Sub main protection!

[TJ Anderson]

Why not? Maybe the seals are not fitted anyway?

I find that seems to be the case rather often

 

[TJ Anderson]

Hoppy, This is what I would do. Buy a 60A KMF. Feed with tails from Split Henley and you can then terminate your 16mm T&E correctly and ensure it is fused correctly. Fill in a MW saying replaced damaged unit like for like. Don't bother adding RCD at the front, if it was to be added in future it would be much better to RCD / RCBO at DB2 where the protection could be split up amongst final circuits.

 

[hoppy]

Thanks TJ, appreciate your reply!

 

[TJ Anderson]

Thanks TJ, appreciate your reply!

No worries, happy to help.

 

[Plonker 3]

You need to check thenmain incoming fuse first to ensure discrimination is met first though.

 

[telectrix]

Hoppy, This is what I would do. Buy a 60A KMF. Feed with tails from Split Henley and you can then terminate your 16mm T&E correctly and ensure it is fused correctly. Fill in a MW saying replaced damaged unit like for like. Don't bother adding RCD at the front, if it was to be added in future it would be much better to RCD / RCBO at DB2 where the protection could be split up amongst final circuits.

i like it. sometimes you can go too far trying to get older installations up to today's regs.

 

[Knobhead]

Hi,

been to job today to install a new bathroom light & fan in a HMO.

The 1st & 2nd floors are fed via a second db on the first floor - db2.

DB 2 is fed using 16mm twin, buried approx. 2m in wall, it is connected to the main tails next to db1 at the intake position, via Henley blocks, so single core insulation exposed (C2 as in kitchen cupboard used for plates etc). DB2 has 100A main switch and mcb's -no RCD protection. DB1 has a 63ARCCB Main switch.
Anyway not doing EICR yet, that is coming though.

My question is, when the EICR is completed it is going to fail due to the 16mm twin being protected by the 100 main fuse, so this will need rectifying, what would you do. Install an RCD unit which would mean a full test on DB2, or Is it ok to install a 2way enclosure with a 63A MCB, but would you still have to do a full test on DB2, or could you just test the sub main.
Not sure if it is acceptable to just protect the cable with an mcb, as I am not doing anyway work on the sub main.
The work I did in the bathroom is protected by a B6 60898, and I have installed an RCD FCU, to cover the light & fan!

Thanks!

Hoppy, This is what I would do. Buy a 60A KMF. Feed with tails from Split Henley and you can then terminate your 16mm T&E correctly and ensure it is fused correctly. Fill in a MW saying replaced damaged unit like for like. Don't bother adding RCD at the front, if it was to be added in future it would be much better to RCD / RCBO at DB2 where the protection could be split up amongst final circuits.

A light and a fan.
The total cost to cover up not doing basic checks beforehand will be?

 

[HandySparks]

You need to check thenmain incoming fuse first to ensure discrimination is met first though.

Why do you need discrimination with the DNO's fuse?

And what rating would you need to guarantee discrimination if the DNO's fuse is, say, 80A?

 

[Leesparkykent]

Why do you need discrimination with the DNO's fuse?

And what rating would you need to guarantee discrimination if the DNO's fuse is, say, 80A?

surely its always best practise to design discrimination in a situation like this?

 

[Plonker 3]

Why do you need discrimination with the DNO's fuse?

And what rating would you need to guarantee discrimination if the DNO's fuse is, say, 80A?

Please tell me this isn't a serious reply.

 

[Darkwood]

In domestic you cannot achieve S/C discrimination and very little partial discrimination, you can usually match your fuse to the DNO as they tend to use a type III or IV against your typical BS88 II so overload discrimation is in your favour.

 

[HandySparks]

Please tell me this isn't a serious reply.

It is a serious reply.

As I understand it, to achieve discrimination under fault conditions, the pre-arc I²t of the higher rated fuse has to be less than the total I²t of the lower rated fuse. Looking at one manufacturer's data for BS88 fuses, it appears that for the ratings we're considering, you need the rating of the smaller fuse to be less than 50% of the larger one, or thereabouts. So, as darkwood says, difficult to achieve in this sort of application.