Author Topic: Lab with NO grd, what about gear?  (Read 2598 times)

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Offline FriedMule

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Lab with NO grd, what about gear?
« on: September 15, 2018, 12:05:14 am »
In Denmark, where I live, it is common to have outlets with no ground / earthing.
What does that mean for a electronic lab?
Meaning oscilloscope, psu, dmm, anstatic mat and yes, it all?
 

Offline ArthurDent

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Re: Lab with NO grd, what about gear?
« Reply #1 on: September 15, 2018, 03:46:07 am »
I live in the U.S. but as I understand it from the following video, there are both grounded and ungrounded outlets available in Denmark for home use but if anyone was wiring a lab or any commercial business they would make sure that all the equipment had ground equipped power cords to plug into ground equipped outlets.

 

Offline t1d

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Re: Lab with NO grd, what about gear?
« Reply #2 on: September 15, 2018, 04:07:11 am »
I think you are going to need the earth ground, rather quickly... Not just for safety, but for dual (positive and negative voltages) circuit designs.

It is not particularly difficult, or expensive, to add an earth ground, for your lab. In the USA, in my part of the country, the earth ground is simply a long, heavy, copper (or galvanized metal) rod, driven into the ground. A heavy gauge wire is clamped to this rod and it goes into the house, to make the needed connections.

https://www.lowes.com/search?searchTerm=grounding+rod
 

Online Ian.M

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Re: Lab with NO grd, what about gear?
« Reply #3 on: September 15, 2018, 07:43:29 am »
Not quite.  The presence or absence of a true Earth ground doesn't affect the ability of a PSU to provide positive and negative rails to a circuit - if it did hand-held electronics that used split supplies would be impossible!  :-DD

However, modern test equipment with SMPSUs, designed to operate with a grounded supply is *UNSAFE* to use without a ground connection, and the risk significantly increases if multiple items have their ground connections tied together, but not grounded, as the permitted leakage currents to ground in their line filters can add up to a dangerous current which will cause any supposedly grounded exposed metalwork to be a serious shock risk.

You either need a proper ground, or equipotential bonding for the whole bench area.   How to do that safely and legally depends on your existing electrical installation and your local electrical regulations, so if in doubt, consult a local electrician.
« Last Edit: September 15, 2018, 07:45:28 am by Ian.M »
 
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Online 0xdeadbeef

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Re: Lab with NO grd, what about gear?
« Reply #4 on: September 15, 2018, 08:12:49 am »
Actually what is called "ground" connector here is the protective earthing (PE) conductor. It's not needed for functionality (as the two other connector are phase and neutral) but "only" for safety. Like devices with exposed metal parts usually need this kind of connector so the metal case etc. is connected to the protective earthing conductor to make sure that even in case of a catastrophic short circuit inside the device, touching the metal parts won't be fatal. Devices without any exposed metal parts can use the "Euro" plug which doesn't feature the PE conductor. There are several safety classes and all but in a nutshell it's about exposed metal parts.
So connecting equipment which would need a PE conductor to a Euro plug socket without one is a safety hazard, not so much a functional problem. Of course, stuff like an ESD mat also needs to be grounded (and you'd usually use PE for this) but there you could also use a water faucet or whatever.
Trying is the first step towards failure - Homer J. Simpson
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #5 on: October 14, 2018, 07:17:13 am »
About safety: In Denmark the circuit breaker trips if there are any different in the power between phase and neutral so if you touch a metal case, it will trip in a tiny fraction of a second.

I am thinking that so many talks about galvanic isulation of the test gear, avoiding grounding anything and so on, but the other hand I do also hear about gear not working correct if not grounded propperly?


 

Online Kleinstein

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Re: Lab with NO grd, what about gear?
« Reply #6 on: October 14, 2018, 07:27:01 am »
Normally modern gear should work without the ground connection. However there is the mentioned leakage current problem and also a possible EMI problem. So especially with many instruments connected this can be a problem.

If you are Lucky it is only the cheaper outlets without the ground. The wiring could be still have ground. At least in Germany one hardly gets 2 wire cables for installation and they tend to be more expensive than the 3 strand ones. So best have an electrician check the wiring.

The Danish plug looks a lot like a  :) .
 

Offline IanB

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Re: Lab with NO grd, what about gear?
« Reply #7 on: October 14, 2018, 07:36:57 am »
About safety: In Denmark the circuit breaker trips if there are any different in the power between phase and neutral so if you touch a metal case, it will trip in a tiny fraction of a second.

I am thinking that so many talks about galvanic isulation of the test gear, avoiding grounding anything and so on, but the other hand I do also hear about gear not working correct if not grounded propperly?

The ground may be used to avoid stray voltages, "floating" voltages, and for interference suppression. Even though it may not be required for human safety, there are other electrical reasons for having it.
I'm not an EE--what am I doing here?
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #8 on: October 14, 2018, 08:09:54 am »
I live in a rented apartment from 1972 and I do not think that there is ground in most of my outlets.

What is your advice, how would you do?
I have a heater but do not know if it is connected to ground.
 

Online bd139

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Re: Lab with NO grd, what about gear?
« Reply #9 on: October 14, 2018, 08:38:20 am »
I know nothing about Denmark mains but is it possible your neutral is grounded?
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #10 on: October 14, 2018, 10:20:06 am »
Great question, I'll try to find out.
 

Online station240

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Re: Lab with NO grd, what about gear?
« Reply #11 on: October 14, 2018, 10:35:45 am »
Do you have any exposed metal water pipes you could clamp an earth wire to ?
 

Offline HB9EVI

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Re: Lab with NO grd, what about gear?
« Reply #12 on: October 14, 2018, 11:09:52 am »
As it was common in the 70' in many european countries, there wasn't installed a separate PE wire to the outlets; installation type was TN-C where the PE pin in the sockets was bound to the N.

Our house dates from the same time and had this type of installation, what made the usage of an RCD impossible - but was my personal requirement. So transferred to whole installation from TN-C/TN-C-S hybrid to all TN-S, what means to add a PE to every outlet; it was quite a work, but it's worth it.

Since you're renting your flat, and you have no PE pin in your outlets, my advice would be installing an outlet with a PE pin and wire it to N - that's still better than running the devices with floating PE. But before, you have to assure that the house installation is a TN-system
 

Online 0xdeadbeef

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Re: Lab with NO grd, what about gear?
« Reply #13 on: October 14, 2018, 11:28:56 am »
Dunno about Switzerland, but in Germany TN-S (separate PE) is mandatory for new buildings since 1973. And using TN-C (neutral used as PE) in older buildings is only allowed for a diameter of 10mm² (copper) or 16mm² (aluminium).
Again, this is all about safety. Using a device designed for an IEC connector on an outlet without or with lacking PE involves risk of death in case of a failure of this device.
Trying is the first step towards failure - Homer J. Simpson
 

Offline HB9EVI

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Re: Lab with NO grd, what about gear?
« Reply #14 on: October 14, 2018, 11:49:22 am »
Good to know; it seems they slept here, since our house dates from 1974 and all outlets were wired as TN-C, while the installation itself is a TN-C-S with a separed earth rod connected to the PEN

Back to the topic; I read this here:
Quote
In Denmark the high voltage regulation (Stærkstrømsbekendtgørelsen) and Malaysia the Electricity Ordinance 1994 states that all consumers must use TT earthing, though in rare cases TN-C-S may be allowed (used in the same manner as in the United States). Rules are different when it comes to larger companies.

This means, you have to add a PE-wire from the house installation to your outlet in the lab; hooking the N to the PE pin in the outlet IS NOT ALLOWED!
 

Online exe

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Re: Lab with NO grd, what about gear?
« Reply #15 on: October 14, 2018, 03:43:44 pm »
Do you have any exposed metal water pipes you could clamp an earth wire to ?

Oh shi~, don't do it. Unless you want to electrocute somebody taking bathroom or something. Not to mention you don't know if part of the pipe was replaced with a plastic one.

I think some idiots did it in my previous appartment. Touching radiator was painful.
 

Online Brumby

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Re: Lab with NO grd, what about gear?
« Reply #16 on: October 14, 2018, 04:34:18 pm »
Do you have any exposed metal water pipes you could clamp an earth wire to ?

You don't do that here any more - and it would be risky to suggest that, IMHO.

Doing so relies on there being very good and reliable earthing of the pipes all the way from the connection point to the soil in which they are embedded.  It only takes some dry dirt around a pipe or one gap in the electrical conductivity to send current in a direction that you don't want.

There is also the very real risk that someone working on a pipe somewhere up the line could find themselves between a fault current and the path it wants to take.  This is no joke - I have seen plumbers with a meter connected across a pipe that needs to be separated for whatever reason and if it measures a voltage above a certain limit, then they have to walk away from the job and get someone in to sort it out.

Separate earth stake is really the only safe way - which is a problem in units/apartments.

Unless you want to electrocute somebody taking bathroom or something. Not to mention you don't know if part of the pipe was replaced with a plastic one.
Exactly
« Last Edit: October 14, 2018, 04:36:14 pm by Brumby »
 
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Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #17 on: October 14, 2018, 05:43:13 pm »
You are all great, thank you!

I have found out that we use something called "TT systemet" does that mean anything to you?
 

Offline helius

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Re: Lab with NO grd, what about gear?
« Reply #18 on: October 14, 2018, 05:51:38 pm »
TT stands for "Terra-Terra", and is the system where each premises is independently grounded (through a copper rod driven into the earth). The electric service connection does not contain a ground conductor, and the protective earth pins in the outlet sockets do not connect back to the power feed, only to the local ground rod.
https://en.wikipedia.org/wiki/Earthing_system#TT_network

The TT system has high ground fault impedance (since the earth between the local building and the utility transformer is part of the loop). That means that in the case of an insulation fault from phase to chassis in equipment, even if the equipment has a grounded plug, the fault current may be too low to trip a breaker. The RCD is what provides safety in this scenario.
 

Online Ian.M

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Re: Lab with NO grd, what about gear?
« Reply #19 on: October 14, 2018, 06:24:37 pm »
If you cant run an adequate ground connector to a ground rod for a TT system because you are on a high floor in an apartment building etc.  its possible to use a large isolating transformer (so the whole bench can be powered through it), then on the secondary side, set up an equipotential bonding zone for supply ground, with secondary side neutral tied to it, followed by a RCD.   Any fault currents are local to the bonding zone, so cannot cause the safety problem with unexpected potentials across any insulating joints or section in water pipes etc.   I must emphasise that *ALL* equipment that is on the bench or connected to it *MUST* be supplied by the same isolating method to avoid endangering plumbers etc.

N.B. This arrangement almost certainly doesn't meet local  electrical code for a fixed installation, so only build it as a plug-in setup.
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #20 on: October 15, 2018, 11:04:43 am »
Since that all gear do not get grounded, why isolate more by using a isolation transformer?

Would it not be enough if I connected all the gear to the same outlet?

By the way, there are no ground rod at all and we have properly te stupdst "safety"! It is posible to put the plug in the socket in two ways, so there about no way to know if phase or neutral is getting the letal power!
 

Online lypse

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Re: Lab with NO grd, what about gear?
« Reply #21 on: October 15, 2018, 11:32:51 am »
Hi FriedMule,

If you cannot use some pipework for the PE connection, consider using a handheld oscilloscope (isolated channels) and skip the anti static mat.. in short, eliminate ways you could shock yourself? :)

/Alex
 

Online 0xdeadbeef

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Re: Lab with NO grd, what about gear?
« Reply #22 on: October 15, 2018, 11:34:58 am »
By the way, there are no ground rod at all and we have properly te stupdst "safety"! It is posible to put the plug in the socket in two ways, so there about no way to know if phase or neutral is getting the letal power!
IMHO that's not a safety concern. It's better to always expect voltage on both, line and neutral. In countries were neutral and line have a specific position on the plug/socket, the installation can still be screwed up and you die on the assumption it wasn't.

Trying is the first step towards failure - Homer J. Simpson
 

Online Ian.M

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Re: Lab with NO grd, what about gear?
« Reply #23 on: October 15, 2018, 11:48:31 am »
The isolation transformer controls where the ground leakage currents and any ground fault currents return to, so its necessary if you can't put in an adequate grounding conductor and ground rod(s) for a TT system.   It also doesn't care which way round Line and Neutral are on its primary or even if the primary is fed between two phases of a three phase supply as long as the voltage is correct, and if wired as described will provide a clear Neutral on the secondary side.   N.B if wired as described it doesn't provide any 'isolation' for working on equipment, its output should be treated with the same respect as any other mains supply.

Due to its surface resistivity and the 1 Meg resistor in its grounding lead, an antistatic mat is not a shock hazard.  However in the absence of a proper ground, you need an equipotential bonding zone, so there is a path for charge equalisation, or even for charge to leak away to true ground via a higher impedance path than would be acceptable for safety grounding if the equipotental bond ties in water pipes, rebar or other structural metalwork.
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #24 on: October 15, 2018, 07:05:58 pm »
1) Am I understanding correct that if I connect all the lab gear to an isolation transformer, I am less in danger?
2) it is a good idea to take a wire and connect all the grounds and antistatic things together even if that wire never are in connection with real ground but only are connecting the gear?


3) would a cheap max 500W isolation transformer be ok? (https://www.amazon.de/gp/product/B0067K0ESA/ref=ox_sc_act_title_1?smid=A3JWKAKR8XB7XF&psc=1)
 

Online bd139

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Re: Lab with NO grd, what about gear?
« Reply #25 on: October 15, 2018, 07:13:03 pm »
If you connect it to an isolation transformer, there's still mains voltage across two points. There's just no path between one terminal and ground through you.
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #26 on: October 16, 2018, 02:21:02 am »
I can see how that would protect me!
What about a common wire so all gear have same voltage like a common ground?
 

Offline CatalinaWOW

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Re: Lab with NO grd, what about gear?
« Reply #27 on: October 16, 2018, 03:09:40 am »
First I will apologize to the safety rules zealots.  Second, I will emphasize that if you don't know exactly how all of this works you can do something dangerous by accident.  So be very thoughtful while reading this.  And then I will proceed with some opinions.

There are two reasons for the earth ground in our test gear and supplies.  One is for safety.  The other is to provide an element of electrostatic shield which can reduce capacitively coupled noise in our systems.  It may also provide a lower impedance path than other grounds directly reducing simple ohms law noise from ground currents.

a.  The safety arguments are valid, but you can limit risks adequately to satisfy most people without earth grounding.  The most important protection is against hot to case shorts.  These are actually uncommon, and can be further mitigated by periodic checks of insulation integrity.  Even when such a short occurs, health hazard is only serious when there is a path to neutral or earth ground through the body of the operator.  In a normal indoor location standing or sitting in a chair you have a very high resistance path to any of the danger paths.  A higher risk occurs from contact with metal benches and the like, but again periodic isolation tests can mitigate the risks.

The only time significant likelihood of injury occurs is when this path is low impedance - in a bathtub, on a wet floor or other circumstance.  And even then, only if the occurances mentioned previously that make tying your ground to a water pipe an inadequate solution are present.

Remember that there is no such thing as completely safe.  Things which comply with electrical codes can still hurt you.  They are somewhat more safe than some practices that violate the codes.  And the codes are designed to keep things safe enough for children and knuckle dragging adults.  You may well be safer operating your equipment outside of approved code but with your eyes open and brain active than the general public is while operating code approved installations.

b.  The noise issues may or may not be important to you.  It depends on what you are doing and how you have set up your equipment.  But if it is an issue it is easily resolved by connecting the grounds of all of your instruments together.  If your instruments have chassis grounded BNC (not uncommon) it is merely a matter of hooking them up with BNC or other shielded grounded connector.  This slightly increases the safety risk because it effectively combines the chance of a failure in any of the connected instruments.  But this chance is low, and you did decide to periodically check isolation didn't you?  Now we just arguing about inspection interval and how much risk you will accept.
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #28 on: October 16, 2018, 07:32:45 am »
CatalinaWOW hmm how risky I want it to be?!

I am willing to know that any lead with voltage and current can hurt me, but I am not willing to get Einstein hair by touching any of my gear!
In other word I want to treat it as anyone would do a blender ore something like that, you don't get hurt by using the machine normally, but don't be an idiot and stick your hands down to the knives!! :-)

So I do get an isolation transformer, make a common wire (floating ground) and everything is fine?

EDIT: About noise, I would low to get as little noise as possible, I will be working a lot with LF analog circuit but will be "fighting" common noise and HF from interfering.
« Last Edit: October 16, 2018, 07:40:45 am by FriedMule »
 

Offline CatalinaWOW

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Re: Lab with NO grd, what about gear?
« Reply #29 on: October 16, 2018, 09:18:53 am »
CatalinaWOW hmm how risky I want it to be?!

I am willing to know that any lead with voltage and current can hurt me, but I am not willing to get Einstein hair by touching any of my gear!
In other word I want to treat it as anyone would do a blender ore something like that, you don't get hurt by using the machine normally, but don't be an idiot and stick your hands down to the knives!! :-)

So I do get an isolation transformer, make a common wire (floating ground) and everything is fine?

EDIT: About noise, I would low to get as little noise as possible, I will be working a lot with LF analog circuit but will be "fighting" common noise and HF from interfering.

Think it through.  In normal situations you will be fine, with and without the isolation transformer.  The isolation transformer provides protection against one class of faults, not all faults.  It also intoduces new possible faults to the system such as winding shorts. 

The ore grinder analogy is a bit strained, but much of the discussion here is not about sticking your hands in the blades, but about what happens when the shield that stops you from doing that fails.
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #30 on: October 17, 2018, 01:34:10 am »
I know that it was a a bit fare out but what I ment was: Security level should be, if I am idiot, its my own fault, if the gear fails, it shall not influence me. :-)

The system in Denmark do the following: It compare the in and out power and if there are the slightest irrigularity it triggers a breaker and the whole house goes dark!
So if a case is live and i touch it, the breaker will trigger in ms.

What I am concerned about is some says that no ground is the best when measuring electronic i.e. Dave's "how not to blow up your scope". On the other hand, there are also a lot about preventing noise, static electricity and so on.

So the "simple" question: "what would you do if you was in the same situation, lots of power but no earth.
 

Offline CatalinaWOW

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Re: Lab with NO grd, what about gear?
« Reply #31 on: October 17, 2018, 02:12:05 am »
What I would do is go ahead and use it without grounds.  But that isn't really useful to guide you.  I have both experience and knowledge that you are apparently lacking, and a hard won understanding of the cosequences of error.

Dave is correct, you can blow up your scope.  If you don't understand the mechanism and avoid it you can do this in many ways, and no single rule will avoid it.

This also plays into noise.  Much noise results from unplanned and/or uncontrolled signal paths.  Which includes multiple returns to ground.  While a brute force, ground everything approach it is quite possible to reduce noise without minimizing.  And making it really difficult to figure out what to do to further reduce it.  "Which path did that ground current take, anyway."

I'm sorry I can't give you a quick and pat answer, but there really isn't any substitute for thinking through all of the consequences of different choices.  A couple of examples.  How much total power will your lab consume.  All instruments, lights, fans solder stations and the like.  The how much does an isolation transformer to support that load cost.  Or, how diligent would you be in periodically checks of case isolation if you choose that method for protecting yourself?  How diligent would you need to be to satisfy your safety requirements.  How do the answers to those questions relate to your standard of "If I make a stupid mistake that's my tough luck, but I don't want to be hurt by someone else's error.
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #32 on: October 17, 2018, 05:11:13 am »
Wow what a lot of great questions!!

As you rightly says, I am to new to this, my first lab, therefor all my "problems" :-)

About noise and signal path, I am thinking of using one power outlet and then one power strip, all without grd connection.

I do not understand the following: "brute force ground" and "Which path did that ground current take, anyway."

About the consequences, that's it, I do not know the consequences or my options at all.

About testing case isolation, all I know is that if I touch a case with voltage in it, the breaker will shut off in milli seconds.

EDIT: would a cheap max 500W isolation transformer be ok? (https://www.amazon.de/gp/product/B0067K0ESA/ref=ox_sc_act_title_1?smid=A3JWKAKR8XB7XF&psc=1)
« Last Edit: October 17, 2018, 05:13:27 am by FriedMule »
 

Offline CatalinaWOW

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Re: Lab with NO grd, what about gear?
« Reply #33 on: October 17, 2018, 01:22:32 pm »
Your ground fault breakers cannot detect all faults in equipment on the output side of an isolation transformer.  Whether that matters depends on details of your lab set up.  Again thinking is the only solution.  What would the current path be if case isolation failed and you touched the case.  Think of all the other things you might be touching at the same time.

A 500W isolation transformer may or may not be fine.  I have no idea how many devices you have in your lab, or what their power requirements are.  If you are testing high power stereos or amateur radio transmitters that unit you are testing could consume all of that with nothing left for test instruments or anything else.
 

Offline Ice-Tea

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Re: Lab with NO grd, what about gear?
« Reply #34 on: October 17, 2018, 01:29:41 pm »
About safety: In Denmark the circuit breaker trips if there are any different in the power between phase and neutral so if you touch a metal case, it will trip in a tiny fraction of a second.

Funny thing though: the RCDs are ussually for 300mA. Too bad that a rule of thumb says things get hairy from, say, 100mA or so.
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Online 0xdeadbeef

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Re: Lab with NO grd, what about gear?
« Reply #35 on: October 17, 2018, 01:46:31 pm »
Dunno about other countries, but in Germany the RCD has to trip for leakage currents <= 30mA if the RCD is used to protect from injury. 300mA are only allowed for fire prevention (i.e. for installations that a normal person could never touch).
Trying is the first step towards failure - Homer J. Simpson
 

Offline Ice-Tea

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Re: Lab with NO grd, what about gear?
« Reply #36 on: October 17, 2018, 02:16:47 pm »
Dunno about other countries, but in Germany the RCD has to trip for leakage currents <= 30mA if the RCD is used to protect from injury. 300mA are only allowed for fire prevention (i.e. for installations that a normal person could never touch).

AFAIK: in "wet" rooms, where injuries are most likely, 30mA or less is required. The "main" RCD, on which a "regular" room would be, 300mA.

So, yeah, what you said ;)

Note, however, that does not help you much anyway if you connect L and N with your body...
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Offline bsdphk

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Re: Lab with NO grd, what about gear?
« Reply #37 on: October 17, 2018, 03:46:34 pm »
Hi from another Dane :-)

First thing:  Determine if your installation has a Residural-Current-Interrupter (Da: HPFI relæ)

It almost certainly does, and that means that the missing earth is not likely to kill you.

If your lab is on the ground-floor and has an exterior wall, call an electrician and get a quote for a ground connection (Da: Jordspyd) for the outlet you plan to use (or a new one next to it.), it probably won't cost as much as you think.  If he asks why, tell him you need it for EMC/EMI reasons.

If you cannot establish a ground connection, for whatever reason, it is *probably* still a good idea to use 3-pin plugs and a distribution-strip which connects all your instruments earth together.

The good idea is that you do not get a voltage potential between your instruments, the "*probably*" is that if any of them has "Y" capacitors, then they will all float around 115VAC above earth.

The easy test is to run the back of a finger lightly along a metal edge on the instruments, if that buzzes, you really need true earth connection (or the courage to fix the power-filtering in the instrument yourself.)

An isolation transformer will just be a waste of time and electricity (unless you plan to work on old tube-based boat-anchors with transformer-less power-supplies).

Feel free to drop me an email.
 

Offline FriedMule

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Re: Lab with NO grd, what about gear?
« Reply #38 on: October 18, 2018, 12:04:31 am »
Thanks a lot you all!!:-)

I do have a HPFI device, so as I understand it, I can connect all my gear to one distribution-strip with ground so that all my gear have a common connection and ignore that it is not grounded in the wall outlet. Unless that I do feel that sudle vibration in one of the cases.
I can maybe try to switch polarety on the plug if I get that vibrating feeling in my fingers.


If I am wrong do please shaut, before I play with the mains!! :-)
 

Offline IanMacdonald

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Re: Lab with NO grd, what about gear?
« Reply #39 on: October 18, 2018, 06:13:06 pm »
Think the point to note is that the main safety purpose of earthing metal cases is to prevent any internal fault from making the case live.  So for the average end user of a toaster or whatever with a metal case, earthing that case is a safety improvement over not doing so.

However, on a test bench where exposed high voltages are present, especially when working on valve (vacuum tube) gear or SMPS, the presence of earthed objects on the test bench is a safety hazard. This is because you can't get a shock from a single connection. So, you inadvertently touch the anode  pin of an EL34 with maybe up to 700v on it, and you might get a slight tingle if your shoes aren't all that good as insulators. However, go to adjust a control on your earthed scope, and you'll be paying Emily van Dort a visit as the earth completes the circuit.

Also, earth fault trips can only protect against current flowing from the mains live to earth. That is because they don't actually measure earth current, they compare the live and  neutral currents and note any difference.  If the HV came from DC PSU in the equipment, then it won't trip. Thus while an RCD/GFI on the bench is a good idea, don't assume it will save your life in all types of mishap.

Basically, when working on exposed circuitry you need to be constantly aware of the risk of a hand-to-hand shock, since this is the most dangerous. Best approach is never to use two hands at the same time. Especially, be careful to keep your non-dominant hand OFF any earthed metal. If you need to adjust test equipment whilst also holding a probe, do so by insulating knobs only without touching the panel.

The danger of not earthing test equipment such as scopes and sig gens is that an internal fault could make the case live. If you only ever work on low voltages this is the best arrangement. For high voltage work the best approach is to run the test equipment via an earth-free isolating transformer. So in fact you need two isolating transformers, one for test gear, and one for the equipment under test. Using the same one for both is not a good idea.

A cutoff button on the bench is also a good idea. Especially if someone else has to come to your aid, it means they don't have to stop and think about how to cut the power. or whether it is safe to touch you.

Oh, and beware of building site 'isolating transformers' which are actually center-tapped autotransformers. Correctly termed tool transformers, these are NOT for testbench use and would actually make things a good deal more dangerous than with no transformer.

Bottom line is that there is no absolutely safe way to work on exposed electricity. Avoiding accidents is a question of using your noggin and being 'ahead of' things that could go wrong. But, isn't that always the case anyway?
 


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