Strange RCBO behaviour (MEM)

[Adam_151]

I generally test RCBOS at the DB rather than at the end of the circuit, but came across something weird today RCBOs failed when I tested them at the board (even with the circuit connected), but worked fine when I tested them from a point on the circuit.

Completely threw me, it doesn't make sense to me, anyone?

 

[ColJack]

these the MEM ones with the "pod" on them?
the rcd works by a non contact CT round the live wire.. ( as I just explained in another thread ) , so testing at what is in effect the breaker screw will miss out the CT "tube"..

 

[JohnD]

what's "CT"?

 

[ColJack]

sorry, CT is Current Transformer.. ( I think.. )

coils that you put around a wire..
a voltage is induced in the coil relative to the current flowing through the wire..

 

[Lectrician]

Current transformer.

This is well known with the MEM RCBO's (even the MCH and ALB off the shelf range rather than field fit pods).

 

[JohnD]

gotcha, thanks