RCD tripping while Zs

rattlehead85 · Nov 24, 2012

rmc86uk said:
We had the exact same problem today doing a Zs test kept tripping the circuit breakers in the control panel. Some of the breakers were 2A and 4A when tried with a higher breaker it wouldn't trip.

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The reason why your MCB rated at 6A or less trips is because your loop tester sends a current of around 25A around the circuit during the test. The MCB recognises this as an overload condition and will trip. To overcome this you need to change the scale setting on your tester from 20 ohms upto 200ohms and repeat the test.
Unfortunately the only way to prevent an RCD tripping under test conditions is to use a loop tester with D-lock facility. Under no circumstances should you 'bridge-out' an RCD as this in breach of the EAW & H&S regulations.

rmc86uk · Nov 24, 2012

Thanks Rattle I will admit I'm not up to scratch with live testing!

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lauriehurman · Nov 26, 2012

A clever person on here once pointed out that you can get Zs by having the main switch on, the RCD off and the MCB on. Then bridge between line-cpc at your light fitting (or wherever) and then Zs test across the open RCD.

Laurie

telectrix · Nov 26, 2012

if i remember correctly, it was sintra who posted that useful tip.

telectrix · Nov 26, 2012

rattlehead85 said:
The reason why your MCB rated at 6A or less trips is because your loop tester sends a current of around 25A around the circuit during the test. The MCB recognises this as an overload condition and will trip. To overcome this you need to change the scale setting on your tester from 20 ohms upto 200ohms and repeat the test.
Unfortunately the only way to prevent an RCD tripping under test conditions is to use a loop tester with D-lock facility. Under no circumstances should you 'bridge-out' an RCD as this in breach of the EAW & H&S regulations.

sorry, you're incorrect. the current generated by the tester on no trip is mA, that's 1 reason why the readings are not always accurate.

rattlehead85 · Nov 26, 2012

We are talking about an MCB tripping here. Not the fact that we are using the no trip setting. The op was asking why an MCB was tripping, not an RCD. The no trip setting sends a reduced current around the circuit to prevent a sensitive rcd tripping during the test. If you refer to your literature or head over to the IET website this is explained.