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)....

- - - Updated - - -

not for measued values of Zs...

 

[singingfraggle]

Then why at my scam inspection do they ask what value do I work to and insist it is the 80% value that I use
I am not disagreeing with you just saying what I have been told to use

 

[GLENNSPARK]

Then why at my scam inspection do they ask what value do I work to and insist it is the 80% value that I use
I am not disagreeing with you just saying what I have been told to use

because (yet again) they are telling you a load of old cak..

 

[GLENNSPARK]

remember this:

if a Zs has been acheaved by calculation...that is to say you have taken a Ze...and added (R1 R2)...to get a Zs...then you have added whats refered to as a `hot` and a `cold` value together...and so 0.8 is applied to the tabulated value of Zs to allow for this

for actual measured values of Zs...then it would be the actual measured value that would apply...without the 0.8 correction factor applied

 

[alanl]

Hmm, I think there is a little confusion going on here, or maybe I'm not reading it right?

The max Zs figures are given in BS7671, eg, a 32A CB is 1.44 ohms.
That figure is the maximum Zs for that CB at 70 degrees.See the note at the bottom of the tables.

The figure at room temperature (~20 degrees) must be lower than 1.44, hence the 80% rule of thumb that you apply to your calculated figures and, the measured figures.

There are calculations you can do to find the exact value at various temperatures, but it takes time to do, and with many electricians not having the skills or time to calculate it properly, the OSG gives the 80% figure, that will assume a worst case scenario, and will give plenty of leeway for temperature rises.

So, yes, if it isnt calculated correctly,(using temperture correction factors), the 80% figure is used for both R1+R2+Ze figure, and the measured Zs figure (when taken at ambient temperature).

If your measured / calculated Zs on a 32A circuit is (say) 1.4 ohms at 20 degrees, then it will not comply at 70 degrees, so the OSG 80% figures should be used as the maximum Zs for the majority of electricians, unless you want to calculate using the temperature correction factors, which do give a more accurate result, and can be used to make marginal figures comply, whereas using the 80% may show the circuit to not comply.

Alan.

 

[mhar]

Not what I was taught. if when measuring Zs you were to apply enough current to raise the conductor temperature to 70C then that would be correct. However your mft only gives short bursts of low current, not enough to raise the temperature so you are effectively still measuring at ambient temperature and must apply the correction factor.

 

[Engineer54]

Haha, ....Never used the 80% value and never seen a final circuit ever coming close to reaching 70%.

As for any 17th edition test meter, ever being capable of taking a conductor to 70C, you're living in cloud cuckoo land!! An ELI tester injects what 20 to 25A into the circuit for around 2 cycles or 4 half cycles, hardly enough time to raise the temperature of anything!! Who telling you this load of old codswallop??

EDIT.... Ooooops, ....Sorry, i misread your post and read ''are'' for ''were''

 

[GLENNSPARK]

,,,,,,,,,,,

 

[sparks1234]

If its Hager, they have a resistance of about 0.67 ohms across the RCD

 

[mhar]

,,,,,,,,,,,

Sorry, don't understand

 

[spud1]

Sorry chaps I haven't had chance to reply to the initial few questions in this thread and I think the conversation has gone askew from my original question as a result.
I have checked connections on both incoming and outgoing terminals on the RCDs in question and they are fine. Also when I have come across it I have carried out EFL tests on both incoming and outgoing also. The reading has tended to be just under an ohms difference. e.g. 0.15ohms (effectively Ze) on the incoming and 0.98 ohms on the outgoing.
At a high load (e.g. 60Amps) that's a potential spot am I right?