Strange RCBO test results??

[dellboy78]

I've never come accross this before so was wondering if anyone here can explain it for me!
Whilst testing new RCBO trip times, one of the RCBO's was tripping too slowly and somethimes not at all, something at 80ms at x1 and >1999 at x5 compared to all the others which were at around 18ms at x1 and x5. Anyway, I replaced the rcbo and tested again - same sort of readings. I replaced it with another new one and guess what....same thing again. I was starting to think that I had a bad batch but I tried disconnecting the circuit and testing the rcbo with no cable connected. Suddenly the readings were as they should be around 18ms. I dont understand how this can affect the speed of disconnection? As soon as I reconnected the circuit the readings went silly again, but it was only this one circuit that was doing it. Any ideas?

 

[Richard]

Did the circuit test out ok otherwise? Was there any sort of load on the circuit when you tested?

Rich

 

[Moses]

I've never come accross this before so was wondering if anyone here can explain it for me!
Whilst testing new RCBO trip times, one of the RCBO's was tripping too slowly and somethimes not at all, something at 80ms at x1 and >1999 at x5 compared to all the others which were at around 18ms at x1 and x5. Anyway, I replaced the rcbo and tested again - same sort of readings. I replaced it with another new one and guess what....same thing again. I was starting to think that I had a bad batch but I tried disconnecting the circuit and testing the rcbo with no cable connected. Suddenly the readings were as they should be around 18ms. I dont understand how this can affect the speed of disconnection? As soon as I reconnected the circuit the readings went silly again, but it was only this one circuit that was doing it. Any ideas?

Have you done IR tests on the affected circuit?

What are the results?

 

[Moses]

RCD test is only valid if the circuit IR tests are within regs.

and no loads are connected.

 

[Paul.M]

As above, you've answered your own question mate. It's the circuit that's at fault and not the rcbo.

IR test, R1+R2, R1+Rn and see what you get. Then test the rcbo, you'll find it.

 

[Adam]

As mentioned above, all loads should be removed.

This isn't always practical, or possible. However, I have found that fridges / freezers almost always give strange RCD trip times.

 

[dellboy78]

A colleague replaced the landlords ccu in a small block of flats but had to fit MCB's as the RCBO's hadnt turned up with the order. He tested the board and left it like that to go away for the weekend, I went back later the same day to fit the rcbos once they turned up and to do the rcbo tests which is what confused me. I haven't seen the other test results yet and wont get to until monday or tuesday now. I gathered that it was the circuit causing the problems but just wondered what could actually be happening to make the rcbo act like that. It's gonna bug me now 'til next week!