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Rcd tripping on no trip whilst doing zs reading

Craig Burton

Did a zs test today on a ring main newly installed, had done end to end & ins res & rcd tripped when i pressed meter to go. Tested on another socket circuit which was existing circuit in the property & it did the same switched off all mcb's apart from mine & it still tripped. Plugged hoover in to new circuit to see if it would trip & it was fine. Never had it before. If i was doing a periodic how can i test zs if it does this & why would it do that. 

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    Yes, it can happen. My installation tester invariably trips a certain brand of RCBOs below 16A. It's one of those never ending battles where on one side RCD makers are trying to make their devices better/more sensitive/less error prone and on the other side instrument makers looking for ways of shoving a high enough current through them that they can get a half plausible measurement that's not swamped by noise or general variation in the supply. In the old days testers used a large d.c. current to deliberately blind RCDs so the test current went unnoticed, then we got A-type ones that can cope with and/or notice the d.c. and still trip.

    If all else fails, it's just a matter of measuring R1+R2 and adding to a measured Ze (or Zdb) and then verifying (e.g. with a continuity meter) that the c.p.c.s have been reconnected properly.

       - Andy.

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  • 0

    Yes, it can happen. My installation tester invariably trips a certain brand of RCBOs below 16A. It's one of those never ending battles where on one side RCD makers are trying to make their devices better/more sensitive/less error prone and on the other side instrument makers looking for ways of shoving a high enough current through them that they can get a half plausible measurement that's not swamped by noise or general variation in the supply. In the old days testers used a large d.c. current to deliberately blind RCDs so the test current went unnoticed, then we got A-type ones that can cope with and/or notice the d.c. and still trip.

    If all else fails, it's just a matter of measuring R1+R2 and adding to a measured Ze (or Zdb) and then verifying (e.g. with a continuity meter) that the c.p.c.s have been reconnected properly.

       - Andy.

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