Earth fault loop ohms

[Rob69]

Hi All,

I have been attempting to do Zs testing on two domestic lighting circuits but the RCD keeps tripping.

My meter is a megger 1502 which is set to "NO TRIP" and performing Three lead test.

Also performed Zs on two ring circuits with no probs and RCD did not trip!!

RCD test also ok and ramp test result was 25mA.....is this ok?????

What am i doing wrong or what have i missed.......?????

Thanks

Rob

 

[AndyD1]

Rob,

Are the RCDs 25mA rated?

If they are (or higher) it appears that they should not be tripping, see below from the manual:

Low current loop test is a 3 wire non-tripping

0.01 Ω loop test designed to prevent RCD tripping by

using a sophisticated 15 mA loop test pioneered by

Megger Limited. This NO-TRIP test ensures the RCD will

not trip whilst performing a loop impedance test.

 

[Rob69]

Thanks Andy,

The RCD is 30mA rated and tripped at 34mS at at nominal current and 4.8mS at five times nominal current.

Like i said the ramp test tripped the RCD at 25mA.Is this ok or should it trip at 30mS.I presume the ramp test increases fault current until the RCD trips so should be its rated value.How accurate does this need to be???????

and Why does it trip on lighting circuit and not on ring circuits????

Cheers

 

[AndyD1]

25mA (sometimes lower) is not uncommon for a 30mA RCD.

Are you testing with all other circuits terminated/ energised and have you tried testing at the RCD itself? You may find that you are picking up leakage from equipment or other circuits installed.

The trip times are fine (200ms @ 100% and 40ms @ 500% of the 30mA rating), is it causing problems with nuisance tripping when in use?

 

[ADS]

Hi,

Could a borrowed neutral be causing this problem.

My way of thinking is:

No-trip loop test trips RCD on lighting circuits only.

Three lead test performed so neutral utilised during test procedure.

If the lighting circuits are on seperate RCDs and have a borrowed neutral between them, then the RCDs might see an imballance during testing and trip out.

Just a thought

 

[Rob69]

Yes i am testing with other circuits energized. So will try with them off.

If i test at RCD then i take it all circuit breakers should be switched off.Test at RCD output which is effectively Ze test?

Will unknown loads cause a problem on the circuit under test?

Also , if my tester on NO TRIP with a test current of 15mA this should not trip RCD at all should it , unless something else effecting the test.....!!!!!

Thanks

Rob

 

[Rob69]

Thanks Andy and ADS,

No borrowed neutrals involved though!!!!!

Rob

 

[Rob69]

No nuisance tripping also.

rob

 

[AndyD1]

As ADS says, it is definitely worth checking the neutral connections, I would still try with other circuits isolated/ disconnected too though.

 

[Rob69]

ADS,

Only one RCD protecting whole installation!!!

Rob

 

[superspark]

hi rob

rcd times and ramp test seem all good so could be nuetral issue maybe borrowed.

secondly have you done ir test on lighting circuits could be low insulation resistance reading nuetral to earth.

and when your testing zs its throwing rcd out.

just a thought.

i very rarely test zs on lights these days because of having to use soft test

i normally do ze then r1r2 and then calculate zs it just works better for me and faster.

 

[superspark]

sorry just seen 1 rcd over whole board guess we can forget borrowed nuetral then!

 

[fletchflash]

could be a neutral and earth mixed up? Will not show up without RCD, ive found sockets wired up like this before when changing fuse boards

 

[Rob69]

Hi ,

All IR tests good.

Yes,when testing for Zs on lighting circuits only its throwing the RCD.

Is it ok then to get R1+R2 and add Ze to get Zs and insert on test schedule.

Just wanted to test for Zs to prove things to myself really ie meter and installation.

I take it bad terminations could cause this too????

Thanks Rob

 

[m4tty]

I always thought the no trip setting will still trip 6a lighting circuits but not normally anything higher than 6a

 

[batty]

I always thought the no trip setting will still trip 6a lighting circuits but not normally anything higher than 6a

Modern testers should not do that it was the old robin testers with d-lok that put 25 amps down circuit that did they also trip Rcbo's.

 

[Rob69]

Hi ,

All IR tests good.

Yes,when testing for Zs on lighting circuits only its throwing the RCD.

Is it ok then to get R1+R2 and add Ze to get Zs and insert on test schedule.

Just wanted to test for Zs to prove things to myself really ie meter and installation.

I take it bad terminations could cause this too????

Thanks Rob

 

[NozSpark]

are you absolutley sure you are connecting the meter up correctly..

i.e.

Red to bottom hole ROTFWL

Green to middle hole

Black to top hole

 

[Rob69]

Yes, NO TRIP test did throw 6A CB's (sorted that)but this is tripping RCD matty

 

[Rob69]

Yes nozspark