HRC fuse Q

[RickH]

Incoming tails from the meter box go indoors to a MEM 100ESSCF 100A SP+N Switch Disconnector.

This has a 100A HRC fuse (labelled as Type CM)
Can this be changed to an 80A fuse ? ........... as Supply Authority fuse is 80A, pointless having the 100A.

Its purpose was to isolate the house during construction.

 

[flameport]

It can be changed, but doing so will achieve nothing.

 

[EFLImpudence]

Can this be changed to an 80A fuse ? ...........

You can if you wish.

as Supply Authority fuse is 80A, pointless having the 100A.

Pointless having another 80A.

Its purpose was to isolate the house during construction.

The main advantage of the switch.

The purpose of the fuse is more likely to comply with a silly rule by the Network Operator which requires another fuse if the tails are more than two or three metres long - they don't even agree about the length.

 

[JohnW2]

Pointless having another 80A.

Indeed - and, I suppose, even more pointless to put a 100A fuse downstream of the DNO's 80A one!

The purpose of the fuse is more likely to comply with a silly rule by the Network Operator which requires another fuse if the tails are more than two or three metres long - they don't even agree about the length.

Quite so.

Kind Regards, John

 

[RickH]

i was just thinking that if it were 80A any additional circuit if I added in could be done in 16mm2, rather than 25mm2

 

[winston1]

i was just thinking that if it were 80A any additional circuit if I added in could be done in 16mm2, rather than 25mm2

It can anyway as it is still protected by the supply authority fuse. Having 2 80 amp in line will mean both will rupture with a fault.

 

[RickH]

My reason for Q.
I had an electrician come to site for another job, he pointed out that a circuit off the switch fuse to an enclosure with 80A RCD and a single circuit via 40 MCB (all via 6mm2) was fine from the RCD onwards, but the 1/2m link between the enclosure and Switch Fuse was incorrect as 6mm2 could not handle the 100A capacity of switch fuse.

The supply authority fuse as mentioned above is 80A, thinking that I would (should) increase the link from SwichFuse to 16mm2. (87A rated)

sketch attached. (Not on sketch but cables to CU are 25mm2)

 

[winston1]

but the 1/2m link between the enclosure and Switch Fuse was incorrect as 6mm2 could not handle the 100A capacity of switch fuse.

It will never have to as it is limited by the supply authority fuse as I said in post 6.

 

[EFLImpudence]

...and more particularly by the 40A MCB.

 

[RickH]

The Electrician agreed downstream of the RCD/MCB was fine, but had the view that the the link to Switch fuse was in 6mm2 under a fault on cct between RCD and 100A fuse then the cable was not sufficiently rated as a full 100A could flow.
RCD prevented downstream but 6mm cable not correct.

I used 6mm as it was a 40A circuit.

I did point out supply authority fuse was 60A, he advised fuses can be changed.
(Right in the point as Western Power updated it to 89A when they switched to PME last week)
He would not include it in the Part P certification … he certified downstream portion only.

 

[EFLImpudence]

The Electrician agreed downstream of the RCD/MCB was fine, but had the view that the the link to Switch fuse was in 6mm2 under a fault on cct between RCD and 100A fuse then the cable was not sufficiently rated as a full 100A could flow.

In a fault condition much more than 100A would flow but it will disconnect the supply "instantaneously".
In an overload condition the 40A MCB will protect the cable.

RCD prevented downstream but 6mm cable not correct.

That is true for the conditions against which RCDs protect.
See above.

I used 6mm as it was a 40A circuit.

Correct - subject to fault protection requirements which will be met.

I did point out supply authority fuse was 60A, he advised fuses can be changed.
(Right in the point as Western Power updated it to 89A when they switched to PME last week)
He would not include it in the Part P certification …

Some typos there, I think.

he certified downstream portion only.

 

[EFLImpudence]

RCD prevented downstream but 6mm cable not correct.

To clarify -

RCDs do not protect against upstream faults so it has no bearing on the size of the cable.

All down to the MCB and the fuses.

 

[JohnW2]

i was just thinking that if it were 80A any additional circuit if I added in could be done in 16mm2, rather than 25mm2

As has been said, electricity knows nothing about 'ownership' of fuses, so the DNOs 80A one will protect the cables to exactly the same extent as your downstream 80A one would.

Kind Regards, John

 

[JohnW2]

The Electrician agreed downstream of the RCD/MCB was fine, but had the view that the the link to Switch fuse was in 6mm2 under a fault on cct between RCD and 100A fuse then the cable was not sufficiently rated as a full 100A could flow. ... RCD prevented downstream but 6mm cable not correct. ... I used 6mm as it was a 40A circuit.

As implied, it sounds as if you need a different electrician - one who has some understanding of what he is talking about.

As has been said, the 40A downstream protection (by MCB) for the 6 mm² cable is perfectly adequate, and would be even if there were NO upstream fuse at all.

Kind Regards, John