Tripping....

[Rammie]

Hi all

Main RCD keeps tripping out on main CU. Think I have eventually traced it to water ingress into an external junction box but why does it not trip out RCD on the small garage CU? Or even the breaker for the external supply on the main CU?

Thank you

Steepest learning curve of my life......

 

[m4tty]

Is the jb between main cu and garage cu if so it will only trip the upstream rcd. The breaker in main cu isn't tripping because it's not overload. It's an earth fault which the rcd will see not the CPD. Hth

 

[ukpablo]

as this is a fault to earth, and there are 2 rcd's in the cct the fault path is going back to the star point of the transformer via the cpc or neutral, and returning back through the ccts phase and tripping the first rcd it comes across which is the one in the main cu.
that seems the only logical answer to me.

look up diagram earth fault path.
As m4tty said, it is not an overlaod fault.

 

[Rammie]

The junction box is between a supply for a pond pump and small cu located in garage, which is why I couldn't understand why it was not tripping this RCD as opposed to main one in house. Have since established customer put jb in to connect the supply cable which had been cut in half?!

Thanks for the help its appreciated.

 

[pushrod]

there should not really be two rcds of the same type on effectively the same circuit as there is no discrimination. If there is i don't think you can predict which will be the most sensitive. If the distribution to the garage CU is in SWA , surface mounted or otherwise mechanically protected it does not have to be on an rcd protected way at the main CU and so it can be moved to prevent the nuisance tripping of the main CU.