Crabtree RCCB and ZS testing

[Chris86]

Hello All,

After a bit of help regards ZS testing on a Crabtree Starbreaker board.

Have replaced 2 lengths of dado trunking in our office block at work and the measured ZS is coming out much higher than the calculated value.

Section 1 - calculated 0.5 ohms, measured 1.92 ohms.

Section 2 - calculated 0.42 ohms, measured 1.68 ohms.

I am convinced it's the RCD causing the higher readings so wanted to check ZS on incoming and outgoing terminals and compare the 2 but as it's Starbreaker with the internal busbar, there are no line terminals to test on. Is there any way I can test this or will I just have to rely on the calculated readings?

I have checked circuits supplied from the non RCD side of the board and the measured and calculated values are equal.

Sorry if it's a daft question but I haven't done a great deal of testing on domestic boards (we are factory based and all our stuff is Schneider or Red Spot 3 phase) and this is the first Crabtree board I have come across.

Thanks and ATB

Chris

 

[Wes1000]

Can you not use the incoming line tail terminal?

 

[Chris86]

Forgot that bit in my original post, i checked the incoming side in the way you suggest but when I try it for the outgoing side, the RCD trips.

ATB

Chris

 

[telectrix]

use this method.it bypasses the RCD and/or the MCB. remember to remiove the L-E short before re-energising. set mft to hi-current, not no trip.

 

[Chris86]

Thanks you your help, much appreciated. I am not back at that site until later in the week but I will give it a try.

Our meter is a Di-log and doesn't have a high current option, will your method still work on low current?

Thanks again,

Chris

 

[telectrix]

should do.but never heard of a mft without hi-current Zs facility.how do you measure Ze?

 

[Ian1981]

use this method.it bypasses the RCD and/or the MCB. remember to remiove the L-E short before re-energising. set mft to hi-current, not no trip.

View attachment 42845

Can’t remember seeing that method in GN3

 

[telectrix]

Can’t remember seeing that method in GN3

you won 't see it there. GN3 per-supposes an ideal world where RCDs and MCBs don't have internal components that f*ck up your reading, the ducks don't swerve as you pull the trigger, and women don't back answer you. .

 

[johnduffell]

Is there any way I can test this or will I just have to rely on the calculated readings?

Not sure if I'm reading this right, but if so you would need to resolve this before filling in the EIC and handing over the circuit as there is an unknown fault somewhere on the circuit that may prevent it meeting the disconnection times.

 

[Chris86]

Right oh, back with a few answers:

Telectrix- MFT is a Di-log 9083P and definately doesn't have a high current option. All loop tests are carried out at 15mA. Photo is attached showing the dial and screen when in loop setting.

Johnduffell- I have calculated the Zs using Ze+(R1+R2) to ensure disconnection times are met. I am under the impression that Zs via calculation can be used to complete the schedule of test results and I have commented on my reason for calculating rather than direct measurement. Happy to stand corrected if the above is incorrect though, every day is a school day in this trade.

ATB

Chris

 

[johnduffell]

I have calculated the Zs using Ze+(R1+R2) to ensure disconnection times are met. I am under the impression that Zs via calculation can be used to complete the schedule of test results

completely agree and if you'd never measured it you wouldn't know, but I wouldn't want to walk away from something if I discovered the Zs of my work was so far from design without understanding why. However I'm not a pro so and this may be a common non-issue!

 

[telectrix]

Right oh, back with a few answers:

Telectrix- MFT is a Di-log 9083P and definately doesn't have a high current option. All loop tests are carried out at 15mA. Photo is attached showing the dial and screen when in loop setting.

Johnduffell- I have calculated the Zs using Ze+(R1+R2) to ensure disconnection times are met. I am under the impression that Zs via calculation can be used to complete the schedule of test results and I have commented on my reason for calculating rather than direct measurement. Happy to stand corrected if the above is incorrect though, every day is a school day in this trade.

ATB

Chris

View attachment 42846

you are correct. Zs can be calculated by adding Ze+ ( R1 + R2). I came across a situation where BG had refused to connect a boiler as the Zs he measured was way over ( socket&seee LED, pass/fail job). Zs measured at a socket was 351 ohms. Ze on incoming side of RCD main switch was 0.22. on the outgoing side of RCD was 349 ohms.

 

[johnduffell]

Didn't realise that was alright, any clue as to why the Zs tester can't get the correct figure?

 

[Chris86]

I have been reading up on the issue myself and it would appear that the Di-log no trip testers have a bit of a reputation for struggling to read accurate Zs with certain brands of RCD.

I have used it over the last few months on Hager and Niglon RCD's with no problem.

I also didn't feel comfortable leaving the circuit without measuring Zs hence the reason for me asking about a method of testing across the RCD.

Thanks to all for your help.

ATB

Chris

 

[HandySparks]

I am convinced it's the RCD causing the higher readings so wanted to check ZS on incoming and outgoing terminals and compare the 2 but as it's Starbreaker with the internal busbar, there are no line terminals to test on. Is there any way I can test this ...

Test at the main switch (as already suggested) and then access the outgoing line by prodding the conductor of the insulated bussbar. Pull the MCB out to do it if necessary.