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EICR and IR Testing

Former Community Member over 4 years ago

Oh the bane of IR testing on EICRs

Now its just impracticle to disconnect all loads to carry out IR testing, I think everyone can agree on that. And that Phase and Neutral are connected then tested to earth.

This is what I do, and then test @250V as to avoid damaging any equipment in the installation. And unless the IR value was <.5 Meg, would not bother me. What BS7671 states, but BS7671 is maybe somewhat unclear in that if that voltage (250V) was used then .5meg value should used, but that value should only be used on SELV, and PELV. Now experience also tells me that even if you had a return 0.0, and then did Kohms by using the ohms setting, even values of 100Kohms are fine. (Actually can be much lower than this) and we are pretty clear that its not the cable reurning these values.

So question is when doing an EICR @ 250V, at what point would you recommend an FI? For me the value would have to be <.5Meg

0 AJJewsbury over 4 years ago

Michael Moore:

Oh thats easy its 7.666Kohm

0.008Megohm

Which is what I alluded to on a Facebook group, and it completely baffled the bafoons there, who were apparently teachers!

Ah, but.. RCDs can validly trip at a residual current of anything between 50% and 100% of I_Δn so you should really be looking at keeping leakage below 15mA rather than 30mA for reliability (so perhaps 15.3kΩ rather than 7.6)

There again d.c. insulation tests only indicate simple resistance - in a real system they'll be some capacitive coupling that'll allow some extra residual current to flow too (so perhaps something significantly higher than 15.3kΩ)

Also there'll likely be leakage from appliances which likely won't be part of a fixed wiring test (even if they many appliances will have components switched off - e.g. pumps and motors of washing machines so likely to avoid the test) - so again you might want to add a significant margin on for that too..

- Andy.
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0 Former Community Member over 4 years ago

Andy

Another great response! much apprecited, yes indeed when doing a ramp test the OPD can open at 15mA.

Brilliant thank you
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0 mapj1 over 4 years ago

From a fire point of view an insulation pass at 10k to 20k would be approaching a more dangerous limit - but only if it really was all resistance, and concentrated at one point, we would be looking at circa 15mA - (10 k ohms would be 23mA) so 3 to 4 watts of dissipation. For sensible volumes (a few cubic mm of material) you do need a few watts to get things hot enough to do damage, but that is in that ballpark.

Of course, the same dissipation spread over several metres of damp mineral insulated cable would not be an issue- you'd need to be losing tens to hundred watts per metre of length to get that hot..

The other problem is that any insulation tester able to hold up the sort of test current you'd need at hundreds of volts, is actually lethal, so having a higher test limit makes the meter safer to use, as it can be designed to current limit at a mA or two and read low resistances unreliably.
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EICR and IR Testing

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