Fuse and main circuit breaker rating

[Ro]

I live in a 20 year old appartment building. The main fuse blew the other day, and when I replaced it, I noticed that the fuse was a 50A fuse, but the main circuit breaker is 63A. There was a bag of assorted fuses left in the apartment when I bought it, and I am wondering if the lower rating fuse was installed by mistake, since there's very little difference between the appearance of the 50A and 63A fuse.

Is it normal for the fuse to have a lower rating than the circuit breaker? I would have thought that the circuit breaker would trip first, and that the fuse was a final fail-safe.

I have also noticed that the lighting circuit mcb trips everytime one of the halogen spots in the kitchen, or living room blows. Is this normal?

(I should mention that everything in the appartment is electric, even the central heating boiler!)

Thanks,

Ro

[Lurch]

On Sun, 09 Jan 2005 14:27:31 -0000, Ro strung together this:

Are you sure it's a MCB and not an RCD. If it's an RCD then the current rating is a maximum working current. RCDs don't trip on overcurrent, only earth faults.

The size of the supply cable will determine the size of fuses\MCBs etc..

Yes, although it can be altered. Usually fitting a type C MCB stops the nuisance tripping, as does changing the MCB for a HRC fuse.

Oh dear!

[Ro]

It is an MCB. There's an RCD on it also.

I'm sure that the MCB is correct, since that's never been changed. What I'm unsure of is whether the correct fuse is fitted, since I suspect that it has been changed.

Is it normal for the fuse to have a lower rating than the MCB?

Ro

[Lurch]

On Sun, 09 Jan 2005 19:59:33 -0000, Ro strung together this:

Could be intentional. It all depends on what size the supply cable is. The different sizes could be due to the fact that that's what the installer happened to have to hand.

[BigWallop]

As Lurch says, the cables from the main fuse might be to small for a larger fuse to be fitted. The fuse is there to protect the wiring directly from it, so it might be intentional for a smaller fuse to have been fitted.

The fact that the switch gear is of a higher rating is of no consequence to the main fuse at the supply head, it just comes as part of the consumer unit when bought, as a standard piece of kit. But the wiring gauge from the main fuse might demand that a certain rating of fuse be fitted for proper safety.

You'd need to find out more about the whole installation for anyone to help you more on this.

[Owain]

"Ro" wrote | I live in a 20 year old appartment building.

In Ireland I surmise from your posting address, so things might be a bit different there but

| The main fuse blew the other day, and when I replaced it,

in UK customers are not supposed to replace the supplier's main fuse.

| I noticed that the fuse was a 50A fuse, ... | since there's very little difference between the appearance of the | 50A and 63A fuse.

If they were British Standard fuses shouldn't they be different sizes and colours? Don't know about Irish fuses.

| (I should mention that everything in the appartment is electric, even the | central heating boiler!)

A 50A supply is woefully inadequate for all-electric. I live in a refurbished-twenty-years-ago flat and my supply is 100A - with gas heating.

Owain

[Christian McArdle]

Why are you convinced that the main switch is an MCB? This would be extremely rare. You mention an RCD as well. The normal situation would be to have a main isolator switch (with a maximum current marked in amps) and an RCD on the other side (also with a maximum current marked in amps). Neither of these devices would trip when the current exceeded the maximum.

What are all the markings on the MCB you mention?

The main cutout fuse is the property of the electricity board. You're not really supposed to touch it. Replacing a 50A fuse with a 63A fuse would be potentially very dangerous, and probably illegal.

It is not normal to have a whole installation MCB at all.

Christian.