Testing an earth. How?

[bernardgreen]

Right Blunt talk

Obviously a couple of metres of under sized CPC will not melt before the protective device operates.

But a thin whisper of copper in a failing connection will pass the basic continuity test but could also melt rapidly when a fault current passes through it.

Yes the discussion can continue onto what specific fault and / or size of whisker could be the problem but as far as I am concerned if a CPC is protecting a circuit with a 32 amp MCB then the CPC must be able to carry the current that the "32 amp" MCB will allow for long enough for the MCB to heat up and trip. If for any reason the CPC cannot carry that much current ( even though it has passed a continuity check ) then the CPC will fail before the MCB operates and then any exposed metal connected to that CPC between the fault and where the CPC failed will be Live due to the now open circuit CPC being Live.

 

[JohnW2]

... But a thin whisper of copper in a failing connection will pass the basic continuity test but could also melt rapidly when a fault current passes through it. ... but as far as I am concerned if a CPC is protecting a circuit with a 32 amp MCB then the CPC must be able to carry the current that the "32 amp" MCB will allow for long enough for the MCB to heat up and trip. If for any reason the CPC cannot carry that much current ( even though it has passed a continuity check ) then the CPC will fail before the MCB operates and then any exposed metal connected to that CPC between the fault and where the CPC failed will be Live due to the now open circuit CPC being Live.

I'm sure that most of us understand that to be your point, and agree with what you are saying. However, I, for one, continue not to understand what you are proposing "to do about it" - i.e. what test(s), beyond a 'continuity' test (and maybe EFLI measurement) you are proposing/suggesting needs to be undertaken to 'ensure' that the CPC would remain intact for long enough for the protective device to operate in the event of a full-blown L-E fault.

As you know, to magnetically trip a B32 MCB will take about 160A. Even to thermally trip it in an hour will take about 46A, much higher currents being needed for more rapid operation than that. You surely are not suggesting that one should attempt to carry out tests using such currents to make sure that the CPC 'survives' for long enough for the protective device to operate?

Can you help me understand what you are suggesting/proposing?

Kind Regards, John

 

[securespark]

I can see both sides here.

In an ideal world, you would want to know by testing and inspection, that the cabling is adequate for any load or fault current.

But I can see what John is saying, yes, Bernard, you are right, a continuity test via a DMM will not flag up a dodgy connection to the cpc but then again neither will a kosher EFLI test with a certificated machine.

An IR test MAY show up a weak or failing joint but then again it may not.

This is why (for example) we fill in the box on an EIC to limit our liability on a board change to just that. We don't accept any responsibility for any faults that may remain hidden elsewhere in the installation (for example under floors) and undetected by the tests required by the EIC.

 

[JohnW2]

I can see both sides here. In an ideal world, you would want to know by testing and inspection, that the cabling is adequate for any load or fault current. But I can see what John is saying, yes, Bernard, you are right, a continuity test via a DMM will not flag up a dodgy connection to the cpc but then again neither will a kosher EFLI test with a certificated machine.

Quite. As I have acknowledged, I also understand bernard's point/concern, but what I don't understand it what he is suggesting we can 'do about it'. As I recently wrote, one simply can't undertake a test at 160A (or even 46A or 32A) to make sure that the CPC doesn't melt or otherwise 'fail' under such a fault condition.

An IR test MAY show up a weak or failing joint but then again it may not.

I'm not really sure about that. Only a pretty unusual fault (or combinations of faults) would result in both a 'faulty joint' and a 'leak' which would be picked up by an IR test.

Kind Regards, John

 

[bernardgreen]

As I recently wrote, one simply can't undertake a test at 160A (or even 46A or 32A) to make sure that the CPC doesn't melt or otherwise 'fail' under such a fault condition.

In its simplest form such a test is performed by using the conductor to discharge a battery. A car battery will give a couple of hundred amps depending on the resistance of the short across its terminals and its own internal resistance.

More comprehensive would be to measure the resistance of the conductor being tested. Then based on that resistance select a suitable voltage to create the required test current through that resistance and then apply it. Given that batteries are made up of cells there is a limited range of available voltages so a precise test current could not be achieved with out the use of some large wattage very low ohm resistors.

Then remove the battery and allow the conductor to cool before re-measuring its resistance. A significant difference between the resistance before the test current and the resistance after the test current would suggest something along the tested conductor did not withstand the test current.

 

[JohnW2]

As I recently wrote, one simply can't undertake a test at 160A (or even 46A or 32A) to make sure that the CPC doesn't melt or otherwise 'fail' under such a fault condition.

In its simplest form such a test is performed by using the conductor to discharge a battery. A car battery will give a couple of hundred amps depending on the resistance of the short across its terminals and its own internal resistance.

This is surely just fantasy? It doesn't matter much what power source one uses, it's the concept at undertaking any test at such high currents which is the issue.

A 12V car battery would not not get anything like 200A through the CPC of a typical final circuit. Just 25m of a 1.5mm² CPC would have a resistance of around 0.36Ω, hence just 33A, even if you used a massive cable (hence negligible resistance) for the return path to the other end of the CPC. With the same length of 1mm² you would only get about 22A under those conditions. In practice, therefore, you're likely to need at least ~100V to get 200A through a typical CPC.

More comprehensive would be to measure the resistance of the conductor being tested. Then based on that resistance select a suitable voltage to create the required test current through that resistance and then apply it.

As above, you would probably end up needing a lot of car batteries!

Then remove the battery and allow the conductor to cool before re-measuring its resistance. A significant difference between the resistance before the test current and the resistance after the test current would suggest something along the tested conductor did not withstand the test current.

That seems to be going even further in the direction of caution. If you undertook your difficult (and potentially dangerous) test at a current which would cause magnetic operation of the MCB and the cable 'stood' that current for more than a few seconds, then you would surely have demonstrated that the CPC was 'fit for purpose'. You seem to be postulating going even further, to determine whether a CPC which is 'fit for purpose' now might possibly cease to be so in the future.

Kind Regards, John

 

[bernardgreen]

A 12V car battery would not not get anything like 200A through the CPC of a typical final circuit.~~hence just 33A,

The point of the high current test is to ensure that there are no weak points in the CPC. Weak points will not be found by a simple impedance test.

25m of a 1.5mm² CPC would have a resistance of around 0.36Ω,

A 20mm long piece of 4 amp fuse wire has a resistance of about 0.014 ohms. Hence if the CPC had been nicked or otherwise reduced in CSA to that of a 4 amp fuse wire then a simple resistance test would be unlikely to find it 0.36 ohms = good and 0.37 = suspect

For the test and inspection of the vast majority of new systems the use of cable from a reputable source and accurate measurement of resistance against length will be adequate given that the regulations are very conservative in respect of how much current a conductor can carry before safety is compromised.

Not really worth the effort for domestic situations where almost all consequential losses from a failed conductor are manageable.

 

[JohnW2]

A 12V car battery would not not get anything like 200A through the CPC of a typical final circuit.~~hence just 33A,

The point of the high current test is to ensure that there are no weak points in the CPC. Weak points will not be found by a simple impedance test.

As I keep saying, I fully accept that - but it doesn't help me to understand what (in practice, in a domestic/commercial installation) one is "meant to do about it". In those environments, tests at 160A-250A are simply not practicol or safe.

25m of a 1.5mm² CPC would have a resistance of around 0.36Ω,

A 20mm long piece of 4 amp fuse wire has a resistance of about 0.014 ohms. Hence if the CPC had been nicked or otherwise reduced in CSA to that of a 4 amp fuse wire then a simple resistance test would be unlikely to find it 0.36 ohms = good and 0.37 = suspect

Exactly, although I would say 'virtually impossible', rather than 'unlikely', on an installed cable. In such a situation, one is very rarely going to know the length precisely, hence whether the resistance would be 0.36Ω or 0.37Ω in the absence of any faults/defects.

For the test and inspection of the vast majority of new systems the use of cable from a reputable source and accurate measurement of resistance against length will be adequate ...

I would be very interested to know whether (m)any electricians actually do that with new cable. I admit that I am in the habit of IR testing a new drum of cable (and I would suspect that very few electricians do even that), but I don't think I have ever measured the resistance of any of the conductors. Let's face it, the 1mm² CPC of a 100m drum of 1.0mm² or 1.5mm² cable will have a nominal resistance of about 2.2Ω (at 70°, a little less at room temp), probably making it next-to-impossible to know whether or not your 0.014Ω discrepancy (or worse) was present.

Kind Regards, John

 

[bernardgreen]

Let's face it, the 1mm² CPC of a 100m drum of 1.0mm² or 1.5mm² cable will have a nominal resistance of about 2.2Ω (at 70°, a little less at room temp), probably making it next-to-impossible to know whether or not your 0.014Ω discrepancy (or worse) was present.

Precisely the problem, there could a weak point in the cable or in a terminal on an appliance. A weak point that melts out before the protective device operates.

 

[JohnW2]

Let's face it, the 1mm² CPC of a 100m drum of 1.0mm² or 1.5mm² cable will have a nominal resistance of about 2.2Ω (at 70°, a little less at room temp), probably making it next-to-impossible to know whether or not your 0.014Ω discrepancy (or worse) was present.

Precisely the problem, there could a weak point in the cable or in a terminal on an appliance. A weak point that melts out before the protective device operates.

Yet again, I agree with you, but don't think there is anything practical we can do about that (exceedingly small) risk.

In reality, the risk you are contemplating is probably incredibly small. You are talking about a bit of CPC having somehow been reduced in CSA so much that it would melt in less than the 10ms or whatever it took an MCB to operate (at the currents we are talking about) - that risk is probably 'vanishingly small'.

I suspect that the risk of an MCB failing to operate as it should, even with an intact and 'adequate' CPC, is probably appreciably greater - and you essentially can't test MCBs, either. However, don't forget, there will, these days, very often be the 'additional protection' of an RCD.

If you cross roads, drive a car, use power tools or go up ladders (let alone smoke or drink alcohol), I would think that the risks you are contemplating probably do not warrant any loss of sleep!

Kind Regards, John

 

[ericmark]

I do see the problem the same problem as testing a MCB we have to trust the device loading to 160A to test is just no going to happen.

I have seen the ELI on two successive tests vary widely and on investigation an earth bolt was lose. But the DIY guy has maybe at best a plug in socket tester with a set of lights able to test earth with a 7mA load and for sockets that's really good enough.

As I said the problem is where the DIY guy wants to fit metal light fittings and having an earth wire present does not mean there is an earth. So some safe method of testing is required.

As an apprentice I had a plug with just an earth wire attached about 10 meters long with a ring terminal on the end with a lump cut out of it so it would fit the AVO Mk8 I used. Latter I used an only battery door bell two reasons could watch where the probe went and still hear the bell and worried that some one would steel my Mk8 on site.

To me belling out the earth was likely good enough. Possibly 1/2 an amp at 6 volt would likely pick up any really poor earths.

Then we all started to walk around with ELI meters and the bell and wire was shoved in a corner.

Is there a cheap ready made belling device of would the DIY guy need to build his own?

 

[JohnW2]

I do see the problem the same problem as testing a MCB we have to trust the device loading to 160A to test is just no going to happen.

Quite. No realistic test is going to pick up a situation in which a CPC was fine at low currents but would fail/melt at 160A. That is simply a (incredibly small) risk (like the risk of an MCB failing) that we have to live with.

As I said the problem is where the DIY guy wants to fit metal light fittings and having an earth wire present does not mean there is an earth. So some safe method of testing is required.

It's not really only the DIY guy that has the problem - an electrician will not really have any appreciably better methods of testing. In practice, I think that one probably has to settle for a low voltage/low current resistance measurement (e.g. a multimeter, or 'continuity' range of an MFT), or (in the case of an electrician) an EFLI measurement (mine uses ~12A for 10ms), neither of which will detect the incredibly rare problems that bernard is contemplating.

Kind Regards, John

 

[ban-all-sheds]

I do see the problem the same problem as testing a MCB we have to trust the device loading to 160A to test is just no going to happen.