RCBO tripping

[cctbreaker]

Hi folks, my first post. Been just lurking in the background for a while so time I joined in.
Had a callout today as lady had no power. Usual isolation identified kitchen ring protected by 32A RCBO. There was an incomer RCD as well (TT installation) that would also get taken out when the RCBO went. Everything unplugged and still tripped out. Narrowed down to section between two sockets but couldn't find any L-E or L-N connections etc.
Then lady remembered something she was using in one of the sockets earlier on had gone pop but didn't cause any trips and then I noticed the kettle live pin nicely pitted and burnt. Anyway, removed sockets, checked them out, reconnected them and no more trips. Happy customer but I'm not sure it's really solved.
Question is - what is likely to have been the cause, loose connection?

 

[WarrenG]

I would assume that as it is a TT system that there is a time delayed 100ma RCD in place and a RCBO with 30mA protecting final circuits.

If this is the case then there shouldn't be a problem with discrimination.

Like you say if the kettle plug's live is pitted then this would be a result of a poor connection (high resistance) made at the socket.

It may have simply been that the plug wasn't plugged in correctly (i.e. not pushed in all the way) or it could be that the socket is old and has a corroded terminal, or the socket is made from cheap quality and has also pitted over time?

I would suggest that you take a closer look at the terminals of the suspect socket (see if the connection is of green colour) and if the kettle plug is pitted, then put a new plug on it.

If the socket is damaged, simply replace.

 

[cctbreaker]

But what about the RCBO tripping? Can a poor connection at socket cause this?
Steve

 

[WarrenG]

The RCBO is really in effect an RCD + MCB.

In other words it will act like an RCD for earth leakage current (to a value of 30mA), and will also work as an MCB for overload / short circuit protection.

So you have good protection on the circuit before reaching the incomer RCD.

My thinking is that if a high fault current / overcurrent were to be created at the socket and was picked up by the RCBO, it would stand a chance that the incomer RCD will have picked up the inbalance of the circuit also.

If for example you do not have discrimination between the RCBO and the RCD (i.e. they are both at 30mA) you will get an up stream effect if there is a fault of say 500mA in the circuit, where both the RCBO the Incomer RCD will trip out.

For this type of install a time delayed RCD is ideal at the incomer.