Slow tripping RCBO

[stilllearning]

Hi, just installed a new CU for someone. ELI, IR, ZS etc for every circuit are fine. They only have one ring for the whole house and I've protected this with a 30ma RCBO. The 2 lighting circuits and cooker circuit are also protected by RCBO's.

When I check the RCBO for the ring circuit it's slow to trip the first time; around 100ms for 1x and 35ms for 5 x. If I check it again immediately after it's fine (25ms and 8ms).

Leave it for an hour or so before testing again and it reverts back to slow tripping the first time and OK after that. I thought it was a dodgy RCBO but after swapping it I still got similar results. I've tested at the CU and at a socket with all the other circuits switched off. A ramp test show it trips at 25ma. I know the times are within limits but I'm just curious as to why this should happen Any ideas??

 

[RF Lighting]

Are you testing the RCBO with the outgoing circuit disconnected from it?

What was the Ze reading (roughly)

 

[stilllearning]

No, I left the circuit connected. Ze was 0.21. Thanks

 

[Spark123]

I take it the device is a BSEN61009?
As 100ms is less than 300ms @ 30mA then that is a pass.
As 35ms is less than 40ms @ 150mA then that is a pass.
Was it tested in isolation i.e. with all loads disconnected?

 

[securespark]

I wouldn't worry as long as it passes.

I've has 4293 devices come in at 199ms on the 1x test!!

 

[stilllearning]

Thanks for the replies. Yes, I know it passes but I was just curious as to why the 2 that I tried were both slower first time round. Thanks again.

 

[ban-all-sheds]

Contact the maker and ask.

I know that over time RCDs get "sticky", which is why people really should regularly operate the test button as specified, but an hour isn't really "over time"...

 

[Click-Sure]

but an hour isn't really "over time"...

It was time and a half when I had a job! Double time on Sundays and Triple on Bank/Public Holidays.