Author Topic: Safety when using a 240V mains inverter in a field or car park  (Read 2754 times)

0 Members and 1 Guest are viewing this topic.

Offline NivagSwerdna

  • Super Contributor
  • ***
  • Posts: 2290
  • Country: gb
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #25 on: July 04, 2019, 07:48:00 am »
Wow... I used to run the technology for sporting events based on 12V batteries and inverters in tents/gazebos in rainy fields and never even considered this!

(At least in my case the bits people touched, laptops and label printers, were powered via transformers which probably has some isolation and low voltage on the person side).

I presume the same earthing requirements would apply to portable generators?
 

Offline tggzzz

  • Super Contributor
  • ***
  • Posts: 13790
  • Country: gb
    • Having fun doing more, with less
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #26 on: July 04, 2019, 09:25:47 am »
Wow... I used to run the technology for sporting events based on 12V batteries and inverters in tents/gazebos in rainy fields and never even considered this!

(At least in my case the bits people touched, laptops and label printers, were powered via transformers which probably has some isolation and low voltage on the person side).

There can be inter-winding capacitance, the significance of which I don't understand.

I believe that main-powered medical devices take great care over that, but I don't understand the topic well enough. I guess that the transformers have sheids between the windings and that shield is connected to protective mains earth (which isn't formally available in this context).

Quote
I presume the same earthing requirements would apply to portable generators?

Quite possibly, but I don't know. If they don't apply, I don't know what's been done to ensure they don't apply.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Shock

  • Super Contributor
  • ***
  • Posts: 3533
  • Country: au
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #27 on: July 04, 2019, 10:28:47 am »
Now, where can I find such authoritative (and preferably comprehensible) guidelines? :(

It's probably in the electrical code somewhere but you would have to read between the lines. But looking it from an electronics perspective:

If the battery, inverter and device under test are not electrically connected to any other circuit (i.e. mains wiring). And your inverter mains output live, neutral and earth are not connected to either the chassis or battery terminals (a fully isolated floating output with no touch risks).

And your inverter has earth/neutral tied together before the RCD protection or RCD protection connected directly into the inverter (rather than on the extension). And you are connecting one DUT on the output. And the DUT is not miswired or has neutral tied to earth or the chassis at the device (which would bypass RCD protection).  And don't let anyone connect any devices already connected to other circuits or mains unless it is floated isolated and not directly connected to the inverter.

And if there is no risk of water, lightening. And you use a very short mains cable. And you observe and question any safety recommendations in the manual. And you don't buy a cheap ass Chinese inverter.  And I'm assuming there is a practical size (current) limit to inverters before you need a grounding rod. And you test it.

You might be ok.
« Last Edit: July 04, 2019, 10:36:34 am by Shock »
Soldering/Rework: Pace ADS200, Pace MBT350
Multimeters: Fluke 189, 87V, 117, 112   >>> WANTED STUFF <<<
Oszilloskopen: Lecroy 9314, Phillips PM3065, Tektronix 2215a, 314
 

Offline capt bullshot

  • Super Contributor
  • ***
  • Posts: 2395
  • Country: de
    • Mostly useless stuff, but nice to have: wunderkis.de
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #28 on: July 04, 2019, 10:40:07 am »
It's probably in the electrical code somewhere but you would have to read between the lines. But looking it from an electronics perspective:

SCNR:
The code is for people that do not understand the laws of physics. If you understand laws of physics and do the right things based on that, you'll be fine. Sometimes one can see code or human regulations or managers or whatever attempt to break nature's laws, but that's just futile. Just look at them and watch'em fail. Even worse are humans believing in the correctness of code or any other human made rules and fail because the rules ignore nature's laws.

Safety devices hinder evolution
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 12554
  • Country: gb
    • Mike's Electric Stuff
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #29 on: July 04, 2019, 10:58:16 am »
Simple answer for short-term use like testing - make sure the generator body and wiring is isolated from earth and anything else. That way any single fault to either AC line will not be hazardous as the whole supply is floating.
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Offline Siwastaja

  • Super Contributor
  • ***
  • Posts: 4098
  • Country: fi
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #30 on: July 04, 2019, 12:13:37 pm »
Simple answer for short-term use like testing - make sure the generator body and wiring is isolated from earth and anything else. That way any single fault to either AC line will not be hazardous as the whole supply is floating.

Note that this is the polar opposite of what Zero999 suggested, yet both types of advice are widely available, and almost always given as a "must".

Zero999's way requires an RCD to be truly safe. Mike's way does not require it, but you have to have full understanding and confidence that the system indeed is fully floating.

It's paramount to make sure whether the inverter is properly isolated (with isolation level safety standards mentioned; functional isolation is not enough) from the battery side or not. If it's not isolated, making a floating system is more difficult.
 

Offline tggzzz

  • Super Contributor
  • ***
  • Posts: 13790
  • Country: gb
    • Having fun doing more, with less
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #31 on: July 04, 2019, 12:39:49 pm »
It's probably in the electrical code somewhere but you would have to read between the lines. But looking it from an electronics perspective:

SCNR:
The code is for people that do not understand the laws of physics. If you understand laws of physics and do the right things based on that, you'll be fine.

There's a few more "and"s you need to add in there, e.g.:
  • you understand your system and its components
  • you understand the failure modes in those
  • everybody else has obeyed the code when installing the system
  • nothing has changed since the system was installed
  • you understand all the subtle failure modes that have killed/injured people in the past
  • you don't make a mistake
  • nobody else makes a mistake

And very very few amateurs won't fall into one of those traps. Even professionals fall into them and die or are injured.

Quote
Sometimes one can see code or human regulations or managers or whatever attempt to break nature's laws, but that's just futile. Just look at them and watch'em fail. Even worse are humans believing in the correctness of code or any other human made rules and fail because the rules ignore nature's laws.

And then the Dunning-Kruger syndrome exists; we've all seen examples of it.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
The following users thanked this post: Siwastaja

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 12554
  • Country: gb
    • Mike's Electric Stuff
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #32 on: July 04, 2019, 01:30:07 pm »

Or you could take along a portable earth
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 
The following users thanked this post: GeorgeOfTheJungle, krayvonk, Alti

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 10220
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #33 on: July 04, 2019, 01:41:53 pm »
Dunning-Kruger strikes again - at least use a *galvanised* bucket!

More seriously, if you are not 100% confident that the Neutral of the inverter is isolated from the DC input,  and you are powering it from the vehicle battery, if at all possible, drive a ground stake and connect it to the vehicle chassis.

Personally, I'd recommend using a separate deep cycle battery for the inverter, in a plastic portable battery box, so you can either guarantee its fully floating, or ground it as per the inverter manufacturer's recommendations (if any), and also so you don't end up flattening your car battery if you use more power than you expected to.   
 

Offline JPortici

  • Super Contributor
  • ***
  • Posts: 2788
  • Country: it
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #34 on: July 04, 2019, 02:51:12 pm »
The mains earth should be connected to the car chassis. Most car inverters connect the neutral and earth connections to the 0V of the 12V, which will be connected to the vehicle chassis anyway, so there's generally no need to do it separately.

Yes an RDC/GFCI is essential to protect against shock.

Do cars have HT (i.e. vaguely 240V) inverters nowadays?
I was referring to the type of after-market inverter designed to be used in a car. They are normally connected via a cigarette lighter plug.

incidentally my car (skoda octavia mk3) have an official option for a 230V inverter
http://www.superskoda.com/Skoda/OCTAVIA-III/Octavia-III-genuine-Skoda-230V-inverter-set
 

Offline Shock

  • Super Contributor
  • ***
  • Posts: 3533
  • Country: au
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #35 on: July 04, 2019, 06:31:52 pm »
Personally, I'd recommend using a separate deep cycle battery for the inverter, in a plastic portable battery box, so you can either guarantee its fully floating, or ground it as per the inverter manufacturer's recommendations (if any), and also so you don't end up flattening your car battery if you use more power than you expected to.

I agree, less chance of someone tripping on your DC cable and can avoid using a mains extension cord as you can plonk everything together as long as it's not going to get rained on.
Soldering/Rework: Pace ADS200, Pace MBT350
Multimeters: Fluke 189, 87V, 117, 112   >>> WANTED STUFF <<<
Oszilloskopen: Lecroy 9314, Phillips PM3065, Tektronix 2215a, 314
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 15809
  • Country: gb
  • 0999
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #36 on: July 05, 2019, 07:50:23 am »
None of the equipment will be earthed per se; it will nominally be floating. But I know that I don't know about the corner cases.

Since independent mains supplies are common (fairgrounds, public events, outdoor concerts, etc.) I am sure there are guidelines and regulations about how to set things up safely. They would be a good reference.

If I were thinking about the problem, I would first make sure all exposed metalwork is equipotentally bonded and call this the system earth/ground. This would mean the car chassis and everything called "earth" in the supply through the third pin of the mains plugs. Then I would tie the "neutral" wire of the inverter to this system ground so that the line voltage can never be more than 240 V away from system ground (ensuring it cannot float freely). Lastly I would install an RCD downstream of the earth/neutral tie, so that the RCD will trip if any current takes a different return path outside the designated neutral wire (e.g. if someone touches the car chassis at the same time as the 240 V line).

This is just my supposition about how I might go about things. I am not an expert, so I don't know about corner cases either. This is where authoritative published guidelines would come in helpful.
The last time I checked the wiring regulations, that's exactly what they said. I have an old copy of the BS7671 somewhere and will have to dig it out.
Floating is of course safe by definition - there is no path for the current.

Question: What happens if there is some inadvertent charge pump mechanism that can raise the floating conductors to some very high voltage relative to the surroundings? In that case touching the floating system could lead to a static discharge that may be uncomfortable or even dangerous if the capacitance of the system is large enough. So is floating truly safe by definition? In some contexts (e.g. aircraft fueling) floating is very dangerous and must be avoided.
The classic example of that would be a high voltage supply which shares its neutral with the mains. If the secondary side becomes connected to earth, then the mains will float at a much higher voltage.

Anyway, if the inverter is designed for automotive use, I suspect it's likely the inverter will have the earth and neutral bonded to the negative connection, which is easy to test.
 

Offline richard.cs

  • Super Contributor
  • ***
  • Posts: 1033
  • Country: gb
  • Electronics engineer from Southampton, UK.
    • Random stuff I've built (mostly non-electronic and fairly dated).
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #37 on: July 05, 2019, 12:36:53 pm »
Anyway, if the inverter is designed for automotive use, I suspect it's likely the inverter will have the earth and neutral bonded to the negative connection, which is easy to test.
Cheap inverters often have input negative common with DC bus negative and and a H bridge output stage, so both L and N are alternately +300 V from battery negative. With the battery negative earthed both output terminals can give you a pulsing DC shock to earth, which may not trip an RCD because common types are not required to be sensitive to DC or unipolar pulses. Class II appliances should be safe because two faults are needed to expose the user to the supply, class I appliances with a L-E or N-E fault will probably crowbar the supply enough to be safe (and the cheapest inverters will probably die spectacularly).

Overall inverters of this type are probably safe for most likely faults, but you have a difficult decision whether to float the battery or not. If you do then a L/N fault to true earth makes the battery and all class I appliances lethal to touch, if you earth the battery then this can't happen but touching L/N whilst standing on true earth will be dangerous and RCDs may not help.
 

Offline tggzzz

  • Super Contributor
  • ***
  • Posts: 13790
  • Country: gb
    • Having fun doing more, with less
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #38 on: July 05, 2019, 02:03:55 pm »
Anyway, if the inverter is designed for automotive use, I suspect it's likely the inverter will have the earth and neutral bonded to the negative connection, which is easy to test.
Cheap inverters often have input negative common with DC bus negative and and a H bridge output stage, so both L and N are alternately +300 V from battery negative. With the battery negative earthed both output terminals can give you a pulsing DC shock to earth, which may not trip an RCD because common types are not required to be sensitive to DC or unipolar pulses. Class II appliances should be safe because two faults are needed to expose the user to the supply, class I appliances with a L-E or N-E fault will probably crowbar the supply enough to be safe (and the cheapest inverters will probably die spectacularly).

Overall inverters of this type are probably safe for most likely faults, but you have a difficult decision whether to float the battery or not. If you do then a L/N fault to true earth makes the battery and all class I appliances lethal to touch, if you earth the battery then this can't happen but touching L/N whilst standing on true earth will be dangerous and RCDs may not help.

Thank you for some solid information about some corner cases and their consequences.

I have insufficent knowledge to have predicted that specific mechanism, but that result is the kind of consequence that concerns me.

It is also the reason why codes of practice exist and it is dangerous to presume that "considering the physics" is sufficient.
« Last Edit: July 05, 2019, 02:07:04 pm by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Siwastaja

  • Super Contributor
  • ***
  • Posts: 4098
  • Country: fi
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #39 on: July 05, 2019, 02:13:06 pm »
which may not trip an RCD because common types are not required to be sensitive to DC or unipolar pulses.

Make extra sure that whenever shopping for RCDs, for any purpose, you get the modern type that trigs on DC or unipolar pulses.
 

Offline Siwastaja

  • Super Contributor
  • ***
  • Posts: 4098
  • Country: fi
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #40 on: July 05, 2019, 02:21:55 pm »
I have insufficent knowledge to have predicted that specific mechanism, but that result is the kind of consequence that concerns me.

Do note that, "lethal to touch" in this context means, lethal only if the device manufacturer has made a lethal mistake, against regulations and common sense, basically bringing you back in time to the level of safety you had in the 1980's: only one layer of safety, so that a single failure within a device makes it dangerous.

It's still two orders of magnitude more probable to die in a traffic accident while bringing your devices with you.

With proper modern RCDs in place, or a properly designed truly floating system, you have the second layer, and the risk is now maybe five orders of magnitude from the traffic accident.

Quote
it is dangerous to presume that "considering the physics" is sufficient.

Indeed - I'm sure even a world-class Nobel-winning physicist of electromagnetics would have no idea.
« Last Edit: July 05, 2019, 02:25:49 pm by Siwastaja »
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 15809
  • Country: gb
  • 0999
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #41 on: July 05, 2019, 09:16:39 pm »
Anyway, if the inverter is designed for automotive use, I suspect it's likely the inverter will have the earth and neutral bonded to the negative connection, which is easy to test.
Cheap inverters often have input negative common with DC bus negative and and a H bridge output stage, so both L and N are alternately +300 V from battery negative. With the battery negative earthed both output terminals can give you a pulsing DC shock to earth, which may not trip an RCD because common types are not required to be sensitive to DC or unipolar pulses. Class II appliances should be safe because two faults are needed to expose the user to the supply, class I appliances with a L-E or N-E fault will probably crowbar the supply enough to be safe (and the cheapest inverters will probably die spectacularly).

Overall inverters of this type are probably safe for most likely faults, but you have a difficult decision whether to float the battery or not. If you do then a L/N fault to true earth makes the battery and all class I appliances lethal to touch, if you earth the battery then this can't happen but touching L/N whilst standing on true earth will be dangerous and RCDs may not help.
The last inverter I looked at wasn't like that: it had the earth and neutral connections tied to 0V, but you're right, it would be cheaper to not isolate the DC:DC converter, so it wouldn't surprise me if some cheap inverters are like that.
 

Offline richard.cs

  • Super Contributor
  • ***
  • Posts: 1033
  • Country: gb
  • Electronics engineer from Southampton, UK.
    • Random stuff I've built (mostly non-electronic and fairly dated).
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #42 on: July 06, 2019, 10:21:32 am »
I've had apart several where the boost stage was an inherently isolating topology but they still connected them together, presumably only to save the cost of a single optoisolator. I've seen plenty advertised as you describe though and I think it's a much better design.

With the N-E bond internal to the device it would be easy to sense leakage current in that rather than as L-N and impliment RCD like functionality at near zero cost. Not something I've ever seen though.
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 12554
  • Country: gb
    • Mike's Electric Stuff
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #43 on: July 08, 2019, 09:34:04 pm »
I've had apart several where the boost stage was an inherently isolating topology but they still connected them together, presumably only to save the cost of a single optoisolator.
Could also have been for EMC reasons
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Offline NiHaoMike

  • Super Contributor
  • ***
  • Posts: 7264
  • Country: us
  • "Don't turn it on - Take it apart!"
    • Facebook Page
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #44 on: July 09, 2019, 03:28:02 am »
In my experience, most cheap inverters are not isolated, but I'm in the US with 120V mains where connecting the output of the converter in series with the incoming 12V gives about 8% of the power rating for "free" and lets them use 160V rated capacitors on the output for some cost savings. (Double that to 16% if combined with the "lossless snubber" trick on the primary side.) That would be halved to 4% with 240V output, which probably isn't worth the downsides.

One notable exception is a CAT 1kW unit that has an isolated output. It even has a built in GFCI circuit that serves no apparent purpose since connecting either side of the output to ground will not trip it - I assume it's a backup in case the isolation fails or just put there to satisfy safety requirements of some construction sites.
Cryptocurrency has taught me to love math and at the same time be baffled by it.

Cryptocurrency lesson 0: Altcoins and Bitcoin are not the same thing.
 

Offline richard.cs

  • Super Contributor
  • ***
  • Posts: 1033
  • Country: gb
  • Electronics engineer from Southampton, UK.
    • Random stuff I've built (mostly non-electronic and fairly dated).
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #45 on: July 09, 2019, 11:38:30 am »
I've had apart several where the boost stage was an inherently isolating topology but they still connected them together, presumably only to save the cost of a single optoisolator.
Could also have been for EMC reasons
Very true, not something I'd considered.
 

Offline electr_peter

  • Supporter
  • ****
  • Posts: 983
  • Country: lt
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #46 on: April 13, 2021, 10:54:20 am »
I know it is old post, but it may be useful for some. John Ward has done video on this very topic (generator earthing).
For the simplest case with one generator and one consumer unit, it is simple IT system. When more connections are made, it gets more tricky and TN-S earthing system should be arranged.

« Last Edit: April 13, 2021, 11:00:24 am by electr_peter »
 

Offline wizard69

  • Frequent Contributor
  • **
  • Posts: 930
  • Country: us
Re: Safety when using a 240V mains inverter in a field or car park
« Reply #47 on: April 13, 2021, 12:06:06 pm »
I might suggest consulting the NEC (National Electric Code) or whatever for your location.   The reality is if you follow the "law" you    are somewhat in a better position if something happens.   I'm on the road at the moment but I'm pretty sure there is something in the NEC with regards to portable power sources.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf