Changing lighting circuit MCB

[wdubbin]

My upstairs lighting circuit is wired in 1mm^2 twin and earth cable, and protected by a 10A MCB (Wylex type B).

This past spring we had new (more) loft insulation fitted, and now the lighting circuit cable is under the insulation through much of its length, increasing the chance of overheating.

So.....I'd like to increase the protection on the circuit by changing the 10A MCB for an 6A MCB, like this one:

CU looks like this:

one of the experts advise, please, on how to go about changing the MCB? Do they come off the busbar easily?

Cheers for any help.

[Dave Plowman (News)]

Yes - depending on make they either just unclip or have a retaining screw other than the cable connection.

[Tony Bryer]

TBH I wouldn't bother. If there's a short circuit you're protected anyway. Unless you do something very odd (plug electric fire into bayonet adaptor as part of a Fifties bash ) you're not going to overload the cable which is probably only carrying 3A if all the upstairs lights are on.

[John Rumm]

Turn off the power at the main switch. Take the cover off the CU. Undo the screw at the top of the MCB and free the live wire going off to the circuit. Loosen the screw at the bottom that holds the MCB to the bus bar (you may need to remove a plastic cover over the bus bar on some CUs to get at these screws). The MCB is held to the DIN rail by a spring clip. With most of them the bottom of the clip hangs down at the back underside of the MCB. You can slip a small flat bladed screw driver into this lug and pull it down (usually by pushing the driver up against the base of the MCB and using it as a fulcrum). This should allow the bottom of the MCB to pull away from the rail (the bus bar will limit how far it pulls away), you can usually then lift it up and off the bus bar.

In true workshop manual tradition: reassembly is the reverse of the above procedure.

(the more prudent will now do a low ohms test from the inlet of the MCB to the live at the first light fitting or switch to make sure they have a good connection at the MCB)

[wdubbin]

Many thanks John - very helpful. If I have time this weekend I may give it a go.

[John Rumm]

Should also say, when you have the CU open, keep well away from the incoming supply end of the main switch (usually the top of it - i.e. opposite end to the input on the MCBs)

[cynic]

There could of course be a reason why you have a 10A MCBrather than the more usual 6A. Swapping the MCB might leave you with a problem of tripping when a lot of light are switched on. Better to properly analyse the set up

[Lurch]

On Wed, 25 Jul 2007 11:39:48 -0700, cynic mused:

There's also the thing that someone may have swapped it for a 10A in the first place without doing the calcs so it may be a bigger job than you want it to be to actually get it correct.

[cynic]

Rearrange the following words to make a well known phrase or saying

Worms can you opened have may :-)