RCD Preventing circuit power on

[DinglyDan]

I recently turned a circuit off at the consumer unit (which has RCDs built into it) and when I turned it back on it did not power up.  After much checking I found this was because an RCD was plugged into a socket on this circuit and only when I removed this did the circuit power on when I switched the switch at the consumer unit.

I've not been able to find an explanation of why this happens.

How does the RCD in the socket prevent the circuit from powering up?

If the socket is turned off, how is the RCD even connected to the circuit?

Also, although I have RCDs built into the consumer unit I've always thought it safer to have additional RCDs in sockets when using lawn mowers and power tools.  Should I continue to do this or would it better to rely on the consumer unit RCD?

Thanks in advance for any answers to these questions?

D

 

[Murdoch]

If the circuit is protected in the fuseboard by a RCD having a 2nd one adds nothing

may be the one you had plugged in had failed ....

 

[binky]

RCD sockets have a test button that trips RCD in board when pressed. When powered down, I think they reset themselves, so chances are you pressed test button rather than the 'go' button to use the socket. 

 

[boltonsparky]

Also, although I have RCDs built into the consumer unit I've always thought it safer to have additional RCDs in sockets when using lawn mowers and power tools.  Should I continue to do this or would it better to rely on the consumer unit RCD?

There's no need really and there isn't any additional protection added by a second RCD. The only advantage would be if the RCD in the consumer unit had become faulty (it does happen) your plug in one would be a back up. 

You'd be fairly unlucky to have a faulty RCD and cut through the cable in a way that would electrocute you but it isn't impossible. 

RCDs should be tested regularly at the test buttons however most will sit there untested for years, the first bit of attention they get is when they trip off in the event of a fault or an electrician carries out an EICR. 

I can't answer why the plug in one prevented the CU one from being turned back on.