Which trips first? ... there's only one way to find out ...

[Addlight]

There must be larger hole in my knowledge than I appreciated, as I can't get my head around why both the mcb and rcd should trip on short to earth.

Situation was wasa taking quick peek into a live double socket to check which way ring ran out the back of the dry wall box. About to push it back when thought - I know, I'll just check the terminals are nice and tight first...

Somehow, shorted live to the socket earth strap. Still don't know how, as I was using proper VDE driver with insulation to within couple of mil of tip, and terminal connector deep and well shielded.

Anyhow - VDE screwdriver, MCB and the RCD (17th ed CU) all did their jobs - just a flash and a slightly shorter screwdriver tip...

What I don't get is why the MCB tripped - or more precisely, had time to trip. Assuming the 30mA 63A RCD in the CU is within spec, it should see the L-N imbalance in around 18mS. So wouldn't it kill the power before the MCB could trip - ~ 18 mS being shorter than 0.4 seconds?

What am I missing or misunderstanding here please ...

 

[Risteard]

What you are misunderstanding is safe isolation!

 

[robsparky1975]

charachteristics of protective device/time curve,loop impedence must be very low i imagine,which is why it tripped at nearly same time,0.4 or 400ms if u like is the maximum,so look up bs7671 current/time as said and that will show u,unless im wrong,if so i need correcting aswell.

 

[Deleted member 9648]

Dont worry about it mate.....as long as something tripped.....As for safe isolation??....who hasn't done it??....usually at five to five on a friday afternoon

 

[Addlight]

Thanks for that - I actually had been starring at the performance charts for some time (table 3A on p. 243 for RCD and Fig 3.4 on p.249 for the B type) before posting, but it doesn't tell me what I need to know.

The PFC is way higher than 100A shown for a 20A breaker. Small print refers to manufacturer's data ... so that'll be my next port of call ...