One three-phase breaker, supplying multiple single-phase loads. OK or not?

[Mark42]

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.

 

[Mark42]

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.

 

[spary]

Have you thought about the PFC complying with the MCBs ?

 

[Mark42]

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

 

[spary]

Yeah looking it the pics the fault currents probably wont be that high

Sent from my SM-G900F using Tapatalk

 

[Mark42]

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

 

[Mark42]

Job done.

Everything's standard except for the one 'Workstation' supplied from a single 16A TPN RCBO (bottom right on the DB). The workstation has provision for single-phase loads, including a hard-wired SP car lift. There's a local TP isolator.

I did this because of existing cable limitations (4 cores only) and there being no room for another DB. (I already have 11 sub-mains on my site and really don't want any more!)

I've researched this in detail and cannot understand why this idea might not be OK electrically. Some members here said it's not, but without giving any explanation as to why not. Convince me, and I'll break up my nice new floor, but I'd really prefer not to

 

[Lucien Nunes]

Electrically, as far as the specific original question is concerned, nothing is going to go bang (provided you don't switch the neutral). Organisationally, there might be minor non-compliance e.g. if the manufacturers' instructions for the lift say that it should be on its own circuit. Not sure about your overall VD situation but with that shared final circuit but the starting current of the lift motor may push the VD over limit on the sockets, and the circuit might be considered notionally overloaded due to the likely application of the sockets. But that is not a 3-phase vs. single thing, it would apply to a shared single-phase final too.