RCCB making the Zs higher than it should be

[The Mains Man]

after finding an unexpectedly high Zs on a shower, I narrowed it down to:

Zs at input of RCD (RCCB) is around 0.18 as would be expected but the Zs at the output of the RCD is 0.96! Both tests done with 3 lead low test on Megger MFT with 3 month calibration.

It is a split load board and both RCDs show similar results. Both pass standard TCD trip tests.

I haven't come across this before although some googling showed an MK article that said the electronics in an RCD can cause this.

It is a pain in the bum to have to bridge out the RCD to do the Zs tests in this case!?

 

[Strima]

Just add your R1+R2 to your Ze(Zdb). Quite a common problem having high resistance through an RCD.

 

[telectrix]

it can be a pain, but you can do it like this on hi-current. option of bypassing RCD or MCB or both. just remember to remove the L-E shrt before energising.

 

[Midwest]

My foggy memory of just last year, suggests the Megger 1700 series, have other options to negate this issue. I just can't remember what it is

Just read the manufacturers instructions.

 

[Loki]

RCD uplift, had it today on MK.
Tested both sides of the RCD & got:
0.22 & 0.75

Just took off the 0.53 difference off the Zs readings for the circuits supplied by that RCD.

 

[telectrix]

alternative is to add the R1+R2 to the Ze.

 

[Charlie_]

Could be very misleading for those that just do the Zs tests on a PI..
Will you leave information for future tests?

 

[Risteard]

As above, the MFT1741 compensates for "RCD uplift".

From what I understand this impedance is not real but is seen by the meter.

 

[telectrix]

real or not,it's a pain in the arse. never caused any problems in the past with the bang test.

 

[Charlie_]

real or not,it's a pain in the arse. never caused any problems in the past with the bang test.

Ahh the good ole days!
We used to call it a flash test.
Passed the bang/flash test then job & knock down to the pub..
Failed the test then still an early finish with exception of the new breed of clever clogs apprentice who was left to do some testing

 

[ipf]

Could be very misleading for those that just do the Zs tests on a PI..
Will you leave information for future tests?

It's a case of R1R2 if RCD involved. Mind you, I usually do, especially on domestic jobs. On industrial, it's get the earth test lead out, if I feel there's any prob.

 

[remedial]

evening guys. im a bit confused by this. isn't the rcd part of the fault path? so if it has a high resistance it should be replaced?

 

[Loki]

I'd be interested in understanding why not all RCD's suffer from RCD uplift?

From what I understand when your measuring Zs on a RCD that suffers from uplift, it is adding the impedance of the coil in the RCD to R1 & R2 which throws out your readings

 

[The Mains Man]

Thanks everyone.
I too had assumed that is wasn't 'real' and shouldn't be included but I would be interested to real a proper technical explanation of why it happens.

I think Loki's idea of deducting the difference of each side of the rcd is the easiest option and at least you have done the test. Also agree that it is confusing on the cert (if the homeowner doesn't loose it before the next test!) but not sure where best to include it.

 

[remedial]

Thanks everyone.
I too had assumed that is wasn't 'real' and shouldn't be included but I would be interested to real a proper technical explanation of why it happens.

I think Loki's idea of deducting the difference of each side of the rcd is the easiest option and at least you have done the test. Also agree that it is confusing on the cert (if the homeowner doesn't loose it before the next test!) but not sure where best to include it.

here you go

Loop Impedance Testing: Have We Seen The End of Uplift? - https://professional-electrician.com/features/the-end-of-uplift/