Electrician Jobs RCBO trip test query - Testing wrong or is there a fault?

[Robert M.]

Hi,

Can anyone please explain why my friend's 30mA trip RCBOs passes the trip test (e.g. x5 is less than 40ms) when the test is done at the consumer unit but fails when the test is done at the sockets.

Is he doing something wrong or is there a fault somewhere (the RCBO or the circuit?

Thanking you in anticipation.

 

[Flanders]

If there is any load on the circuit then you can get false readings
Rcds should be tested with no load in the circuit.

 

[stevethesparks]

If there is any load on the circuit then you can get false readings
Rcds should be tested with no load in the circuit.

That's absolutely correct.
It just makes you wonder if they will work properly in an actual earth fault situation.

 

[Flanders]

I beleave its the way the tester works to simulate the fault , if there is already leakage on the circuit, for example a fridge or computers , thats when the tester gives the false readings .

 

[Pete999]

Are you not testing the actual RCD, not the circuitry?

 

[stevethesparks]

Yes, if there are other loads you might expect the rcd/rcbo to trip before its supposed to due to the cumulative effect, but there are also times when they won't trip at all when tested end of circuit.

 

[Midwest]

Has he done an insulation resistance test?

 

[UNG]

I would suggest ramp testing the RCD

Had this problem a little while ago fitted a new consumer unit and everything tested ok, went back 6 months later to do some additional work and the RCD would not trip on the 1x test and the 5x result was a bit close to the limit when tested at a socket same tests at the CU gave satisfactory results although I was not too happy so decided to ramp test at the CU and the socket and both results were very similar with an indicated trip current of 35ma. The result after a phone call to the manufacturer was a replacement RCD in the post

I now always ramp test RCD's before any other tests and I have had quite a few that trip above the rated tripping current