Multiple single phase power from 3 phase supply

[DHC]

Hello All, A bit of advice required if possible.

I have a workshop unit within a large industrial warehouse, and we have a 3 phase feed from the main power setup within the warehouse. The company that the main warehouse belongs to has a huge power demand as they make steel tools and have plasma cutters, 3 x 10t cranes etc... This is not a problem as they recently had extra power units put in (8 KVA). My problem within my humble little workshop was that I needed single phase power as I am IT engineer and do electrical repairs. So I installed a single phase consumer unit and took a feed from 1 leg of the 3 phase (Red Wire) and the neutral (Black Wire) and the earth from the steel shielding around the cable. This large shielded cable runs to a box on the wall within my workshop and terminates there, the box has a on/off handle and is marked with 600V 30A. This works without any problems and I have a full consumer unit with an MCB and an RCB (80A 30ma) which tests OK.

The problem is that we use a lot of devices with a high peak power demand within this workshop, and these devices added together often near the 80a max of the RCB. I have made sure that the loads are spread over individual fuse breakers in the consumer unit (IE: A row of 10 mains plug sockets on the back of a workbench on its own 32A fuse, lighting on its own 6A etc..) and this works fairly well apart from such as the lighting level dropping when the fan heater kicks in or someone boils the kettle!

I have a couple of spare consumer units which I would like to install and use the other 2 legs of the 3 phase supply, but this where my knowledge and experience is a little light. The two remaining legs are coloured yellow and blue, I tested both these wires with a multimeter across the neutral and they both read between 240v and 250v.

My Question

Is it possible to install a 2nd or 3rd single phase consumer unit given the situation explained above, and if so how would this be wired?

I have been an electronics engineer for some years and I have a some understanding of single phase power within a household wiring setup, but I don`t have much experience of 3 phase. I am aware that there is no second chance with 3 phase power and would get a qualified sparky to come and check my work before switching back on, but work is a little slack at present and I would like to do the donkey work to keep the bill down. I do know for a fact that there is an isolation switch for the main 3 pahse power cable running into my workshop, so I can work without risk. The main reason for this extra consumer unit is a hand water heater I recently purchased to give us some hot water as we only have a cold water feed, and I sure this would push things over the top.

Any advice would welcomed.

Danny.

 

[spinlondon]

There's no reason why what you propose cannot be done.

Your problems will be with the electricity supplier and the network distributer.

From what I understand, although you have a 3 phase supply into your workshop, you only in fact have a single phase metered supply?

You would have to arrange for the other 2 phases to be metered and before connection can be made, the distributer will very likely require an installation certificate along with appropriate inspection and testing schedules.

Do you feel you are able to provide this?

At the very least, I would imagine that a Periodic Inspection Report will be required, this will again need to be accompanied with inspection and testing schedules.

This could be provided by the qualified sparky, once you have finished the installation.

As for the actuallities of the installation, the 2 wires yellow and blue are live conductors, what we term 'Line' just the same as the red wire.

They will need neutral and earth conductors which will need to be fed from your existing neutral and earth conductors.

This is done usually by the use of Henly Blocks and an Earth Marshalling Terminal block.

The existing neutral conductor is cut, connected into a Henly Block and a feed taken to each of the three boards.

The same is done with the earth conductor, and this MET can be used not only for the feeds to the three boards, but also for any equipotential bonding conductors (you mentioned water).

There may already be a MET, but you may find you need a larger one. You can either cnange the existing one, or just connect a new one to the old using a suitably sized conductor.

It is quite important to try to evenly spread your load or current draw across the 3 phases, so you may well have to alter some of your existing circuits and spread them across the new boards.

Hope this is helpfull.

 

[DHC]

Thanks for the reply Spinlondon.

First of all, the power supply to my workshop is not metered and is part of the main warehouse power supply. The workshop I have is just a bricked off area in the bottom corner of a large warehouse, and there is only one electricity bill for the premises.

I look after the computer network for the firm that is within the main warehouse free of charge and in return they allow me to use their electricity. I believe that their last quarterly bill was just under 250k, so my usage is like a dripping tap next to a water fall!

As for the certificates, the firm within the main warehouse have an electrician that does any work they need and any inspections required. So I will be asking the same guy to check it over and give it the OK.

Many thanks for the info with regards installing additional consumer units.

As you suggested there is already a henly block for the neutral cable with enough spare sockets for another 2 wires.

As for the earth, when I first moved into the workshop I installed my own earth rod just outside of the warehouse with 15mm earth cable running into the workshop. I needed a reliable independent earth for myself as for more sensitive circuits etc..

Again, I will get the OK from the electrician for using this earth. I take your point about spreading the load across the 3 phases as evenly as possible, but if the large shielded cable that supplies the 3 phase power to the workshop is part of the main warehouse supply I would imagine that the balancing of the load is out of my control. I notice within the main warehouse there are several of these large shielded cables running to various points with some large machinery connected, and also to the three overhead cranes. Please correct me if I am wrong but would my attempts to balance the load within my workshop be in vain?

Last question: Will there be any problems with the 3 consumer units each containing an RCD whilst using the same neutral?

Many thanks for your advice so far.

Danny.

 

[spinlondon]

I thought in your initial post, you stated that you took an earth to the consumer unit from the Steel Wired Armour of the supply cable.

It is this earth that I am suggesting you fit into a MET, and then feed the three Consumer units from that. The MET can be placed anywhere that is convenient. Perhaps next to the box that the supply cable goes to.

You also mentioned a water supply. This should be bonded back to the MET by using a BS951 earth clamp attached to the pipe, with an appropriately sized earth conductor connecting the clamp to the MET.

The site electrician should be able to give you some advice on appropriate sizing for the Main earth (which is the conductor attached to the SWA) and the Main Protective Bonding Conductor.

There should be no problems with the RCDs sharing the same neutral. This is what most houses do. Each house in groups of three are on different Phases.

A word of warning, the voltage between any two of these phases will be a nominal 400V, could be more, could be less but not by much. There used to be a Regulation that accessories fed from different phases should be 2m apart, with a warning notice between. The Regulation now only requires the warning notice if the accessories can be touched simultaneously.

 

[ADS]

Hi mate,

Would it not make more sense to do away with the single-phase DB that you have already installed and replace with a three-phase DB.

Feed a supply to this from the three-phase isolator that you describe in your post.

That way all circuits can be neatly contained in one board and there no worry of crossing neutrals etc.

You can then, also, distribute your loading more evenly.

Just a thought.

 

[Andy™]

^ what ADS says. much better way

 

[binky]

[quote=DHC;183138

.. but this where my knowledge and experience is a little light. The two remaining legs are coloured yellow and blue....

.... but I don`t have much experience of 3 phase.

Danny.

Are you really sure you want to be doing this???? At least get someone to fix 3 ph DB and final connect circuits into it. Running lighting and power circuits isn't particularly difficult and at least you recognise the danger you are in. But not many sparks will wont to check your work and you need certification for insurance purposes.

 

[Manator]

As ADS has said I would install a 3ph board and run your DB,s from that. The site already have an electrician on board who will know the installation so could give the best advise. Lights dimming when you put the kettle on is a bad sign TBH

 

[sellers]

DHC, while I am impressed with your knowledge, I think on this occassion it may be best to get an electrician in. 3phase is a bad'n if you get it wrong. Even something as simple as forgeting to put the neutral link back in a 3phase board, will say bye bye to most of your expensive equipment.

Before going down the expensive route of upgading your installation it may be worthwile doing a clamp test to see how many amps your pulling, often when clicking on high powered items it can dim lights due to the quick power surge.

A couple of things that have been missed, can you find out what is protecting the incoming SWA? I would be concerened that your isolator below the board is rated at 30A and also is your RCB rated at 80A overcurrent or for switching 80A.

Tom

 

[kme]

Concurring with ADS & Manator here mate.

As you accept, your 3 pahse knowledge is sketchy. Get someone in to do the distribution side at least - if you are confident you can install the final ccts, that is up to you. Just make sure all ccts are checked for compliance - please?

KME

 

[steptoe]

do we have a rod and a DNO earth here?

or have I missed something?

NOT good.

Im not sure if this is something that should be discussed much further,

we are now going into the realms of helping people with 3phase, which as OP himself knows there is no 2nd chance with,

its not bob and his kitchen light here folks,

 

[ADS]

Kind fo agree with Steptoe here, and I meant to mention it earlier - before installing any further, you need someone to look at your earthing/bonding requirements and to assess the arrangements that you have at present - in particular the fact that you installed a rod and seem to be utilizing the supplied earth as well.

I'm guessing that, with it being a warehouse, you could have a lot of metal about - you really need someone with experience to look at this.

It's not just a case of adding a new DB or couple of circuits.

 

[SLIPSHOD & SLAPDASH]

Apart from the main switch only having a 30 Amp max load, there is an obvious volt drop if the lights dim when the kettle is switched on. It sounds like the original circuit was installed for a smallish 3 phase machine and you are already overloading the cable and you should get the factory sparky to check this out straight away