UK best way for isolation

[DPG]

Each flat will have its own consumer unit covering 3/4 circuits, there not large flats, more like studio flats, there will be a means of isolation in each flat via the flats cu , just wondering the best method of providing isolation at the main board, does there need to be individual isolation there to as it is now, each flat will be on its own MCB at the main board. Also at the landlords main board there will be the F/A and two landlords circuits for communal hallways. The card meters in the flats are sub meters, the landlord sells the cards to the tenants .

Have you read the comments above regarding feeding the flats via MCBs?

 

[telectrix]

this is your most cost effective way. however, if using SWA to feed flats, some steel trunking ( to gland SWA ) will be required.

Click DB701 80A Fused Main Switch (KMF)

Scolmore DB701 80A SP Lockable Switch Fuse

www.etcelectrical.co.uk

 

[pc1966]

Do your diversity/load calculations on the whole block, is it more than 100A?
What is the main fuse?

Have you measured supply impedance (for PSSC) and ground fault impedance (Zs for PFC)?

If the landlord is a tight-wad and not going to do it properly, and it is below the 100A limit, so a MCB board is the only option then at least get something like a 12-way or 14-way so you can put blanks between each of the flat's MCBs (and final landlord's circuit) to reduce mistakes in switching two at once, etc. Also get one with a 300mA type-S RCD incommer to protect the feed cables, and put all flats on RCBO (or worst case a local 30ma RCD) for safety on their own boards. Also landlord's circuits on RCBO as well.

Probably worth putting in an SPD there as well, that takes 2 of the ways up.

If you want any sort of reasonable discrimination with a MCB you will need the upstream one (i.e. per flat in the main board) to be something like a 50A D-curve and/or to set the flat's own rings on a 20A B-curve breaker instead of the "normal" 32A.

From that upstream MCB you get another constraint on cable size based on final (flat consumer unit) unit Zs for adequate fault clearance time, above the rating for 50A thermal (or whatever). Hence your measured supply Zs from the earlier comment!

Check the regs, crunch the numbers, and don't believe me as I'm just a passing stranger...
[automerge]1584620505[/automerge]
Actually the flat feeds would be "distribution circuits" I guess so a 5 sec disconnect time is OK, that would ease the max Zs value for a 50A D-curve to somewhere around 0.4 ohm.

Again, check that and compute max length for your cable of choice and its supply and earth impedance per meter values. You are the professional who is doing it!

 

[shaun1]

The first port of call should definitely be to calculate max demand for each flat and then check the total for the building doesnt exceed that supply. Then you probably need to replace that T&E with something a bit more suitable. And also add some overcurrent protection on the supply end (unless that isolator has a fuse in, which I dont think it does).

 

[L1Jay1]

Do your diversity/load calculations on the whole block, is it more than 100A?
What is the main fuse?

Have you measured supply impedance (for PSSC) and ground fault impedance (Zs for PFC)?

If the landlord is a tight-wad and not going to do it properly, and it is below the 100A limit, so a MCB board is the only option then at least get something like a 12-way or 14-way so you can put blanks between each of the flat's MCBs (and final landlord's circuit) to reduce mistakes in switching two at once, etc. Also get one with a 300mA type-S RCD incommer to protect the feed cables, and put all flats on RCBO (or worst case a local 30ma RCD) for safety on their own boards. Also landlord's circuits on RCBO as well.

Probably worth putting in an SPD there as well, that takes 2 of the ways up.

If you want any sort of reasonable discrimination with a MCB you will need the upstream one (i.e. per flat in the main board) to be something like a 50A D-curve and/or to set the flat's own rings on a 20A B-curve breaker instead of the "normal" 32A.

From that upstream MCB you get another constraint on cable size based on final (flat consumer unit) unit Zs for adequate fault clearance time, above the rating for 50A thermal (or whatever). Hence your measured supply Zs from the earlier comment!

Check the regs, crunch the numbers, and don't believe me as I'm just a passing stranger...
[automerge]1584620505[/automerge]
Actually the flat feeds would be "distribution circuits" I guess so a 5 sec disconnect time is OK, that would ease the max Zs value for a 50A D-curve to somewhere around 0.4 ohm.

Again, check that and compute max length for your cable of choice and its supply and earth impedance per meter values. You are the professional who is doing it!

yea im a professional, but not taken on this type of job before, the landlord is trying to save money, i think hes realised that hes going to have to get this work done, i also need the work at the moment so its difficult to turn down, i trying to go along with what he wants

Do your diversity/load calculations on the whole block, is it more than 100A?
What is the main fuse?

Have you measured supply impedance (for PSSC) and ground fault impedance (Zs for PFC)?

If the landlord is a tight-wad and not going to do it properly, and it is below the 100A limit, so a MCB board is the only option then at least get something like a 12-way or 14-way so you can put blanks between each of the flat's MCBs (and final landlord's circuit) to reduce mistakes in switching two at once, etc. Also get one with a 300mA type-S RCD incommer to protect the feed cables, and put all flats on RCBO (or worst case a local 30ma RCD) for safety on their own boards. Also landlord's circuits on RCBO as well.

Probably worth putting in an SPD there as well, that takes 2 of the ways up.

If you want any sort of reasonable discrimination with a MCB you will need the upstream one (i.e. per flat in the main board) to be something like a 50A D-curve and/or to set the flat's own rings on a 20A B-curve breaker instead of the "normal" 32A.

From that upstream MCB you get another constraint on cable size based on final (flat consumer unit) unit Zs for adequate fault clearance time, above the rating for 50A thermal (or whatever). Hence your measured supply Zs from the earlier comment!

Check the regs, crunch the numbers, and don't believe me as I'm just a passing stranger...
[automerge]1584620505[/automerge]
Actually the flat feeds would be "distribution circuits" I guess so a 5 sec disconnect time is OK, that would ease the max Zs value for a 50A D-curve to somewhere around 0.4 ohm.

Again, check that and compute max length for your cable of choice and its supply and earth impedance per meter values. You are the professional who is doing it!

yea i am a professional but ive not taken on this type of job before, the landlord is trying to save money, im trying to go along with what he wants but i have to make sure its all inline with the regs.

 

[pc1966]

@L1Jay1 sorry if I implied you are not, that was not my intention!

There are a few useful suggestions here, just take your time to go through the steps laid out by the regs and you should be fine.

If (as is likely) the block of flat's total demand is within the DNO supply fuse you have any of the options here to look at and decide what you think is the best combination that is acceptably safe and reliable.

 

[davesparks]

does there need to be individual isolation there to as it is now, each flat will be on its own MCB at the main board.

So you'll be ignoring the above advice, and the regulations, regarding discrimination and protecting each submain with an MCB?
[automerge]1584624357[/automerge]
From the picture it looks like the label says there's and 80A fuse in the cutout.

I'd want to carry out a load study first off to assess the actual demand on the supply to see if there is not a bigger problem.

 

[L1Jay1]

So you'll be ignoring the above advice, and the regulations, regarding discrimination and protecting each submain with an MCB?
[automerge]1584624357[/automerge]
From the picture it looks like the label says there's and 80A fuse in the cutout.

I'd want to carry out a load study first off to assess the actual demand on the supply to see if there is not a bigger problem.

im not ignoring the advice , clearly protecting each sub main with an MCB is not the way to go about it.

 

[pc1966]

So you'll be ignoring the above advice, and the regulations, regarding discrimination and protecting each submain with an MCB?

It is possible, but tricky.

Safe isolation would mean using lockout on the MCB tabs, should be OK for anyone who ought to be working on the feed cables, as the flat occupiers have their own 2-pole CU switch for isolation.

Discrimination is harder, from some doc I have from ABB they give fault currents for 2.5kA for a 50A D followed by a 20A B, so rings are fine if that is used as I can't imagine seeing PFC above that by time you get to the 13A sockets in the flats. Usual 32A B is only 1kA which is likely to be seen.

Shower/cooker on 40A are harder, the ABB doc has 0.5kA for 50A D followed by 40A B (not really useful), or 1kA if 63A D upstream (to me barely adequate) and that has more constraints on the final board's Zs.

From the picture it looks like the label says there's and 80A fuse in the cutout.

I'd want to carry out a load study first off to assess the actual demand on the supply to see if there is not a bigger problem.

Wow, your eyes are better than mine for sure!

 

[westward10]

This is 2020 discrimination is now selectivity

 

[pc1966]

This is 2020 discrimination is now selectivity

I stand corrected

In fact the doc I referred to is "Coordination tables 1SDC007004D0204" so yet another term to apply!

 

[snowhead]

Wow, your eyes are better than mine for sure!

For those of us with less than perfect vision;

 

[pc1966]

Blown up photo helps, but a closer/sharper one would be better. Maybe its a trick of the photo, but it makes the earth connection to the cutout look p-poor to me.

Also looks like rust stains on the isolator switch, never a good sign (just where the seal wire is fixed). Wonder if it is 100A rated if the DNO would grace the supply with a fuse upgrade?

 

[buzzlightyear]

Also looks like rust stains on the isolator switch, never a good sign (just where the seal wire is fixed). Wonder if it is 100A rated if the DNO would grace the supply with a fuse upgrade?

single phase to that lot ,more like 3 phase is needed .

 

[shaun1]

How is the house heated? If there is gas central heating controled by the landlord, could there be a danger of tenants topping it up with electric heaters?

 

[ipf]

This is 2020 discrimination is now selectivity

If you talked selectivity politically, it would be discrimination.

 

[Zerax]

How is the house heated? If there is gas central heating controled by the landlord, could there be a danger of tenants topping it up with electric heaters?

No risk of that if he's charging 'em £1 per kWh...