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Topic Title: EICR Forms
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Created On: 09 December 2015 08:03 PM
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 09 December 2015 08:03 PM
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20did99

Posts: 12
Joined: 24 June 2012

I was wondering why on EICR forms we are not expected to record a Zs between line and neutral.

We have all heard of the RCDs should not be used as sole means of protection, however this always to seem's to be the quick fix for a loop reading that exceeds that permitted for the protective device.

Granted, if the Zs reading between line and cpc exceeds, then an RCD is the way to rectify this. However we should now prove that the device will still operate in the required time if short circuit fault between line and neutral should occur.

In my opinion, I think recording loop reading between L - CPC on a TT system is pointless. If you have proved that the fault protection and additional protection RCDs will operate (and the Ze is well below 200 ohms), then any fault to earth will operate these devices, but a fault between L - N will not. Therefore the Zs readings throughout an EICR should taken between L - N not L - CPC.

I hope this make sense
 09 December 2015 08:24 PM
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spinlondon

Posts: 5494
Joined: 10 December 2004

The Z in Ze and Zs is referring to the earth fault path.
You therefore cannot measure the Zs between line and neutral.
An RCD operates by detecting earth leakage, it will not operate if there is a line to neutral fault.
In normal circumstances the resistance of the Line and earth conductors will be equal to or greater than the resistance of the Line and neutral conductors.
Therefore if the resistance (Zs) of the line and earth conductors is low enough for an overcurrent device to operate, it stands to reason the the resistance of the line and neutral will also be low enough.
A test similar to the Ze test is usually conducted to determine the prospective fault current (PFC), this test will indicate whether any overcurrent protection devices will operate.
 09 December 2015 10:24 PM
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sparkingchip

Posts: 10187
Joined: 18 January 2003

Why not both?

Andy
 09 December 2015 10:28 PM
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spinlondon

Posts: 5494
Joined: 10 December 2004

Why indeed.
 09 December 2015 10:55 PM
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monk

Posts: 69
Joined: 08 December 2012

Well I see your point. But with a high l-n path you'd find a number of electrical items in the house simply not working... a flag would be raised before an EICR takes place

...yes that depends on the actual value... someone will soon put more thought into it... I do see your point though
 10 December 2015 09:08 AM
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mapj1

Posts: 9711
Joined: 22 July 2004

voltage drop is normally noticed during operation - the earth can drop off and all appears well, but we have lost our 'defence in depth' we have gone from double fault to danger, to single fault to danger.
A loss of neutral may stop operation, but has no potential to make the equipment live to touch and no ADS to turn it off, unlike lost earth.
Honeslty, in a perfect world we'd test it too, but test time is money, and we accept the trade off time saved and a reduced test coverage, verus relative risk to life - testing R2 is important.
Testing Rn may be desirable, but not being backed by the same strength of safety argument, never made it into the final round....

-------------------------
regards Mike
 10 December 2015 09:26 AM
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AJJewsbury

Posts: 16114
Joined: 13 August 2003

However we should now prove that the device will still operate in the required time if short circuit fault between line and neutral should occur.

But what is the required time? (Rhetorical as BS 7671 doesn't specify one for L-N faults)

While L-PE faults cause exposed-conductive-parts to become live and so have to be disconnected within a very specific time limit the dangers from electric shock, L-N faults don't of themselves pose a shock risk - so disconnection time is much less of an issue.

BS 7671 only requirement is that disconnection occurs before the conductors (or more specifically, their insulation) suffers permanent damage. Usually, where the protective device is rated less than or equal to the cable rating, the protective device provides that protection - it might take minutes or even hours to disconnect, but the cable should be protected. Typically calculation is only really needed where the cable is rated lower than the upstream protective device (e.g. industrial motor circuits, or the domestic ring circuit).

Also the UK tradition of using smaller c.s.a. conductors for c.p.c.s than live conductors (both in the installation and the supply network) means that Zs is likely to be higher than L-N loop impedance. So measuring L-N alone is no guarantee that ADS will operate within the required time for a L-PE fault. Typically if L-PE loop is OK for ADS, then L-N loop will typically be OK for thermal protection of the conductors (as the protective device will be the same, the permitted disconnection time can't be shorter and the conductors normally no smaller).

What all that doesn't consider is the possibility of a bad (high resistance) joint in N. To cover that possibility I would agree that a L-N loop test would be useful - although I tend to cover that possibility (for new work) with a R1+Rn dead test myself.

- Andy.
 11 December 2015 05:13 PM
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leckie

Posts: 4453
Joined: 21 November 2008

Well dead testing has its own little posers.

If you have an off-peak supply and you are doing an EICR, you are probably only going to be able to do dead tests without some very special arrangements.

I tested a storage heater supply recently that gave an R1+R2 value of about 30 ohms. I noticed that the cable started as a red and black but came to the heater switch as brown and blue. There was nothing else connected to the supply from what I could see, so there was obviously a joint. First thought was the hidden joint probably had a bad connection either on the cpc. So I ran a wander lead out and measured R2, cpc was fine. Did the same to measure R1, it was about 30 ohms. Now I had been doing this with the storage heater isolator on, and a little thought popped into my head I turned off the switch and measured R1 again, open cicuit. So now I knew! A cross polarity; I had been measuring through the heater element. Confirmed this by test to the "neutral". So presumably in the hidden joint, which I could not find, black when to brown and red went to blue!

Just shows you, don't jump to conclusions.
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