA which is considered as the lethal limit.
For voltages as high as 700V, the only valid safety is a visible disconnection of both poles (disconnector) and a circuit / load earthing.
All subway cars have such a device, it is mandatory.
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I didn't come here to discuss about safety issues as my system has been designed by following all safety procedures. I don't need to go into details how the system works and about all safety matter but I can say that safety is not what I wanted people to be talking about in this thread. So thank you all for your safety concerns and worries, but they aren't the problem here. The system is normally de-energized, there is a discharge resistor and there is as well a visible one pole disconnection. There is insulation measurement device and so on..
An engineer is a person who is supposed to think outside the "box" and try to be creative. Using ready made solutions is not really engineering.
If everyone would just use what already exists, our world would be still stuck with technologies from few centuries back.
Lets stick to the topic of this thread and leave safety issues and concerns aside. I could have perhaps made this problem look much simpler if I didnt mention that it is used in 400-800Vdc. People would then try to think about possible solutions and leave safety aside.
So for those with safety concerns: You have a system that has a low voltage (0.5V) and high voltage (24V). On the high voltage side positive pole as well as on the negative pole you have a relay without auxiliary contacts. You need to detect whether there is continuity on the terminals of each relay (not the coil) and the detection circuitry should be totally isolated from the low voltage system.
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You posted a question in the beginner section of the forum about a circuit with a battery and a voltage of up to 800V and you don't want to discuss about safety issues ? You don't think there is something wrong ? |O
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@oldway. You are not contributing to this thread at all. I haven't asked for evaluation of the safety of my system. The only wrong thing that I see here are your unrelated comments for solving the presented problem(apologies if this sounds harsh).
I am very far from being a beginner but as I am new to this forum I thought it would be wiser to start here.
Heard enough comments about safety concerns, but not many solutions to the actual problem. All safety concerns have been looked through and are taken into account in every step.
Please, no more comments about safety concerns.
Lets stick to the topic.
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I have already 1615 posts on this forum and I don't have nothing to proove anymore.
Some electronic forums forbid posts concerning high voltages because the forum is read by hundreds of thousands of people including beginners.
It is clear that a question about 800V voltages is not an innocuous question and that safety is a major concern for all who are reading this topic.
You are posting on a public space, we are not talking in personal messages.
So, your thoughts on the help or not that I bring in this topic are totally inappropriate.
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Here is a circuit that could work.
Leakage currents remain below 30mA.
I supposed that the load has a relatively low resistance.
But I decline all responsibility, you will use this circuit at your own risk .... You say you are qualified to work with circuits of high voltages, you are therefore fully responsible for your acts.
For the operating principle, the position of the contacts of the 2 reed relays are different if one contactor is faulty.
For correct operation, the contacts of the 2 reed relays must be in the same position.
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Bypass current or what I was thinking of as a test current is allowed. I think we might have the same thinking direction at this point.
I was thinking of shunting each of the series connected contactors with a high voltage resistor (or 4 high voltage fusible resistors in a series and parallel arrangement for higher reliability) so they can drive an optocoupler with a current or 1 milliamp or even lower. A quick search found some optocouplers specified down to 100 microamps or an even lower current could drive a relaxation oscillator to pulse an optocoupler (or transformer) at a higher current.
I am a little leery of high voltage disconnect circuits which have such a leakage current however they may be suitable in some circumstances.
My idea is to inject a test current through the contacts once in a while (at some frequency) but I can't figure out the whole idea of the circuit.
I could see doing that as well and it may be what max_torque referred to. Use a small coupling transformer with the contactor side AC coupled across the contacts with a high voltage capacitor. Now an actual resistance measurement may be made with AC current excitation.
This is both safer with no contactor side leakage and perhaps more reliable without any temperamental optocouplers.
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Is some quiescent current allowed?
Or must this be zero-drain circuit when the contactors are off?
Why does the battery bank "float" with respect to ground? This is unusual and not permitted with 125VDC battery banks I've worked with in buildings.
A sensing circuit will probably add enough leakage current across the contactor's contacts to keep things unsafe. I don't know your application, but there must be a disconnect upstream of the contactor for people to work safely on the HV bus. It sounds like you know this.
If some quiescent current is allowed, I can post some ideas.
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There is a circuit for continuity detection that is suitable for contact with voltages (certified for 600V in form of the Unitest V1x200). It is astonishingly simple, as it was build around three transistors. The´test connection forms part of an oscillator circuit of some sort. I just cannot find it, but I remember to have retraced the circuit one time.
Anyway, it will come probably come out as something using a small AC signal across the contacts. So you would need well isolated supplies for both of them and then use capacitive or inductive coupling in such a way, that you can recover your signal and yet withstand the full voltage, even with a transient on top. So it might be helpful to raise the frequecy beyond the audible range. Any objections to the use of such signal from the other aspects of your setup?
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Similar to the previous post, I was thinking of testing continuity by injecting a small test current across the contacts from an isolated and floating supply. The system would be set up in such a way that the test current can be distinguished from the normal system current, either by polarity or by frequency.
However, that system is not fail safe, since a fault in the detection circuit would give a false impression that the contactor is open.
The engineer in me would prefer a mechanical system. When the contactor is open something must move and provide a positive indication that the contactor is in the open position. If the contactor can "stick" closed and yet still enable a mechanical indication of open, then I start to have my doubts about the design of the contactor. Since I am unfamiliar with this field, I do not know what such contactors look like or how they are constructed.
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The ideal solution would be some kind of mechanical detection for closed contacts, since you cannot rely on any system voltage or current being present with the contacts welded closed (other isolators or fault detection circuits may have tripped).
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Diode's?
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I agree way to much juice could be drawn, called ARC FLASH. 75ft away if testing.
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This is an idea that's been floating around in the back of my head - and something similar has been mentioned.
A non-contact system that relies on the difference in capacitance between the system when the contactor is open versus closed. The "capacitance responsive circuit" could be anything that takes your fancy.
The capacitive plates could possibly be something as simple as foil wrapped around the input and output power cables. Set your capacitance by how big they are.
Certainly there would need to be some characterisation of such a circuit to identify open and closed contactor conditions. There would also need to be some protection from transients - and if the detection circuit were to inject an AC signal, then perhaps some ferrite beads either side of the detection system to prevent escape of the injected signal and protect it from spurious signals being picked up. With this approach, frequency of operation is a key consideration.
There's probably a lot of issues that I don't have credentials to address ... but it's just an idea...
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Why not ask Andy (photonic induction)? He has a lot of experience with power systems.
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Don't forget that there is always some inductance in the circuit (wiring) and that you can expect an overvoltage (U=dI/dt) when you are interrupting current.
The simple circuit I designed is well protected against such overvoltages.
That's the reason why I choosed to use 6 wirewounds resistors 5W in serie with the reed relay.
To be reliable, the circuit must be as simple as possible.....that's what I tried to do.
Max leakage current is 21.7 mA at 800V which is < 30mA.
Primary safety must be done by protection against contact with live conductive circuit, then a max leakage current < 30mA is correct as secondary safety.
NB: If battery is earthed, you can't use 2 contactors....the earth connection can't be interrupted.
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Think it's also generally the case that contactors are not considered safety isolation devices. There are manual isolators available that have forced contact break mechanics and are designed for this type of situation. Though, the safest way to work on any high power equipment is plug out of socket.
Short of someone spilling patent snake oil on the cable, it stays disconnected. ;)
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Think it's also generally the case that contactors are not considered safety isolation devices. There are manual isolators available that have forced contact break mechanics and are designed for this type of situation. Though, the safest way to work on any high power equipment is plug out of socket.
Short of someone spilling patent snake oil on the cable, it stays disconnected. ;)
Yes indeed, I already wrote this.
For voltages as high as 700V, the only valid safety is a visible disconnection of both poles (disconnector) and a circuit / load earthing.
All subway cars have such a device, it is mandatory.
But elima is a "safety expert" he said, he does not need to discuss about safety concern.... :-DD
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Thank you all for the replies. The systems I work on consist of low voltage DC and high voltage DC. The low voltage DC controls of course the high voltage system with a very complicated circuitry and lots of safety measures taken. Never said that I am a safety expert and whether I am or I am not, I can always learn from others.
Attached is a very simplified schematic of the HV system side with the contactors that I am talking about so that people can have a slightly better understanding of the needed circuitry.
Mechanical contacts have proven to be reliable this far but they can also fail. I am looking into improving the systems from several points as per the requirements. In some systems expenses and weight are not an issue while in others they can be. Designing such circuitry can be used to increase the diagnostic coverage of the systems with aux. contacts and thus make them more safe. I would basically like to have an electronic circuit backup for the mechanical contacts as well in systems with mechanical contacts.
Having a test current controlled from the LV system(isolated..) is the preferred approach as I would like to have the system as safe as possible. AC impedance measurement might be the right approach but I am still open for other suggestions. I have seen myself few years back such circuit with few transistors, diodes and isolated DC-DC.
For the safety experts here: with the electric cars of these days in which voltages are higher than 400Vdc, where does the end-user have those visible disconnections for both poles? Enlighten us.
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For the safety experts here: with the electric cars of these days in which voltages are higher than 400Vdc, where does the end-user have those visible disconnections for both poles? Enlighten us.
I never worked with this technology, only on DC drives up to 550Vedo 5000A for metalurgy , choppers (subway), high power No-breaks, SMPS, thyristorized industrial battery chargers up to 330V 600A, rectifiers up to 3000Vdc 1500A and electrical distribution cabinet up to 120Kv....
But I imagine that both polarities of the battery are fused and that the visible desconnections are done by removing both fuses.
Your design has an obvious safety failure as only one polarity of the battery is fused.
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Electric cars do have disclosed and labelled disconnect loops in multiple places, and any one of them being removed or cut disables the high voltage circuits. these are generally known to the fire services and first responders who attend to the accidents.
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Why does the battery bank "float" with respect to ground? This is unusual and not permitted with 125VDC battery banks I've worked with in buildings.
One of the issues with an international forum, rules may be different :)
I've worked with floating systems up to about 900VDC in Australia. In fact, as personal safety went I always felt way more assured with that style of system. Could lick the battery terminal at any battery without an issue.
As this is the beginners page I will now add, "kiddies, do not try this at home".
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There are pro's and con's of earthed neutrals and floating supplies.
Earthed neutrals increase the risk of a shock if you touch the live whilst earthed. Which is why isolated supplies are best for opened equipment on the electronics test bench.
On equipment in use, floating supplies create uncertainty as to whether a voltage (to earth) is to be expected on any given cable, and that may depend on whether a parasitic earth has arisen somewhere else in the circuit. An earthed neutral avoids that uncertainty. Of course that is only true on single phase supplies.
The other issue with floating supplies is the need for double pole switching and/or double pole fusing. A circuit in which only one conductor needs to be fused and switched is simpler and less likely to be subject to unexpected fault conditions such as one pole of the double pole switch failing, leaving an inoperative but live piece of equipment. Early UK supplies had neutral fuses, and it was found that they caused accidents rather than preventing them.
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Thank you all for the replies. The systems I work on consist of low voltage DC and high voltage DC. The low voltage DC controls of course the high voltage system with a very complicated circuitry and lots of safety measures taken. Never said that I am a safety expert and whether I am or I am not, I can always learn from others.
Attached is a very simplified schematic of the HV system side with the contactors that I am talking about so that people can have a slightly better understanding of the needed circuitry.
Mechanical contacts have proven to be reliable this far but they can also fail. I am looking into improving the systems from several points as per the requirements. In some systems expenses and weight are not an issue while in others they can be. Designing such circuitry can be used to increase the diagnostic coverage of the systems with aux. contacts and thus make them more safe. I would basically like to have an electronic circuit backup for the mechanical contacts as well in systems with mechanical contacts.
Having a test current controlled from the LV system(isolated..) is the preferred approach as I would like to have the system as safe as possible. AC impedance measurement might be the right approach but I am still open for other suggestions. I have seen myself few years back such circuit with few transistors, diodes and isolated DC-DC.
For the safety experts here: with the electric cars of these days in which voltages are higher than 400Vdc, where does the end-user have those visible disconnections for both poles? Enlighten us.
Thanks for posting the information. Those are totally enclosed KILOVAC relays made by Tyco, very nice. No visible moving parts. May be difficult to find the components (appropriate rating of diode and resistor) but a snubber circuit across the contacts would help, reducing the chances of welded contacts. As far as sensing whether it is open or not is a challenge. I believe you said in an earlier post that there may not be current running through the contacts even if relay is closed? A few coils of wire around the lead wire would sense current, but if you have no current, that will not work. A voltage divider across the contacts (high value resistance) would be a check, but if there is no current load, again may not work and is not isolated. You could make it isolated by firing a opto-isolator with the voltage/current from voltage divider and that would tell you the contactor is open.
The diagram helped, some of the letters are mirrored but I was able to read them, curious?
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In my application, the end-user has an access to a high voltage disconnect which basically cuts one pole of the battery. Same is done in electric vehicles. There is a high voltage disconnect connector that basically cuts the loop of one pole wiring. If there are multiple battery containers, each should have a high voltage disconnect. For the people that will work with the high voltage system, there are internal interconnects that can be removed and they basically divide the battery into smaller stacks with voltages below 60Vdc for each stack and each stack has multiple small fuses.
I have been working with such systems for 3 years now and never had any welded contactor but this doesn't stop me from trying to improve my system.
To avoid problems with earthing/grounding, high accuracy insulation measurement device is used between the HV and LV system and can easily detect any problem. In case of a problem, the HV system is cut immediately and latched to that state.
How would a second fuse help in this application?
Thanks.
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I have a few ideas, roughly drawn here. I'm assuming a power system for a nuclear submarine or similar lol.
The basic principle is a balanced bridge.
Any one contactor failing closed will imbalance the bridge and energize the opto-coupler(s) LED. Could add a CCS to the LED portion to take the 2:1 voltage range expected.
I target for about 1mA of LED current, to keep heat low.
Another building block is the isolated HV switch, if you must have zero quiescent current. Use a photo-voltaic opto to drive a HV IGBT or MOSFET acting as a switch, with a series-diode to prevent backfeed through the body diode. Yes it's a DC system and reverse polarity never exists in theory- but in reality I find faults/arcs make -ve spikes due to stray inductance and cause drama. Add the switch in series with battery side resistor i.e. R1 to give zero quiescent current.
In your FMEDA you get good diagnostic coverage if you don't close both relays simultaneously. Switch one and contactor fault should be asserted (=self-test).
A bit of a hassle to test each polarity. Or use one opto and a bridge rectifier driving the opto's LED, but you lose the ability to tell which contactor is stuck.
Ground-fault detection exists somewhere in your system? I see that on floating systems, so this circuit might affect.
Floating-ground systems are problematic for common-mode EMI due to stray capacitance and leakage currents to chassis/earth ground.
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@floobydust. Liked your idea but the main working principle limits a bit the use of such circuitry in my application.
The idea is to be able to detect the status of each contactor without relying on other parts of the circuitry.
The way I do it now with the auxiliary contacts is feeding a constant isolated signal (lets say 3.3V) with a very short pulse(just to eliminate the problems of signal stuck to high..).
There is a specific order in closing/opening the contactors and none of these operations happen at the same time.
For example:
1. Voltage/current across the HV bus is checked, should be ZERO(or very very close). States of all contactors are checked using auxiliary contacts. If everything is ok, discharge contactor which is normally closed is opened, voltage/current across the HV DC bus bar is checked again to verify that everything is ok, next.
2. Check that the rest of contactors are in the state they are supposed to be using the auxiliary contacts. Everything is ok? next.
3. HV- contactor that is normally opened is closed, verified with auxiliary contacts that everything is ok including the rest of the contactors, voltage/current measurement on HV bus..all good? next.
4. Testing circuit that ensures that the precharge resistor is still connected and has the right value is used. If all seems ok, precharge contactor is closed.
5. Precharge process in monitored voltage/current/time, when the precharge is over, the whole precharge circuitry is bypassed with the HV+ contactor.
and so on...
Perhaps there are some more inner checks that I am forgetting now, but the idea is that diagnostic coverage is quite high and basically everything is "doubled" in that sense. Haven't had a single failure during my 3 years working on this project and it has been daily used. Having another circuit for detection will improve the diagnostic coverage and also offer different ways for checking the contactors.
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I'm not clear what "...without relying on other parts of the circuitry" means.
You have to end up relying on some hardware to work. The only way around this is self-test which I see you have in your start-up sequence. Whether it's mechanical aux. contacts or opto-couplers reporting back to your controller, self-test gives 100% coverage on that part of the safety function and eliminates the need for redundancy in the FMEDA as I recall.
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I'm not sure why you can't use a contactor with the circuit built-in for this but maybe you could mount a proximity detector to the outside.
If you could fire the solenoid, you could try and detect the back EMF when the contact bounces. Some fuel systems work like this to detect the injector response time. Tthere may be reasons for not doing that (like the other contactor has failed).
If we assume when the contacts are welded, it changes the location of the moving contact. I wonder what the inductance of the solenoid is with the contactor closed vs open. I just tried it with a contactor I had laying around and indeed, pick the right frequency, the inductance changed more than 50%. You would just switch out your normal driver when making the measurement. Maybe a train mode to work with different contactors.
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@floobydust. The solution offered relies on the states of both contactors and not one. I would prefer a test circuit powered from the LV system(isolated) that is able to check each and each contactor individually without the need of closing/opening other contactors as I do now with the aux. contacts.
I am quite impressed with the level of knowledge on this forum. Having self-tests with high Diagnostic Coverage(99%) eliminates the need for redundancy in the FMEDA. Having redundancy doesn't harm though.
I have already mentioned that I am trying to offer different solutions for different requirements. In some cases I need to cut with expenses and weight and in some cases the requirements are quite strict.
I will try to get forward with AC impedance measurement. Will see where this leads me unless someone comes with a brilliant idea(which can be very simple).
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Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.
You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct
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Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.
You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct
Under a fault condition there may be no voltage present.
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Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.
You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct
Under a fault condition there may be no voltage present.
Under a fault condition one would think/hope the appropriate fault clearing mechanism would do ITS job (aka fuses)
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Maybe there is still some misunderstanding about the specific fault condition to be detected and avoided:
it results from being previously switched on (both rails:complete circuit) and occurs on switching OFF.
The aim is to detect this condition in order to avoid any subsequent switching ON, because that would be unsafe.
So, when the sought condition appears, no current will flow and no fuse will blow.
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Maybe there is still some misunderstanding about the specific fault condition to be detected and avoided:
it results from being previously switched on (both rails:complete circuit) and occurs on switching OFF.
The aim is to detect this condition in order to avoid any subsequent switching ON, because that would be unsafe.
So, when the sought condition appears, no current will flow and no fuse will blow.
Not sure if you are referring to the cct I gave. If you are and we assume there is some logic involved energizing the contactor then...
If coil is turned on and output from both resistive networks is present then all is good.
If coil is turned off and no output from restive networks is present then all is good.
If coil is turned off and output from only one restive network is present then flag fault and don't turn on contactor
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Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.
You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct
Under a fault condition there may be no voltage present.
Under a fault condition one would think/hope the appropriate fault clearing mechanism would do ITS job (aka fuses)
Which is the point.
The contacts may be welded/stuck after a fault condition. There may be no voltage/current present on the line and closed contacts need to be detected.
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Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.
You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct
Under a fault condition there may be no voltage present.
Under a fault condition one would think/hope the appropriate fault clearing mechanism would do ITS job (aka fuses)
Which is the point.
The contacts may be welded/stuck after a fault condition. There may be no voltage/current present on the line and closed contacts need to be detected.
If that's a concern then place another resistive detection string accross the feed side. If no voltage is present then fault.
At some point somebody has to open the the device to see why it's faulted
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I'm not sure why you can't use a contactor with the circuit built-in for this but maybe you could mount a proximity detector to the outside.
If you could fire the solenoid, you could try and detect the back EMF when the contact bounces. Some fuel systems work like this to detect the injector response time. Tthere may be reasons for not doing that (like the other contactor has failed).
If we assume when the contacts are welded, it changes the location of the moving contact. I wonder what the inductance of the solenoid is with the contactor closed vs open. I just tried it with a contactor I had laying around and indeed, pick the right frequency, the inductance changed more than 50%. You would just switch out your normal driver when making the measurement. Maybe a train mode to work with different contactors.
This is not correct.
Pressure on the contacts is applied by a spring.
If the contacts are welded, the magnetic circuit is NOT fully closed.....there is an air gap of some millimeters.
There is is a little change of inductance of the coil, but far less than 50%
So this way to detect welded contacts is not safe at all.
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I'm not sure why you can't use a contactor with the circuit built-in for this but maybe you could mount a proximity detector to the outside.
If you could fire the solenoid, you could try and detect the back EMF when the contact bounces. Some fuel systems work like this to detect the injector response time. Tthere may be reasons for not doing that (like the other contactor has failed).
If we assume when the contacts are welded, it changes the location of the moving contact. I wonder what the inductance of the solenoid is with the contactor closed vs open. I just tried it with a contactor I had laying around and indeed, pick the right frequency, the inductance changed more than 50%. You would just switch out your normal driver when making the measurement. Maybe a train mode to work with different contactors.
This is not correct.
Pressure on the contacts is applied by a spring.
If the contacts are welded, the magnetic circuit is NOT fully closed.....there is an air gap of some millimeters.
There is is a little change of inductance of the coil, but far less than 50%
So this way to detect welded contacts is not safe at all.
I guess I am not sure what part is not correct. All of it?
I did mention I had tried it and I really have no reason to provide false information, so maybe I missed something. I had already considered that the return spring may cause some gap. Obviously I don't have their contactor. The one I had laying around to try uses a 24V 60Hz/ 22V 50Hz coil.
Fully open I measured 11.59mH
Fully closed I measured 66.25mH
Relaxing the bar 7mm, I measured 20mH.
From a safety side, I am not familiar with the regulations but I assume based on the OP's posts that they are. They were just looking for ideas. Regardless, I assume that your last sentence suggests that the safety was not the part you suggest was incorrect, it was measuring the inductance. It's possible their contactor will not see this sort of change but it's easy enough for the OP to check.
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You spoke about measuring "the inductance of the solenoid with the contactor closed vs open" and it is not really closed when the contacts are welded.
But if you already "relaxed the bar 7mm" when you measured the inductance, that's alright.
I made a very simple circuit with resistors and reed relays measuring voltage on the contacts.
No information has been given on the load, but it seems that both polarities of the load are short circuited.
Then the very simple circuit I proposed will work fine.
Measuring the inductance is not a simple circuit.
It must be done at low level and when the coil is not energized....It can't be safe because it is not simple.
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Measuring the inductance is not a simple circuit.
It must be done at low level and when the coil is not energized....It can't be safe because it is not simple.
It is no more complex than measuring the contact resistance through a transformer. And if the inductance measurement is made using a self oscillating circuit, lack of oscillation can be used to indicate a failure of the sensing circuit.
Some fail-safe drive circuits use an AC signal from the controller so that a stuck high or stuck low output results in an off condition. High reliability sensor circuits may use AC excitation for the same reason so they do not have to rely on stuck on low or high detection which is easier to fool. For instance LVDTs which use AC excitation have an advantage over hall effect sensors in high reliability applications. AC excitation also allows synchronous demodulation for increased noise immunity.
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....It can't be safe because it is not simple.
::)
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Measuring the inductance is not a simple circuit.
It must be done at low level and when the coil is not energized....It can't be safe because it is not simple.
It is no more complex than measuring the contact resistance through a transformer. And if the inductance measurement is made using a self oscillating circuit, lack of oscillation can be used to indicate a failure of the sensing circuit.
Some fail-safe drive circuits use an AC signal from the controller so that a stuck high or stuck low output results in an off condition. High reliability sensor circuits may use AC excitation for the same reason so they do not have to rely on stuck on low or high detection which is easier to fool. For instance LVDTs which use AC excitation have an advantage over hall effect sensors in high reliability applications. AC excitation also allows synchronous demodulation for increased noise immunity.
Not that easy at all....I wonder how you will make a self oscillating circuit with an inductor which has a free wheeling diode to suppress dV=dI/dt when current is interrupted in the coil.
And inductance varies, then you have to measure the frequency of oscillation....
A lower frequency than expected should indicate welded contacts.
How much components to make such a circuit ?
In principle, safety should not depend of a µC, nor of a PLC.
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The explanations given by elima are very nebulous.
As far as I guess what it does not clearly explain, the battery would consist of low-capacity elements connected in series / parallel with multiple fuses protecting each group of battery cells.
The battery would be inspired by the batteries manufactured by Tesla.
The choice of a voltage of 800V is abnormally high ... It is probably imposed by the operation of the circuit that it supplies.
The battery is not grounded, at least elima does not say it.
According to the (very) simplified diagram, only the negative pole of the battery is protected by a fuse.
It seems very strange to me, because for a not grounded battery , it would be necessary to protect the 2 poles of the battery by a fuse.
The battery could then be safely isolated by removing the two fuses.
This is certainly not a common application, such as electric vehicles.
For this type of application, a much lower voltage is chosen.
The load has a capacitor of high capacity since there is precharge and descharging circuits.
The voltage is too high to be used in a defibrillator ... I would rather think of a generator of high voltage and high nergy pulses but for what purpose? I do not know.
For the problem of welded contacts, it is logical because it seems that elima would like to use a compact contactor type that does not have auxiliary contacts.
On the other hand, the welding of the contacts is a possible event in case of non-operation of the precharging circuit.
But before looking for a solution for detecting such an event, we need detailed informations about the project, inclusive informations about the load, about the contactors, and so on....
For example, it is not certain that the operation of the special contactor causes a change in inductance of the solenoid.
One of the contactors cited by elima looks like a power reed relay, in this type of contactor, the welding of the contacts does not lead to variation of inductance of the coil.
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It's very difficult to beat the simplicity and low cost of a mechanical (auxilliary) switch.
I would probably look to save dollars elsewhere. Tesla Motors developed their own electronic fuse, for example.
Why does the nuclear submarine project have such a low budget lol.
Back to OT:
You could measure contactor inductance. This is done with automotive fuel injectors and peak/hold drive, and automotive ignition system's misfire detection looking at primary voltage.
OP did not say if PWM is used on the contactor's coils, as they are piggies for power probably 1-2A coil current.
I would add A/D channel sampling coil voltage and look at the slope during PWM to infer inductance.
Depending on the magnetic circuit, most contactors have 2-3:1 change at least. But the solenoid-types do not have much inductance change. You would have to measure things. There is probably an arc blowout magnet too. Should be no back-EMF diode across the contactor's coil; this would slow the decay time unless a zener used too.
It might be possible to inject a HF carrier (say 100kHz with current-transformer excitation) on the battery's leads and look for the carrier after the contactor.
The contactor will get stuck during release, so I'm not sure why all the start-up procedure and testing. What about shutdown? You'd already know when de-energizing the contactor that it failed.
I wonder about the fault-chain where a DC bus event/fault happens, the contactor is opened and interrupting high-current. Not this nicey nice pre-charge current switching.
That's when the contacts will actually weld.
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As usual, simpler is better.. You don't need to monitor contacts, measure impedance... You need to make sure there are no dangerous voltages/currents behind contacts after relays are off...
(https://www.eevblog.com/forum/beginners/circuit-for-detecting-openclosed-relaycontactor-in-400-800vdc-applications/?action=dlattach;attach=354670)
So all you need are two voltage measuring circuits.. I drew simplified, no fuses or other protection, just relay monitoring concept.
You can make it low or high impedance whatever you need. It will monitor if there is any voltage after relays...
If all is allright neither meter will show voltage, if either or both do, danger....
By the way, safe way to do switching is to have two pairs of contactors in series. And you switch them with a slight delay, so one of them is newer carrying current. So if working one gets welded, chance is that the other one will disconnect. And you put monitoring on both so you can detect life threatening malfunctions. In which case you have to permanently disable device until repaired...
Also you want to have manual master switch that also disconnects both sides. That is for maintenance work, if battery cannot be safely disconnected.
My 2c.
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Not to the circuit, but to the statement about the fuse.
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As usual, simpler is better.. You don't need to monitor contacts, measure impedance... You need to make sure there are no dangerous voltages/currents behind contacts after relays are off...
(https://www.eevblog.com/forum/beginners/circuit-for-detecting-openclosed-relaycontactor-in-400-800vdc-applications/?action=dlattach;attach=354670)
So all you need are two voltage measuring circuits.. I drew simplified, no fuses or other protection, just relay monitoring concept.
You can make it low or high impedance whatever you need. It will monitor if there is any voltage after relays...
If all is allright neither meter will show voltage, if either or both do, danger....
By the way, safe way to do switching is to have two pairs of contactors in series. And you switch them with a slight delay, so one of them is newer carrying current. So if working one gets welded, chance is that the other one will disconnect. And you put monitoring on both so you can detect life threatening malfunctions. In which case you have to permanently disable device until repaired...
Also you want to have manual master switch that also disconnects both sides. That is for maintenance work, if battery cannot be safely disconnected.
My 2c.
When both contacts are open, the load is short circuited or has a low impedance (descharged capacitor).
Then, with 800V battery voltage, both voltage sensors P1 and P2 became a voltage divisor and will measure 400V each, what should be considered as a faulty condition, when it is not.
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When both contacts are open, the load is short circuited or has a low impedance (descharged capacitor).
Then, with 800V battery voltage, both voltage sensors P1 and P2 became a voltage divisor and will measure 400V each, what should be considered as a faulty condition, when it is not.
Two diodes take care of that...
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I do not agree.
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And you are right..
But nevertheless, in that case voltage would be half per meter so state would be recognizable..
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The first schematic I posted was the same as your's, but I changed my mind and I edited my post because it is not a safe design.
I prefered to measure voltage across the contacts, because voltage across welded contacts is 0V, and that's 100% sure.
Fault detection is easy: there are welded contacts if both reed relays are not in the same position. (both on or both off = no fault, one on and the other off = fault)
It is also a safe design because it will detect welded contacts if one of the coils of the reed relays is defective. (interrupted)
For safety, it must detect a fault if there is something wrong in the detection circuit.
NB: values of the resistors must be calculate again when the 24V reed relays model is chosen and their technical caracteristics are known....
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Sorry for the late response. There are plenty of replies already and most of those totally ignored something that I mentioned.
The test current should be provided from the LV system.
I can't elaborate more on the systems I am working on (hopefully you can guess why..).
It was my mistake to mention anything about high voltage dc systems because it seems that it made people over-complicate things.
The same question could be asked if you had a basic mechanical switch, connected at one side to 24V supply and the other side continues to some circuit. You don't know anything else about the rest of the circuitry/dont have access to it but would still like to detect the state of the switch. Simple continuity test. How would you do that?
I am already working on a solution and it is actually simple, very simple. Will update when I get it tested.
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I would imagine, that the switch of a contactor which is able to trip 500 Ampere is not a tiny little part. So I would have a look, if it is possible to install a simple light barrier sensor at the contactor, so that the switch of the contactor will interrupt the light barrier when in open position.
The circuit needed to trigger a simple LED flashing up when the contactor is closed (light barrier is open) is in fact ultra simple and therefore very reliable - and durable too.
Using this construction, the sensor circuit is feed only from the LV-side and the isolation to the HV-side will be 100%.
Also this LED-indicator circuit will be safe to be interpreted:
LED on = contactor closed
LED off = contactor opend
And it will be also safe for failure interpretation: If the LED will not flash up when the switch for the corresponding contractor will be closed to "on", there must be a failure and you will call for service.
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Sorry for the late response. There are plenty of replies already and most of those totally ignored something that I mentioned.
The test current should be provided from the LV system.
I can't elaborate more on the systems I am working on (hopefully you can guess why..).
It was my mistake to mention anything about high voltage dc systems because it seems that it made people over-complicate things.
The same question could be asked if you had a basic mechanical switch, connected at one side to 24V supply and the other side continues to some circuit. You don't know anything else about the rest of the circuitry/dont have access to it but would still like to detect the state of the switch. Simple continuity test. How would you do that?
I am already working on a solution and it is actually simple, very simple. Will update when I get it tested.
If you have 2 separate LV power supplies both isolated at 5KV, there is a very simple solution: see schematic.
I supposed that the load has no EMF higher than the battery voltage !
As long you refuse to give us informations about the load, we can only guess .... |O
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The whole things looks like kind of automotive / traction stuff, and several safety standards (e.g. 26262) apply.
I'm familiar with industrial functional safety, and IMO the simplest solution would be to go for safe monitor contacts within the contactors (that's the standard method in my field of experience). Including no lengthy discussions with the certification authority.
One other way I can imagine would be a high impedance voltage monitoring at four points in your circuit: the battery side terminals and the load side terminals of the contactors. Use any suitable reference point, doesn't matter as long as there's no large AC common mode signal present.. Depending on the load (some kind of inverter, I guess), you may need a high impedance discharge / bleeder path to ideally "middle of battery", other potentials may also work.
Now measure all four voltages, and from the differences between them you can derive the states of the contactors. Yes, some MCU power may be required, but often enough some kind of MCU is already there ...
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Maybe another way:
use two small pulse transformers decoupled by diodes to inject a test current across the contacts.
Apply pulses to the LV side, reflected current gives information if the contact is open or closed.
Risk: you put some components across the contacts, that may rise safety concerns (diodes fail short, ...)
Using high impedance voltage sensing, these risks can be easier mitigated (just use enough suitable resistors connected in series)
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i am working on a circuit with a similar idea presented by oldway. First board tested and works nicely, now have to make it fail-proof and add all the needed protection.
Thanks.