Earthing/bonding of independent system inside existing building

[Mark42]

I've volunteered (!) to install a complete theatre lighting system in my local sports hall, so the stage can be used for theatrical productions, bands and so on. It should be fun.

Part of the deal is that the new system is to be completely independent of the existing installation.

It will be fed from a new Hager three phase board, with RCBO circuits going out to the amps, stage rack, various lighting trusses and so on. The board will be at the rear of the hall, where the stage snake and other infrastructure will terminate for the audio and lighting control desks.

A 5-core flex, either from elsewhere in the building or an external generator, will be used to energise the installation only whilst it’s in use. It’s analogous to a mobile studio with an appliance inlet on the side.

Here’s the question: there’s no easy route from the new board to the building incomer. Do I need to earth the installation separately, or can I rely on the inlet earth which of course will be present only when connected?

I’m guessing the whole thing becomes extraneous metal parts when not connected to anything?

 

[ImpededLoop]

It'll only be extraneous if it's connected to true earth such as via an underground metal conduit etc. If the building is steel framed, the lighting frames may well end up being bonded by being bolted to the steel framework.

Another way to think of it is that if you dump a fridge in the middle of the hall and don't plug it in, does it need bonding?

Without seeing the building its difficult to give a definitive answer. If you're planning to use a generator then careful thought will need to be given to having two different earthing systems in the same building and potential differences between them.

 

[Mark42]

It'll only be extraneous if it's connected to true earth such as via an underground metal conduit etc. If the building is steel framed, the lighting frames may well end up being bonded by being bolted to the steel framework.

Another way to think of it is that if you dump a fridge in the middle of the hall and don't plug it in, does it need bonding?

Without seeing the building its difficult to give a definitive answer. If you're planning to use a generator then careful thought will need to be given to having two different earthing systems in the same building and potential differences between them.

All excellent points. Thanks for your insight.

The building is brick block and wood so there's no true earth anywhere.

Your fridge analogy is bang on. The reason I'm considering this at all is because I forsee getting a lot of verbal about an 'unearthed electrical installation' from somone who hasn't thought it through.

 

[Lucien Nunes]

The earthing systems of the two installations most certainly need to be interconnected, otherwise each system is a massive bunch of extraneous parts to the other. FWIW I've had more equipment blown up by independent, non-interconnected earthing systems on film sets than any other cause (possibly with the exeption of missing neutrals). I'll happily expand on that if your're interested.

Why would you install the distribution at the desk end of the hall... surely it wants to be at or near the stage end? Are you having dimmed circuits for generic fixtures or only hard power? Contactors? Many Q's I have.

 

[pc1966]

All supplies must be reliably referenced to the Earth, it is part of the ESQCR and in the wiring regs.

While having a separate rod for the generator (if used) technically meets that goal, really you will have all sorts of trouble and risk if they are not reliably bonded together. Even if you don't plan on using the incoming supply as such, run something like a 10mm CPC (assuming here typical 100A with 25mm line/neutral sort of supply and it is bonding, not main earth conductor) around the side/edge/etc of the building to connect them together.

 

[Mark42]

The earthing systems of the two installations most certainly need to be interconnected, otherwise each system is a massive bunch of extraneous parts to the other. FWIW I've had more equipment blown up by independent, non-interconnected earthing systems on film sets than any other cause (possibly with the exception of missing neutrals). I'll happily expand on that if you're interested.

Why would you install the distribution at the desk end of the hall... surely it wants to be at or near the stage end? Are you having dimmed circuits for generic fixtures or only hard power? Contactors? Many Q's I have.

Thanks. And yes, interested. But of course that would only apply if we are forced to use a generator, which would be an extreme and unwelcome choice. If we simply plug the new system into the old, everything, including the earthing, would be parallelled.

It's being considered because the hall has only a 100A single phase supply, so we are pushing the load limit. I need to do a proper load schedule first, and maybe plan to turn off the hand dryers, some of the infra-red heaters and so on whenever we run a gig. I'm wiring our bit in three phase (single phased for now) in anticipation of the supply being augmented next year.

And yes, I have thought about not overloading the neutral core of the flying lead if the phases are temporarily parallelled. I'm using a 32A x 3 inlet and cabling, when only 16A x 3 would ever be needed.

At the stage end would be ideal electrically, but I need to keep all 'our' kit in one place, so putting it with the mixing and control gear is more politically acceptable.
There are no circuits 'dimmed' from the control end, in the old way. All that will come from there is about five separate 16A feeds, one to each lighting truss, plus independent feeds to a projector, electric screen, haze machine pre-heats and so on. I want to have easy power switching to these individual units, whereas everything else will stay live all the time, with every fitting controlled via two DMX 'universes' (Up to 512 channels each).

All fittings will be LED. There will be no old-school tungsten sources which run hot, use loads of power, need colour gels, and are in nearly every respect obsolete (except maybe for film).

It's a lot to plan and install, and as usual with me - not being a committee man* - I'm doing it alone. I'll post pics of progress if anyone's interested. I haven't messed with theatre/club lighting since schooldays, and it's amazing what's available now. I have three months. I welcome suggestions, especially those which might save time!

* For ease in making decisions, committees should always have an odd number of members.
(And note that three people is too many.)

 

[Mark42]

All supplies must be reliably referenced to the Earth, it is part of the ESQCR and in the wiring regs.

While having a separate rod for the generator (if used) technically meets that goal, really you will have all sorts of trouble and risk if they are not reliably bonded together. Even if you don't plan on using the incoming supply as such, run something like a 10mm CPC (assuming here typical 100A with 25mm line/neutral sort of supply and it is bonding, not main earth conductor) around the side/edge/etc of the building to connect them together.

Agreed. that's now in the plan, thanks. IF we use a generator (I'm absolutely trying not to), I'll certainly bond the supplies' earths together.

My main question was do I need to bond the unconnected second system when it's not energised, in use, or even connected to anything.

 

[pc1966]

Agreed. that's now in the plan, thanks. IF we use a generator (I'm absolutely trying not to), I'll certainly bond the supplies' earths together.

Yes, generators are a huge hassle unless absolutely necessary.

My main question was do I need to bond the unconnected second system when it's not energised, in use, or even connected to anything.

If (as is likely) the new system's CPC is less than 20k ohm to true Earth and any part of it is within hand-hand distance for touching any part of the main installation and that is TN-C-S, then you have extraneous stuff that needs bonding due to the risk of an open-PEN derived shock.

Personally I always go with Prince's 16th edition song "(were going to bond it like its) 1999"

Or Velvet Underground's "Venus in green/yellow Sleeving"