This is the final step in a design issue for my house rewire. The
situation is that I end up with a CU which can potentially pull enough
current to take out the company service fuse. Likelihood of this
happening is small, however... I'd like to protect against taking out
the service fuse with a 100A switch-fuse unit between the meter and CU.
Clearly this is not much help unless the switch-fuse trips out before
the service fuse. Simple - install a quicker blowing fuse, problem, I
would rather this fuse didn't go before the MCBs in the CU. Solution,
find a 100A fuse which will blow quicker than the service fuse but
slower than an MCB.

Any suggestions/alternatives?

Fash

"Fash" wrote in message
ups.com...
This is the final step in a design issue for my house rewire. The
situation is that I end up with a CU which can potentially pull enough
current to take out the company service fuse. Likelihood of this
happening is small, however... I'd like to protect against taking out
the service fuse with a 100A switch-fuse unit between the meter and CU.
Clearly this is not much help unless the switch-fuse trips out before
the service fuse. Simple - install a quicker blowing fuse, problem, I
would rather this fuse didn't go before the MCBs in the CU. Solution,
find a 100A fuse which will blow quicker than the service fuse but
slower than an MCB.

Any suggestions/alternatives?

Fash

Do you need 100A? Why not install say 80A? I can't comment on the speed of
trip but no doubt others can.

hzatph wrote:
"Fash" wrote in message
ups.com...
This is the final step in a design issue for my house rewire. The
situation is that I end up with a CU which can potentially pull enough
current to take out the company service fuse. Likelihood of this
happening is small, however... I'd like to protect against taking out
the service fuse with a 100A switch-fuse unit between the meter and CU.
Clearly this is not much help unless the switch-fuse trips out before
the service fuse. Simple - install a quicker blowing fuse, problem, I
would rather this fuse didn't go before the MCBs in the CU. Solution,
find a 100A fuse which will blow quicker than the service fuse but
slower than an MCB.

Any suggestions/alternatives?

Fash

Do you need 100A? Why not install say 80A? I can't comment on the speed of
trip but no doubt others can.

I definitely want 100A, the point is that I need overload protection
rather than fault current protection as this will be provided by the
MCBs/RCBOs.

Fash

I'd like to protect against taking out the service fuse
with a 100A switch-fuse unit between the meter and CU.

The alternative solution is to buy a supply of fuses that will fit the
electricity cutout. Naughty? A little. Effective? Yes.

Christian.

On 18 Jul 2006 04:26:28 -0700 someone who may be "Fash"
wrote this:-

This is the final step in a design issue for my house rewire. The
situation is that I end up with a CU which can potentially pull enough
current to take out the company service fuse. Likelihood of this
happening is small, however... I'd like to protect against taking out
the service fuse with a 100A switch-fuse unit between the meter and CU.
Clearly this is not much help unless the switch-fuse trips out before
the service fuse. Simple - install a quicker blowing fuse, problem, I
would rather this fuse didn't go before the MCBs in the CU. Solution,
find a 100A fuse which will blow quicker than the service fuse but
slower than an MCB.

Any suggestions/alternatives?

One can generally have two out of three criteria, but not all three.
In this case I think there is no way to have the sort of
discrimination you are after. The "electricity board" fuses do not
obey different laws of engineering to the ones you can buy.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Christian McArdle wrote:
I'd like to protect against taking out the service fuse
with a 100A switch-fuse unit between the meter and CU.

The alternative solution is to buy a supply of fuses that will fit the
electricity cutout. Naughty? A little. Effective? Yes.

Christian.

I already have one! I do have a solution that is 'proper' but I think
it's probably overkill and I should get over worrying about it.
Solution would be: Cut-out -- Meter -- CU1 with 100A mainswitch and
2x 63A Type D MCBs --- 2xCU with 100A mainswitch and mix of MCBs and
RCBOs.

Resulting effect would be that the Type D 63A MCBs would give overload
protection to the service fuse but would not trip ahead of the
downstream MCBs. Each of the child CUs would then have overload
protection as well.

It just feels a bit ridiculous given that there appears to be nothing
wrong with relying on the service fuse and putting in a standard albeit
large CU.

The current set-up is awful: Cut-out --- Meter -- Henley block --
split to 1. Rewireable Fuse CU, 2. Another Henley block which then
splits to 5 switch-fuse units each with no more than two fuse-ways in
them, all rewireable. Pretty terrible.

Unless someone comes up with the killer suggestion I'm going with
Christian's proposal with the fuses.

Fash

Christian McArdle wrote:
I accept that I can't get two fuses with the same rating that
will discriminate at very high fault currents, but it should
be possible at lower overloads which is what I'm talking about.

What lower overloads do you anticipate?

How many electric hobs and showers do you have?

Christian.

Realistic overload condition: It's christmas, evening time. The cooker
is running all 3 electric ovens (6kW), the lights in all 14 rooms are
on (2kW), the wife has decided it's a good time to fire up the kiln
(3.5kW) and the underfloor heating is on in the cellar (3.5kW). The
washing machine and dishwasher are running (3.5kW), someone is drying
their hair (1kW) The TV and all associated gubbins are on (1kW), Granny
has decided it's cold in her room and turned on the 3kW fan heater.
(We're now up to 23.5kW - 98A). Someone boils the kettle and we're now
over our 100A. This assumes that the electric shower has been removed.

OK so it's not that likely it's just that as discussed in another post
there will be 300A worth of breakers in the CU and somehow it feels
like there should be something between all of that and the service
fuse.

I am starting to get over it, particularly whilst racking my brains to
think of more things to turn on to take me past 100A. Particularly
given that I could probably pull 120A for quite a while without
creating a problem for any of the components.

Fash

Fash wrote:
This is the final step in a design issue for my house rewire. The
situation is that I end up with a CU which can potentially pull enough
current to take out the company service fuse. Likelihood of this
happening is small, however... I'd like to protect against taking out
the service fuse with a 100A switch-fuse unit between the meter and CU.
Clearly this is not much help unless the switch-fuse trips out before
the service fuse. Simple - install a quicker blowing fuse, problem, I
would rather this fuse didn't go before the MCBs in the CU. Solution,
find a 100A fuse which will blow quicker than the service fuse but
slower than an MCB.

What rating is the service fuse I wonder?

I'm asking because recently when we had our meter replaced routinely,
the guy said our fuse was not up to modern standards and needed
upgrading - I can't remember what ours is, and what he said it should
be, but maybe you are in a similar situation and all you need to do is
whinge at your supplier and they'll uprate your fuse FOC, end of problem?

David

Realistic overload condition: It's christmas, evening time. The cooker
is running all 3 electric ovens (6kW), the lights in all 14 rooms are
on (2kW), the wife has decided it's a good time to fire up the kiln
(3.5kW) and the underfloor heating is on in the cellar (3.5kW). The
washing machine and dishwasher are running (3.5kW), someone is drying
their hair (1kW) The TV and all associated gubbins are on (1kW), Granny
has decided it's cold in her room and turned on the 3kW fan heater.
(We're now up to 23.5kW - 98A). Someone boils the kettle and we're now
over our 100A. This assumes that the electric shower has been removed.

That lot is easily dealt with by diversity. It is very unlikely that most of
those heating appliances will be on simultaneously as they will largely be
thermostatically controlled. The system is designed to cope with short term
overloads where by fluke all the thermostats are engaged.

Christian.

On Tue, 18 Jul 2006 15:59:57 +0100, David Hansen wrote:

snip

The "electricity board" fuses do not
obey different laws of engineering to the ones you can buy.

Err, yes they do. Or to be more accurate, they will tend to have a
different fusing characteristic. In very general terms, the Distribution
Companies will be using fuses that are capable of passing a sustained
overload without undue degradation but will be fast-operating in the event
of a short-circuit.

Memory fails me now, but it's all to do with fuse-class ratings. I had a
browse but info about it is sparse on the web.

--
the dot wanderer at tesco dot net

Even an ordinary BS1361 fuse will carry its rated current indefinately,
will sustain a 30% overload for an hour, will trip in one second on a
3 times overload and will trip 'instantaneously' only on a ten times
overload.

Service fuses have an even more forgiving characteristic than this so
OP has little to worry about. Except his electricity bill.

On Tue, 18 Jul 2006 18:19:35 +0100, David Hansen wrote:

On Tue, 18 Jul 2006 16:59:05 +0100 someone who may be The Wanderer
wrote this:-

The "electricity board" fuses do not
obey different laws of engineering to the ones you can buy.

Err, yes they do. Or to be more accurate, they will tend to have a
different fusing characteristic. In very general terms, the Distribution
Companies will be using fuses that are capable of passing a sustained
overload without undue degradation but will be fast-operating in the event
of a short-circuit.

That is different to other cartridge fuses in some way?

Yes, it's a very long time since I had to know about fuse classes for
interviews :-). I think the classes are P, Q & R, BICBW. Each fuse class
has different operating characteristics.

--
the dot wanderer at tesco dot net

On 18 Jul 2006 10:13:27 -0700, dcbwhaley wrote:

Even an ordinary BS1361 fuse will carry its rated current indefinately,
will sustain a 30% overload for an hour, will trip in one second on a
3 times overload and will trip 'instantaneously' only on a ten times
overload.

Service fuses have an even more forgiving characteristic than this so
OP has little to worry about. Except his electricity bill.

Err, yes, that was more or less what I said. The theory is many years
behind me, sort of thing that was asked for a job interview.

--
the dot wanderer at tesco dot net

Fash wrote:

Realistic overload condition: It's christmas, evening time. The cooker

I think you are overworrying!

I am starting to get over it, particularly whilst racking my brains to
think of more things to turn on to take me past 100A. Particularly

Remember that a 100A cartridge fuse will supply 150A steady for at least
an hour or two without blowing. That ought to deal with any overload
scenario you can imagine.

given that I could probably pull 120A for quite a while without
creating a problem for any of the components.

Which the main fuse will happily supply indefinitely. ;-) (although glad
I am not paying your bill!)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Fash wrote:

Realistic overload condition: It's christmas, evening time. The cooker
is running all 3 electric ovens (6kW), the lights in all 14 rooms are
on (2kW), the wife has decided it's a good time to fire up the kiln
(3.5kW) and the underfloor heating is on in the cellar (3.5kW). The
washing machine and dishwasher are running (3.5kW), someone is drying
their hair (1kW) The TV and all associated gubbins are on (1kW), Granny
has decided it's cold in her room and turned on the 3kW fan heater.
(We're now up to 23.5kW - 98A). Someone boils the kettle and we're now
over our 100A. This assumes that the electric shower has been removed.

OK so it's not that likely it's just that as discussed in another post
there will be 300A worth of breakers in the CU and somehow it feels
like there should be something between all of that and the service
fuse.

I am starting to get over it, particularly whilst racking my brains to
think of more things to turn on to take me past 100A. Particularly
given that I could probably pull 120A for quite a while without
creating a problem for any of the components.

Fash

as others have said, you dont have a problem to solve. Not many
families run the kiln while cooking 6 turkeys simultaneously for
christmas. But any that do certainly wont have any need to switch on
extra electric heaters, not with most of 100A already heating the house
up.

And as also said, you can use 100A anyway


NT

wrote:

as others have said, you dont have a problem to solve.

I tend to agree - but in any case the most likely choice for the fuse in
the main switch fuse is a BS 88 gG (non-motor rated) type. This will
almost certainly operate more quickly under overload than the DNO's BS
1361 Type 2 service fuse.

For the time/current characteristics of the service fuse Google for
"LR85" or "KR85".

--
Andy

On Tue, 18 Jul 2006 18:37:46 +0100 someone who may be The Wanderer
wrote this:-

Each fuse class has different operating characteristics.

Yes, I knew that. However, they are not dramatically different as
they all obey the same laws of engineering.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54