UK best way for isolation

[L1Jay1]

ive been asked to rewire 5 flats , each flat is on a card meter, a new cu is to be installed in each flat to cover 3/4 circuits,
a new landlords main board is to be installed by the front door, can each flat be isolated from the main board or does each flat need individual isolation like it is now . What size run of cable to each flat 6mm or 10mm, only one flat has a electric shower , two flats have an electric oven. The flat with electric shower doesn't have electric oven.

 

[pc1966]

Well at least there is a main isolator.
Is that 10mm T&E from the main isolator?
Is the T&E earth used at all?!

 

[Zerax]

Be interesting to know how the electric is being charged if tenants have card meters and it all runs through the Landlords meters.

Reselling and making a profit ??

 

[snowhead]

Reselling and making a profit ??

That was stopped in 2003, in theory.

 

[pc1966]

I guess that a Ryefield board is the dog's proverbials for this sort of job, but not exactly cheap:

RYEFIELD TP6AHS60 TPN Board 60A Fuse Units - 6 Way

Product Description The Ryefield Tpn 6 Way Board 60A Fuse Units Tp6Ahs60 is used to provide separate circuit protection for each dwelling in a multiple dwelling setting. Provides 6 x single ways Suitable for up to 6 flats 60A Fuses L1/L2/L3 in a 2/2/2 combination Horizontal arrangement...

www.electricals247.co.uk

Mind you I doubt a MCCB board and matching breakers would be much cheaper either.
A consumer unit and MCBs would work, but as already pointed out you won't get much in the way of discrimination for any high current faults (typically only to 300-500A or similar), as well as the extra issues of safe isolation. Schneider normally have an on-line tool to calculate discrimination between breakers and/or fuses, but for now it seems down so they offer the guide book instead:

https://download.schneider-electric.com/files?p_Doc_Ref=LVPED318033EN

A set of basic switched fuses is another good suggestion already made, though you will have the issue of Henley block capacity to face!

 

[L1Jay1]

All of the flats could be fed from a single DB, however it should not be an MCB distribution board. For any hope of discrimination each flat needs to be fed via a fuse or possibly an MCCB.
With that in mind it will almost certainly be cheaper to install a switched fuse for each flat supply.
Then there is also the future convenience of operation to be considered, it will be far better to have seperate switchgear for each flat to allow any future work to be done without disturbing other flats.

The size of the supply for each flat needs to be calculated and the cable size calculated.
If you have been a trainee but aren't now then that implies you are now, and fairly recently, fully qualified so you should have the knowledge to carry out cable calculations far better than just guessing at 6/10 maybe.

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 .

 

[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