Fuse and main circuit breaker rating
 

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

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

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.

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..

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?

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.

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

Oh dear!
--

SJW
Please reply to group or use 'usenet' in email subject

On Sun, 09 Jan 2005 17:26:46 +0000, Lurch
wrote:

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

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.

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.


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

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

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?

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.
--

SJW
Please reply to group or use 'usenet' in email subject

"Lurch" wrote in message
...
On Sun, 09 Jan 2005 19:59:33 -0000, Ro
strung together this:

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?

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.


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.

"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

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

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.

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

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

Christian.