Earth fault loop impedance test (Ze) with Multimeter?

[eveares]

I know that you can't use a cheap multimeter for carrying out a earth fault loop impedance test, but what would happen if your tried to get the value of Ze with one.

Would the reading be wrong, if so what would it show and why?

I am guessing it has something to do with the test voltage and current used by a multimeter being to low and it not being able to supply enough current to safely test the main earth?

So from a theory point of view, is it solely because a multimeter can not supply 25A that it can not be used to get Ze or because some other reason?

Also, not an electrician my self, just interested in the theory behind it.

 

[Spark123]

Don't try it, you may break your meter, blow it up and even do yourself harm.
A bog standard multimeter isn't made to measure resistance on a live line.

 

[333rocky333]

putting your multimeter across 240v unless on the VOLTAGE range would not do it much good

Ze is the external part back to the supply transformer.

maybe you mean Zs

 

[eveares]

I won't and had no intent to! I know it's a live test and am aware of the dangers.

Just what to know the theory as originally stated in my question.

 

[eveares]

I won't do this! But in theory what would my multimeter read on the ohms range if I connected one probe to the main live in and the other probe to the disconnected main earth going back to the local transformer.

 

[JohnW2]

I won't do this! But in theory what would my multimeter read on the ohms range if I connected one probe to the main live in and the other probe to the disconnected main earth going back to the local transformer.

If the meter was on a low ohms range (which is presumably what you mean), it would read nothing - at least, not for more than a few milliseconds. If you were lucky, a fuse (in leads or meter) might blow. Otherwise, it would either explode, burst into flames or, at the least, get otherwise destroyed - and there would be potential for it to destroy, or at least seriously damage, you as well.

Kind Regards, John

 

[eveares]

I think I have solved my question with ohms law.

A normal multimeter simply works out the impedance/resistance by dividing the fixed output voltage of its self by the current it measures and any external current by external live sources would be added to this native current provided by the multimeter, thus giving a false reading. Not to mention the circuits involved with measuring resistance could likely not handle the mains voltage or AC type wave forms.

Where as with a proper rated tester, not only can it handle the mains AC voltage, but also subtracts the mains voltage from the fixed output voltage of the tester, meaning it can accurately work out the impedance between the main line and the local transformers earth.

In essence the proper tester is rated for mains voltage and takes the mains voltage into account when working out the impedance.

Am i right in my theory?

 

[ericmark]

You can find Ze in two ways.
1) Special Meter.
2) Inquiry.
The problem is to measure involves time. Within a very short time the meter measures voltage with a load and without a load and calculates from those readings the earth loop impedance.

Since measured to earth during the test it will raise the earth voltage towards line voltage which means if the earth is not good it could have dangerous voltages on the earth. To reduce these dangers the meter measures in less than 40ms this also means it will not trip the RCD.

So in real terms if you don't have special meter then it's inquiry from DNO. Once you have Ze which should be entered on installation certificates you can work out Zs by measuring the resistance with a low ohm meter and adding it to Ze.

The low ohm meter will use at least 200ma for two reasons with three the third is to comply with regulations but using a high current means it will more likely find bad connections and also easier to measure 0.01 ohms accurate enough to give a reasonable reading.

Modern meters have improved and using a constant current source rather than simple battery and variable resistor likely they don't any longer need 200ma but when you buy the meter you don't really know how it will work.

I have asked the question before "Can one be certain using an earth loop impedance meter will do no damage?" maybe when mine was new it did have the test current marked but when I looked I could not find it so when testing on a boat I would take reading of shore supply and use low ohm meter on boat.

I was and still am uncertain as to how one should calculate and add the 1.2 volt drop between boat and shore due to diode but since allowed 200 ohms when using a RCD (Table 41.5 note 2) I was not really too worried.

I would say in real terms the same applies today with homes since all circuits now RCD protected you are allowed 200 ohms so unlikely to be a problem.

With a TT system we use a Wheatstone Bridge type circuit and yes I suppose you could measure this using a modern multi-meter as they are centre zero.

I have never put my mind to designing a circuit to measure the resistance of an earth rod although done it many times with a propriety meter. Although you still need to add DNO figure to find Ze.

 

[JohnW2]

I think I have solved my question with ohms law. A normal multimeter simply works out the impedance/resistance by dividing the fixed output voltage of its self by the current it measures ....

Indeed so.

...and any external current by external live sources would be added to this native current provided by the multimeter, thus giving a false reading. Not to mention the circuits involved with measuring resistance could likely not handle the mains voltage or AC type wave forms.

There is no "likely not" about it - most definitely not. As well as not being able to handle AC, an incredibly high current would try to go through the meter (on its ohms range) if it were connected between 'live' and earth - as I said before, probably destroying the meter and possibly also you.

Where as with a proper rated tester, not only can it handle the mains AC voltage, but also subtracts the mains voltage from the fixed output voltage of the tester, meaning it can accurately work out the impedance between the main line and the local transformers earth.

As eric has explained, they don't work in the same way as multimeters. The tester itself provides no voltage - only the mains is used. As eric said, they measure the (small) drop in voltage between 'live' and earth when a load (a low value resistor) is applied between 'live' and earth for a tiny fraction of a second (that to avoid the tester being destroyed). The tester can then calculate loop impedance (Ze or Zs) from the size of the load used and the (small) drop in voltage it results in.

In essence the proper tester is rated for mains voltage and takes the mains voltage into account when working out the impedance. Am i right in my theory?

As above, it's more complicated than that - these testers work in a totally different way from multimeters measuring resistance.

Kind Regards, ohn

 

[bernardgreen]

You can get a rough idea of impedance of the supply by measuring the voltage when a a known heavy load such as a 3 kilowatt fire or similar is switched on.

Do not measure voltage drop at the appliance as the impedance of the MCB, internal wiring and the appliance's lead will be included. Measure at the CU or a socket on a different and unloaded circuit.

Take the average of several measurements.

It will be more accurate, ( or rather less in-accurate ) if done when other people in the neighbour hood are not likely to be switching appliances on and off which will alter the current in the local network.

 

[JohnW2]

You can get a rough idea of impedance of the supply by measuring the voltage when a a known heavy load such as a 3 kilowatt fire or similar is switched on. Do not measure voltage drop at the appliance as the impedance of the MCB, internal wiring and the appliance's lead will be included. Measure at the CU or a socket on a different and unloaded circuit.

Indeed so - but if one does that, one will be obtaining a measure of the (L+N) loop impedance which, other than for TN-C-S supplies, will not (unless by coincidence) be the same as the Ze.

Kind Regards, John

 

[bernardgreen]

one will be obtaining a measure of the (L+N) loop impedance which, other than for TN-C-S supplies, will not (unless by coincidence) be the same as the Ze.

Yes in the majority of so called PME installations where the incoming supply cable has only two conductors and "earth" is derived from neutral then the loop impedance is for all practical purposes the same as the impedance that limits fault current until an over current protective device operates.

If the supply cable has three conductors then the test has to be the load applied between Live and "Earth" but this creates a hazard of lifting the local "earth" wires to a potential above ground while the test load is connected. How high the potential on the "earth" wires goes will depend on the quality of the "earthing" system.

DO NOT ATTEMPT THAT TEST

Personally I feel that provided the supply impedance is high enough to ensure maximum fault current is low enough that disconnect devices will not be welded closed then the system for over current protection is adequate.

The protection against any currents to ground should be from residual current devices detecting that Live and Neutral currents are different. With 30 mA sensitivity these devices will remove supply from a circuit even if the "earthing" arrangement is so poor that Ze is as high as 7666 ohms albeit with a transient voltage on the "earth" wire of 230 volts.
The transient voltage will not exceed 50 volts if the Ze is less than 1666 ohms.

 

[JohnW2]

one will be obtaining a measure of the (L+N) loop impedance which, other than for TN-C-S supplies, will not (unless by coincidence) be the same as the Ze.

Yes in the majority of so called PME installations where the incoming supply cable has only two conductors and "earth" is derived from neutral then the loop impedance is for all practical purposes the same as the impedance that limits fault current until an over current protective device operates.

Quite. I think that's what I said!

If the supply cable has three conductors then the test has to be the load applied between Live and "Earth" but this creates a hazard of lifting the local "earth" wires to a potential above ground while the test load is connected. How high the potential on the "earth" wires goes will depend on the quality of the "earthing" system.

I hope and presume that you're not suggesting that anyone does that! Apart from the dangers, if the circuit was RCD protected, the RCD would obviously immediately operate.

Personally I feel that provided the supply impedance is high enough to ensure maximum fault current is low enough that disconnect devices will not be welded closed then the system for over current protection is adequate. ... The protection against any currents to ground should be from residual current devices

That might be your personal feeling (and, personally, I'm not necessarily going to disagree) but readers need to understand that, other than in TT installations, the regulations essentially require that earth fault loop impedance is sufficiently low that over-current devices (e.g. fuse, MCB) provide adequate protection in relation to 'faults to ground' as well as L-N faults. In other words (albeit there is some vague talk about using an RCD to achieve required disconnection times if they can't be achieved with an OPD), the regs are not happy with L-E fault production 'relying on' an RCD (other than in TT systems).

Kind Regards, John

 

[bernardgreen]

I hope and presume that you're not suggesting that anyone does that!

Definitely NOT and so a warning has been added to the post

In other words (albeit there is some vague talk about using an RCD to achieve required disconnection times if they can't be achieved with an OPD), the regs are not happy with L-E fault production 'relying on' an RCD (other than in TT systems).

I cannot see why RCD protection is not acceptable. Unless it is because the modern RCDs are considered to be less reliable than the older designs. True that nothing can be 100% reliable but the use of electronic amplifiers to allow for less sensitive current transformers to be used is ( in my opinion ) a cost cutting but backward step.

The earlier RCDs had several turns of Live and Neutral on the toroid and the secondary produced enough energy from a 30 mA difference between Live and Neutral to reliably trip a mechanical latch . But the toriods' Live and Neutrals had to be hand wound and the mechanism was delicate and needed care in assembly and setting up. But they seldom failed to operate when needed to.

RCDs with amplifiers and half turns for Live and Neutral through the toroid can be assembled by robots so are very cheap to make.

 

[JohnW2]

I hope and presume that you're not suggesting that anyone does that!

Definitely NOT and so a warning has been added to the post

Thanks.

... the regs are not happy with L-E fault production 'relying on' an RCD (other than in TT systems).

I cannot see why RCD protection is not acceptable. Unless it is because the modern RCDs are considered to be less reliable than the older designs.

It wouldn't be the first time we'd come across a situation in which we "couldn't see" why the regs are such as they are. I basically agree with you, but I think you may have identified a 'reason' for the regs being as they are. I don't know whether it's necessarily got anything to do with modern RCDs being regarded as any less reliable than older ones, but there is this seemingly fairly widely-held belief that modern RCDs are not all that reliable in-service (we've discussed the {limited amount of} available statistics many times before). Whether they are actually any less reliable than the magnetic parts of MCBs, I just don't know. It could just be that whilst it's easy to test in-service RCDs (and hence identify 'failures') it's essentially impossible to test MCBs - so, for all we know, they could possibly be just as 'unreliable' in service as RCDs.

Given potential uncertainties about the in-service reliability of both RCDs and MCBs, I suppose the one thing to be said for the regs' approach (that one should not rely on an RCD for fault protection, unless that is 'unavoidable') is that it represents 'belt and braces'.

Kind Regards, John