Unusual Zs reading when testing circuit with RCBO

[Octopus]

Been testing a modified circuit today.

Ze = o.20
R1 + R2 = 0.77
Zs = 1.77

So the reading is about 0.80 higher than I was expecting... Nothing on this (lighting) circuit switched on..... I was testing at the end of a smoke alarm addition.

I've read elsewhere, that its a good idea to do a Zs on the outgoing terminal of the RCBO to look for the "addition" this device is giving.

Is this correct? Anyone be able to point me in the direction of any previous threads, or guidance?

Thanks

 

[Andy78]

Never seen the internal resistance of a device to be that high, but it would certainly be worth investigating.

I get a bit of variance with the low current Zs test on my MFT, but nothing in the order of 0.8 ohms

 

[Ian1981]

Been testing a modified circuit today.

Ze = o.20
R1 + R2 = 0.77
Zs = 1.77

So the reading is about 0.80 higher than I was expecting... Nothing on this (lighting) circuit switched on..... I was testing at the end of a smoke alarm addition.

I've read elsewhere, that its a good idea to do a Zs on the outgoing terminal of the RCBO to look for the "addition" this device is giving.

Is this correct? Anyone be able to point me in the direction of any previous threads, or guidance?

Thanks

I take it this was on a non trip loop setting?
I.e. 15mA test current?
Perhaps 'noise' on the circuit or the internal components of the rcbo causing it

 

[westward10]

Type B 6A and below can cause this on high current tests but that is a lot. Ditch the rcbo and retest on high current.

 

[Octopus]

I take it this was on a non trip loop setting?
I.e. 15mA test current?
Perhaps 'noise' on the circuit or the internal components of the rcbo causing it

Yes - non trip setting

 

[Ian1981]

I take it this was on a non trip loop setting?
I.e. 15mA test current?

The

Yes - non trip setting

Three-wire no-trip testing requires connections to be made to the live, neutral and earth conductors, and typically uses a test current of 15 mA or less. Its benefits are that there is no need to bypass RCDs and RCBOs during testing, which saves time, and that it does not trip MCBs. There are, however, a few limitations.


The results are not as consistent as those obtained with high-current testing, the instrument takes longer to make the measurements, the results can be affected by external factors; also the RCD internal impedance may occasionally affect the result. In addition, even though this is a no-trip test, it sometimes happens that existing and test earth currents add together to trip the RCD or RCBO.
These limitations only affect a minority of cases, however, and three-wire non-trip testing is the preferred technique for circuits protected by RCDs and RCBOs where access to the live, neutral and earth conductors is readily available.

Taken from the megger site.

 

[soulman]

Hi,
I have had this problem before, spoke to megger, if the Zs is much higher than anticipated at the end of the circuit, Take a Zs at the incoming side of the rcd/rcbo & take a Zs at the outgoing side = resistance of the rcd device. Deduct this resistance from your measurement at the light.
Cheers

 

[LankyWill]

I would investigate further, seems a bit high even for RCD/RCBO malarky. Im with @westward10 chuck a standard B- type in and retest. Try another circuit on the same board and see if you get a variance there. The joy of testing ey..

 

[Strima]

If you need to then measure the Zs. However it is still accepted to add your R1+R2 to your Ze.

A lot of people are now being trained to keep live testing to a minimum to reduce risk. As long as you have Ze/PSSC/PEFC/phase rotation etc then you can use your dead test results to complete your paperwork.

 

[telectrix]

once has a Zs on all circuits 350 ohms. suspected a TT with a crap rod, but investigation found that it was PME .Ze of 0.2 ohms.was an upfront RCD causing this. test on hi current this way.diagram courtesy of Richard Burns.:

 

[davesparks]

If you need to then measure the Zs. However it is still accepted to add your R1+R2 to your Ze.

A lot of people are now being trained to keep live testing to a minimum to reduce risk. As long as you have Ze/PSSC/PEFC/phase rotation etc then you can use your dead test results to complete your paperwork.

Which leads to poor earth connections going undetected and makes the whole point of testing rather pointless.

 

[Ian1981]

Which leads to poor earth connections going undetected and makes the whole point of testing rather pointless.

i would say dead testing will pick up poor earth connections as you are disconnecting any parallel paths which may influence the test.
A simple zs will involve leaving all cpcs and bonding conductors connected and will generally give lower than the calculated value of zs due to parallel paths and may conceal the fact that a cpc is not continuous on the tested circuit.
Both tests are applicable in initial verification except zs can be determined by calculation alone eliminating the need of live testing and keeping it to a minimum

 

[Deleted member 9648]

TBH I'm so used to inaccurate no trip low current tests (they are the rule rather than the exception) that I'd just accept the result is way under the maximum for the OCPD and crack on. As already stated dead tests have verified low resistance R1 and R2.
Spending time farting about bypassing RCD's on one circuit may be viable. But if you've got a large installation to verify.....it's going to be a late one.

 

[Octopus]

FWIW I'm back on site tomorrow so will test the Zs on the outgoing side of the RCBO out of interest.

Thanks for the input so far.

 

[Strima]

Which leads to poor earth connections going undetected and makes the whole point of testing rather pointless.

I sort of agree with you, but if the dead testing is done correctly then any loose earths should be identified at that stage.

Also live testing may not detect a poor earth if only testing at the furthest point, any spurs on the circuit may get missed.

TBH both methods have their merits and downfalls.