High Earth Fault Loop Impedance readings caused by RCDs

[spud1]

Hi everybody,

I have encountered a problem with EFL (Zs) testing a couple of times recently.
I have had unusually high readings for the length of circuit and earthing system type and the increase in impedance seems to be coming from resistance across the line pole of the RCD in the consumer unit?

Has anyone else had this and have any explanation for it? as I am worried about he circuits in question achieving their disconnection times with such high readings!

Thanks

 

[Marvo]

Are you sure it's not just the clamp termination a bit loose on the wire or busbar?

 

[Octopus]

How does the live test reading vary from the calculated "reading"?

 

[guest119]

have you measured r2 for the circuits?

Boydy

 

[R G]

I would carry out the EFL test each side of the RCD and compare ,,,


Check all connections , do an RCD test it may be faulty..

 

[singingfraggle]

have had same problem as ray says check incomer of rcd and outcoming side if vastly different then could be loose conection inside rcd and as these have limits that they must meet .if new send it back and they will replace. newlec rcds and megger testers just dont mix. but you still have to meet your 80% values to meet disconnection times

 

[GLENNSPARK]

have had same problem as ray says check incomer of rcd and outcoming side if vastly different then could be loose conection inside rcd and as these have limits that they must meet .if new send it back and they will replace. newlec rcds and megger testers just dont mix. but you still have to meet your 80% values to meet disconnection times

so..if carrying out a measured value of Zs (he must be for RCDs to be in the equasion)..is this a true statement?

 

[GLENNSPARK]

still waiting for a reply to #7

 

[alanl]

Higher Zs test readings than the calculated figure (from R1,R2&Ze), are quite common.
It is a by-product of not tripping the RCD during testing, the tester cannot put a big enough current through the RCD to accurately assess the Zs, so it calculates the result from the low current test it does.
Some testers are better than others, and I believe the Fluke range are the most accurate in this test as they give very short bursts of high current to bypass the RCD tripping problem, and using an higher current give a more accurate reading.

I've experienced a number of different results that were higher than expected, MK were the worst, giving a ~0.50 ohm higher reading. The internal electronics of the RCD also affect the result when using the 'no trip' testing function.

 

[GLENNSPARK]

the point i`m making is that the 0.8 correction factor is used when Zs is calculated from Ze+(R1 R2)....

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not for measued values of Zs...