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Electrical

High impedence reading on sockets

Hi. Anyone know the answer to this. An electrical test was conducted in a flat and the kitchen circuit on 2 sockets were above the allowed readings according to BS7671. The readings were in the range of 1800 Ohms. The circuits are RCBO protected and when a times 5 test is conducted the RCBO does not trip. But it trips at times 1. Rather strange as the socket faces are new so no issue there. The other sockets namely 2 of them give readings within the allowed tolerances

Any ideas

Thanks
Raj

6 Answers from MyBuilder Electricians

first thing is to check all the connections are tight . the RCBO may not trip if the resistance is too high, the lower the resistance the faster it will trip, is possible a leg of the ring has come unattached thus giving you a higher reading or it could be a problem with the wiring or design of the circuit, the more cable used the higher the resistance will be.... you may have to re wire!

2020-01-14T11:45:01+00:00

Answered 14th Jan 2020

I would re-do the tests as not tripping on x5 but tripping on x1 does not make any sense at all. You have not mentioned dead tests for the whole circuit R1, RN, R2? all OK? You mention 1800 Ohms, to what measurement does this apply? Suggest you do all the tests which might give us a clue.

2020-01-14T13:40:02+00:00

Answered 14th Jan 2020

How have you attained the rest of the readings r1r2 insulation tests, zs readings, when you test the RCBO is everything disconnected from that circuit? It make a difference. As a guide you do not have to do a 5x reading under the 18th Edition. That’s if all the afore mentioned tests are satisfactory.

2020-01-14T14:41:17+00:00

Answered 14th Jan 2020

Surgest redoing test tripping on x1 but not x5 makes no sense.

2020-01-14T16:15:02+00:00

Answered 14th Jan 2020

Just to clarify
BS7671 askes for domestic circuits up to 63amp are now required to be 30 m/a rcd protected and trip within 40 m/s
You carry out a 1 x rcd test to ensure the rcd trips within its rated current (<30 ma ) but you should also carry out the 5 x rcd trip test (150 m/a)to ensure your complying with the 0.4 sec disconnection requirements.
Any spark that is just leaving out the 5 x test isn’t complying with requirements.

Just to add the above is assuming the circuit impedance are within permissible tolerances
And the rcd is there for additional protection. So to the question you need to have the installation professionally inspected to find why your impedance values are so high,it’s like you have no main earth connected and relying on some other high impedance earth paths,

2020-01-14T19:15:02+00:00

Answered 14th Jan 2020

A reading like this means there is no effective means of earth on that part of the circuit (or the entire dwelling)

The person who changed the sockets should have tested but either did not or did and couldn't care less
Is there a certificate for the consumer unit alteration?

2020-01-17T17:30:02+00:00

Answered 17th Jan 2020

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