Discuss One three-phase breaker, supplying multiple single-phase loads. OK or not? in the Electrical Forum area at ElectriciansForums.net

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Mark42

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I’m fitting a single-phase car lift in my own workshop.

New three phase sub-main panel 10m from the lift, 16A TP MCB (with TP 30mA RCD upstream) feeding car lift control panel the in middle of the concrete floor, via existing underground (non-ducted) 4c SWA.

Is it OK to mount the following on the control panel, without any additional, separate breakers?

L1 / L2 / L3: TPN red socket, with local TP switch.
L1 only: 16A industrial socket with local SP switch.
L2 only: 13A Switched double socket.
L3 only: The car lift itself.

It’s very useful to have a range of sockets local to the lift, without trailing leads across the floor.

I don’t want to use three separate SP MCBs in the DB because that would mean a shared neutral, with all the issues that brings. Additional cable runs are not possible.

I do want to have TP available at the lift for future-proofing, load balancing, and for the possible future exchange for a three-phase car lift.

It feels a clumsy design, but is it OK electrically?

An overcurrent fault of any one phase will kill the whole panel, that’s fine. An earth leakage fault on any one phase will trip the upstream TP RCD, killing the whole area. Not ideal but OK.

Ideally I’d use a 16A 3-pole RCBO, but such a thing does not appear to exist. I need to limit current to no more than 16A per phase for cable back-protection, as it’s a very long run of 10mm from the supply.
 
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No! No! And not Okay electrically!


EDIT:

What do you mean here:
"[FONT=&amp]I don’t want to use three separate SP MCBs in the DB because that would mean a shared neutral, with all the issues that brings." [/FONT]
 
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No! No! And not Okay electrically!

Thanks, but why not? I like to understand things.


What do you mean here:
"I don’t want to use three separate SP MCBs in the DB because that would mean a shared neutral, with all the issues that brings."

Because there are only four cores available. If I used three MCBs, one on each phase, it would be effectively multiple sources of supply into one panel, with a shared, single neutral conductor.

Consider a fault which breaks one phase only. Not only would there still be two phases live in the panel, but the single neutral would also become live were it to be broken while any load remained connected on either of the two other phases. Very poor and potentially dangerous practice! (Unless I’ve misunderstood something).
 
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Oops, I missed something, apologies. The 16A and 13A sockets would be wired on the same phase as they are provision for portable equipment and are adjacent.
 
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Sounds very messy would you not be better off fitting a 4 way 3 phase distribution board at the end of the cable and supplying your circuits from that, It would not be ideal feeding a distribution board via an MCB but better than the way you are suggesting
 
Sounds very messy would you not be better off fitting a 4 way 3 phase distribution board at the end of the cable and supplying your circuits from that, It would not be ideal feeding a distribution board via an MCB but better than the way you are suggesting

But he wants wants a neutral for all 3 phases for whatever reason.

This needs to be in the DIY section as he's not a spark obviously.

"Consider a fault which breaks one phase only. Not only would there still be two phases live in the panel, but the single neutral would also become live were it to be broken while any load remained connected on either of the two other phases. Very poor and potentially dangerous practice!

(Unless I’ve misunderstood something)"


Yes.
 
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But he wants wants a neutral for all 3 phases for whatever reason.

This needs to be in the DIY section as he's not a spark obviously.

"Consider a fault which breaks one phase only. Not only would there still be two phases live in the panel, but the single neutral would also become live were it to be broken while any load remained connected on either of the two other phases. Very poor and potentially dangerous practice!

(Unless I’ve misunderstood something)"


Yes.

I Don't see your point What I have suggested is use triple pole and neutral supply to supply a 3 phase and neutral DB then fedd the circuits from them , it would be the same as any 3 phase and neutral DB with single phase or triple pole MCBS fitted in it ,
circuit 1L1 16amp RCBO supply socket
Circuit 1L2 16 amp RCBo supplying Twin socket
Circuit 1 L3 16 AMP MCb feeding single phase isolator
Circuit 2 TP 16 amp MCB feeding tripel pole isolator
So tell me how would the neutral become live if one of these MCBS trips ( although the neutral is a live conductor buts being pedantic :) )
 
Your suggestion is obviously spot on.

My point is, your suggestion is wasted when reading his reply in the quotation marks.
 
But he wants wants a neutral for all 3 phases for whatever reason.

This needs to be in the DIY section as he's not a spark obviously.
That is a little belittling of OP tbh. He obviously does understand the theory if you read his post fully. The OP should do as Flanders has suggested. You could also argue that as neutrals for the SP sockets will terminate in control panel enclosure anyway, then you would isolate all three circuits before working on it and would label it as having multiple points of isolation.
 
That is a little belittling of OP tbh. He obviously does understand the theory if you read his post fully. The OP should do as Flanders has suggested. You could also argue that as neutrals for the SP sockets will terminate in control panel enclosure anyway,then you would isolate all three circuits before working on it and would label it as having multiple points of isolation.

Thanks, TJ (and Flanders). Ad Hominem attacks can be revealing.

My initial posts were written late at night and I don't really understand them myself now :)


I agree that another TPN dis board on the car lift post would be ideal but there just isn’t room, plus it’s a bit of an overkill (and overspend) for a few local sockets.


I’ll give up trying to find an imaginative solution, and wire two single-phase circuits down the four-core (blu & brn sleeved), from two single pole RCBOs: one for the lift, one for all the sockets together, and have no TP supply there at all. If needed for portable equipment, an extension lead can be run across the floor.
 
That is a little belittling of OP tbh. He obviously does understand the theory if you read his post fully. The OP should do as Flanders has suggested. You could also argue that as neutrals for the SP sockets will terminate in control panel enclosure anyway, then you would isolate all three circuits before working on it and would label it as having multiple points of isolation.


According to his profile he is a skip robber, best stick to that
 
For the record, I’ll try to be more clear about what I was getting at:

One three-pole 16A MCB feeding potentially single-phase loads. One neutral core. Deliberately protected together to avoid ‘borrowed’ neutrals.

I accept that on first reading that sounds daft, but what is the difference between that and a TPN supply to say a lathe, with a 230V light incorporated into the machine?

Or using, as many people do, an adaptor lead comprising a 16A 5-pin red plug to a single-phase 16A or 13A socket or sockets? Event suppliers sell such things: wired between one phase pin and the neutral. So long as the upstream three-phase protection does not exceed 16A how is that in any way incorrect?

As most on here will know, there’s nothing magic about 3-pole MCBs, they’re only three singles ‘glued together’ and constructed internally so that one pole tripping also trips the other two. If that’s in response to a single-phase fault, then so what?

You could think of my little panel of sockets as a discrete 'appliance' with its own TP supply and local TP isolator.

See picture below of a commercially-available adaptor. There are no incorporated SP breakers, and certainly not an inline distribution board! Consider the car lift to be plugged in to one SP socket, with the other sockets being used for tools. That, electrically, is exactly what I was proposing. Why all the abuse? I really don't understand.
 

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Can anyone, please, answer my question properly without just taking the p!ss? Whilst entertaining, that’s not helpful to anyone. :)

I’m building a new workshop this week and need some guidance. I’ve had a bit of a mind blank on this one. Here’s another very similar scenario I’m considering: There will be a number of workbenches, each needing one three-phase socket and various single-phase sockets for portable tools, all in a group on a small panel.

16A TP MCB > TP 20A isolator > 16A 5-pin TPN socket (L1/L2/L3/N/PE),13A socket (L1/N/PE) and two 16A sockets (L2/N/PE and L3/N/PE).

Each supplied by one 5-core 2.5mm SY. Each ‘workstation’ is therefore protected by one three-pole MCB, including the single-phase loads.

I usually work alone so it’s unlikely that more than one individual socket will be on-load at any one time.

I’m very short of ways and want to save cable and time: the scissor lift disappears after the weekend.

Despite you lot jumping on my back, I can neither think of nor find anything in the regs that says this is unsafe.

Immediate isolation is available locally by the TP switch; there in no inconvenience in tripping the whole panel at once if there a fault on any one socket.

Have I missed something?
 
I can see no reason why you cant do as you suggest, apart from the provision of RCD protection which is required for all sockets rated at 20a or less. While you could fit a RCD 13a socket at reasonable cost by the time you've added RCD protection to your 16a socket at each station costs will be mounting.
Far better to replace the DB for one of adequate capacity and run out an RFC for 13a sockets and individual radials for 16a sockets etc. RCBO's could then provide the required RCD protection.Probably wont cost a great deal more.
 
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I can see no reason why you cant do as you suggest, apart from the provision of RCD protection which is required for all sockets rated at 20a or less. While you could fit a RCD 13a socket at reasonable cost by the time you've added RCD protection to your 16a socket at each station costs will be mounting.
Far better to replace the DB for one of adequate capacity and run out an RFC for 13a sockets and individual radials for 16a sockets etc. RCBO's could then provide the required RCD protection.Probably wont cost a great deal more.
Thanks Wirepuller, for your sensible and helpful reply. It's appreciated.

I’ve not kept current for the last two years (I’ve been doing other things - that’s why I’m checking before I proceed with something potentially non-standard) and had completely missed the amendment removing the ‘skilled person’ exemption for certain non-RCD sockets.

OK, you’re right, I’ll shell out £300 and buy a new 16-way board, and a load more SY. I was just trying to use up stuff I have already, at zero cost :) I’m over-budget already on my new playroom (see pic), so do need to be sensible!

Final design as follows:

1. 13A
2No. 16A(B) SP RCBO (L1/L2) radials to a selection of 13A sockets, in trunked 2.5 T&E. (I don’t like RFCs.)

2. 16A
2No. 16A(C) SP RCBO (L3) radials to a selection of 16A sockets, in trunked 2.5 T&E (only one socket will be in use at one time)

3. TPN 16A sockets
3No. 16A(C) TP MCB radials to 3No. interlocked 5-pin TPN switched sockets in girder-clipped 5-core 2.5 SY. (For portable equipment, and electric car charging, giving flexible parking options.)

4. TPN Roller shutters
3No. 10A(C) TPMCB radials to 3No. TP roller shutter motors, in girder-clipped 5-core 2.5 SY, with local TP isolators.

5. Main lighting
3No. 10A(C) MCB radials (L1/L2/3) to 3No. 400W Metal halide Low-bay fittings, in 3No.trunked/girder-clipped 1.5 T&Es. 3-gang switch by personnel door, labelled as having multiple sources of supply.

QUESTION PLEASE (5): What is standard practice with factory lighting? Might this be better with a single 3-pole MCB, then run the supply to the switch and the outgoing switch lines in 5-core, looped through each fitting, with one neutral core only? No need then for the multiple supply label.

As these three main lights will be on all the time, I want them on separate phases so as not to asymmetrically load up one phase before I’ve even started. (Note the supply is limited.)

6. Car lift workstation (Design changed again!)
1No. 16A(C) TP RCBO(see pic) to car lift post via existing 4-core SWA in concrete.
RCBO fitted in Hager TP board by cutting off one L3 busbar tag (to be confirmed that it will fit, otherwise in separate enclosure). Neutral supplied for test button operation, but outgoing neutral permanently connected, not switched.
Workstation on car lift post as follows:
Local 20A TP isolator, then:
TPN 5-pin 16Asocket (L1/L2/L3/N)
Car lift (L1)
1No. 2-gang 13A socket (L2)
1No. 16A socket(L3)

QUESTION PLEASE (6): Former abuse notwithstanding, is this circuit really not OK? And if not, why not?

7. Other small stuff for office, store etc, not worth boring you with. Standard domestic install. All SP. Mainly RCBOs.

NOTES
a) Max supply available (via long run of existing 10mm) 20A x 3. Supplied from 20A(C) TP MCB at upstream disboard, for cable protection. Therefore no local breakers over 16A to ensure discrimination.

b) TN-C-S,‘exported’ from main house, all neutrals unswitched. Steel building bonded,plus local earth spike (yeah, I know, but very remote building, only three buildings from single (new) tranny. Was 480V split-phase, currently being upgraded).

Any design defects anybody? I do a shed-load of other trades as well, and do sometimes forget stuff.
 

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Have you thought about the PFC complying with the MCBs ?
Thanks, Spary, good question. Some of these are indeed only 6kA units.

But it's at the end of nearly 150m of 10mm so I doubt it will be possible to push more than 6000 amps down that lot. Time (and a test once installed) will tell ...

Am understanding this correctly?

Cheers, Mark
 
Oh, and to anyone who thinks why the hell did I build such a remote workshop supplied by only 10mm, here’s the reason.

The smaller storage building was put there ages ago, with not much power requirement apart from lighting, so was supplied in 10mm.

The new bigger workshop was supposed to be nearer to the house, but the planners insisted it was remote and hidden. I can’t face another 120m of trenching and messing up of my garden, all over again. At least not yet.

A prime example of how hassle cascades and costs escalate once the planners kick off.

I know, I should have applied the well-tested and often successful JFDI approach :)
 
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