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.

 

[Upton Sparks]

I've come across this as well on RCD and RCBO,

my understanding is that when doing the RCD test on most Meters on the non trip setting, the meter does this test by letting a very small current through down the earth conductor about 5 mill amps ish , and does this several time and then takes the mean value as the reading, therefore give a reading that doesn't always fall into the normal expectations

My advice would be to do your R1 + R2, record this value then take the Ze/ Zs at the DB and add these values together and record this as your value for the Zs for your circuit.

Just perform the Zs at the final point of the circuit for more of a confirmation that there is return path and piece of mind.

 

[davesparks]

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.

Dead testing does not pass enough current to highlight any fault in the earth path, the 200mA of the continuity test will not reveal poor connections. The usual 6A of the high current Zs test used these days is only a little better, but unless we are to return to using the test rigs of the past or the ductor type tester then it's the best we can do.

A method which relies on calculation doesn't prove anything at any point in the circuit, it's purely mathematical and entirely pointless in that respect.

I can't see any merit to the calculation method, modern test equipment and good working practice reduces the risk associated with testing live equipment.

 

[davesparks]

I've come across this as well on RCD and RCBO,

my understanding is that when doing the RCD test on most Meters on the non trip setting, the meter does this test by letting a very small current through down the earth conductor about 5 mill amps ish , and does this several time and then takes the mean value as the reading, therefore give a reading that doesn't always fall into the normal expectations

My advice would be to do your R1 + R2, record this value then take the Ze/ Zs at the DB and add these values together and record this as your value for the Zs for your circuit.

Just perform the Zs at the final point of the circuit for more of a confirmation that there is return path and piece of mind.

Better to leave the box empty than to record a calculated value which gives a false impression of a measured value.

 

[Upton Sparks]

Better to leave the box empty than to record a calculated value which gives a false impression of a measured value.

not at all.. and I disagree, leaving a blank box is worst thing to do... unless you put a long comment on as to why you've left it blank. .

there are 3 ways to obtain a result,
1.Measurements
2.Calculation
3.Enquire

your perfectly in your rights to calculate the result, as long as you do your dead test correctly.

Also as someone else had already mention above the point about this method being pushed as best practice for testing as it reduce the need for live testing, and anyone who's had to write some RAM's (risk assessment and method statement) will know the grief you've got in justify why your working live.

 

[davesparks]

not at all.. and I disagree, leaving a blank box is worst thing to do... unless you put a long comment on as to why you've left it blank. .

there are 3 ways to obtain a result,
1.Measurements
2.Calculation
3.Enquire

your perfectly in your rights to calculate the result, as long as you do your dead test correctly.

Also as someone else had already mention above the point about this method being pushed as best practice for testing as it reduce the need for live testing, and anyone who's had to write some RAM's (risk assessment and method statement) will know the grief you've got in justify why your working live.

I don't my see the issue in risk assessing live testing, it's certainly not the same thing as working live.
The risk is controlled by a high level of training and competence along with modern test equipment having being built to a high standard of safety. Meters have built in sensing to detect incorrect connection and not perform the test, along with fused shrouded leads.
Then if you identify a particular risk from say the design of some old equipment you need to test then you can use the alternative Zs test method to move the point at which you connect to the live supply to a point where it is safer to do so.
I'd be more concerned about the assessed risk of not carrying out live testing, the risk of poor earth connections going undetected because the calculated value makes it appear to be all ok.