Central heating MCB

[PM]

I don't expect it to blow at all. But then I've been disappointed in the past...

The posts from NP have convinced me to run 2.5mm^2 to the switch and 1.0mm^2 from then on.

[PM]

I'm getting confused between MCBs and RCDs then. I know the fundamental difference is that the RCD trips on current to earth and MCB trips on too much current. I just thought the quick trip was a feature of both.

[The Natural Philosopher]

I think you can find examples in both directions. i.e slo blowing fuses and fast acting ones that beat mechanical trips.

[David Hansen]

On Thu, 21 Sep 2006 10:40:03 GMT someone who may be marvelus wrote this:-

The Wiring Regulations disagree with you.

Protection by earthed equipotential bonding and automatic disconnection of supply is one of the recognised methods of providing protection against indirect contact.

[David Hansen]

On Thu, 21 Sep 2006 15:55:38 +0100 someone who may be "PM" wrote this:-

It is a feature of neither, though it is a common misconception that a gadget full of coils, linkages and contacts will somehow always operate more rapidly than a fuse element will melt.

As I said there are circumstances where an MCB will operate more rapidly than a fuse. There are circumstances where an RCD will never operate, but a fuse or MCB will operate. There are circumstances where an RCD will operate but a fuse or MCB will not operate.

Electrical systems are more complicated than they first appear. This can cause danger to people who don't know much about them, including people who have a background in electronics. As I occasionally mention, someone with an electronic background once asked me how many milliamps would flow in a certain situation. He was gobsmacked when I said that many thousands of amps would flow. He was even more gobsmacked when I said how many amps would flow if there was a fault in the nearby 11kV system.

[The Natural Philosopher]

Indeed. But MCB's dont do that do they?

Not in a 'I touched live and earth' situation.

[David Hansen]

On Thu, 21 Sep 2006 21:23:19 +0100 someone who may be The Natural Philosopher wrote this:-

They do the automatic disconnection of supply bit.

If someone touches a live part then the measures to protect against direct contact have failed.

Measures for protection against direct contact are (usually) different to those for protection against indirect contact. Direct contact is self-explanatory, indirect contact is when a part that is normally not live is made live because of a fault.