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[SOLVED] [MOVED]RCD and MCB questions

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Newbie level 4
Jun 2, 2011
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Hi, Is that correct that there are two types of protection in the every electrical consumer unit? RCD and MCD?

I was asked to help replace the spoilt electrical outlet at my good friend's home in the night. After checking with MCB and electrical outlet, I switched off MCB that is connected to that spoilt electrical outlet. Why does RCD trip while I was replacing the outlet? I managed to replace the outlet throughtout the blackout in the night with torch.

Your help will be appreciated

Re: RCD and MCB questions

sorry for my bad english.
did you say about circuit breaker type Residual Current Devices (RCD)
if we have leakage in the electrical network then RCD may break electricity .
i think that wire or device maybe have a little leak to ground (home structure) in the time of replacing.

Re: RCD and MCB questions

No They are different for each purpose. They are put together in the same unit. I thought that with MCB off, would be sufficient and safe to replace electrical outlet. The Live wire should not be alive with MCB off. Does shorting neutral and earth wire cause RCD tripping?

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Re: RCD and MCB questions

How come no replies?
Am I in the wrong place?
I thought that anything not so close to electronics would be considered as off topic.

Oh come on..... if you know the quick answer but don't give, I will be dead the next day!




It depends on if the MCB is single or double pole. If it is single pole then it will only turn off the live. That can be risky if you don't know for certain that live and neutral are the correct way round.

Neutral and earth are not necessarily at the same potential - you can get a few volts difference. The RCD detects a difference in live and neutral currents. The small voltage on neutral will create a current when you touch it. That current won't be matched by the same amount in live so it will trip.

I have moved this thread to a more appropriate forum. "Off Topic" chat is for discussions not about electronics such as "What mobile phone do you use?", "What's your favourite movie?" etc. People who are here to answer electronic questions don't normally look there.


Thanks Keith,
Phew... My legs are still kicking alive


---------- Post added at 16:13 ---------- Previous post was at 16:05 ----------

Hi Keith,

So does that mean everytime, I make any changes to electrical network, I must set RCD off?


Yes, if the MCBs are only single pole. If you look at the part number of the MCB then you should be able to check if it is single or double pole but if in doubt, kill the whole panel rather than yourself.

In my house I have these but they are only single pole.


---------- Post added at 09:22 ---------- Previous post was at 09:19 ----------

By the way, the double pole ones "look" double pole: double pole

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