Dimmers and RCDs and earth bonding of exposed metal

[ramdram]

Have been reading the forum for some months and found the general advice very interesting not to mention invaluable, so thanks to all for the gen.

 

This is my first post/question on the forum; I have searched the forum but not found the answers I am looking for, so apologies up front if I did not search using correct parameters.

 

I do "stuff" (lights and sound, cleaning the auditorium, rodding the drains, extreme gardening, etc, etc.) at a theatre staffed almost exclusively by volunteers. We have just installed two vertical booms (aka scaffold poles) to be used for lanterns and flying the spkrs.

 

It is my contention that all the new exposed metalwork must be earth bonded everywhere, from boom to stanchion (to which it is attached) and metal trunking (for lantern supply cables) both tray and cover. These bonds to be 10mm cable back to "known" earth points in auditorium.

 

One of our volunteers, a qualified pro sparks as it happens, says the RCDs, supplying each dimmer pack, should be protection enough for earth leakage and the dimmer fuses (10A) for over current situations. A local contractor, doing other works on site, advised that earth spikes should be used, as near as practical to the booms, obviously to ensure a very, very low resistance earth path in the event of an over current situation thus ensuring the fuses blow.

 

However, just to muddy the waters further, the H&S booklet: "Electrical Safety for Entertainers", mentions that some RCDs cannot always be relied on to cut the supply when connected to dimmer circuits owing to "stray" DC, generated by certain dimmer packs, interfering with operation of the RCD.

 

Opinion is divided on the "correct" way of working and I'm anxious to resolve the problem before the season starts.

 

Despite the old saying, "The show must go on" ... this is absolute lunacy if you have a power supply with questionable protection, probably on a par with disconnecting the safety earth to "cure" a hum loop.

 

Anecdotal evidence says that folk have experienced belts off the lighting circuits at our theatre and the RCDs did not cut the supply. The lanterns in question were Parcans and the lamp holder was being rotated to change the beam spread. (you may infer the lantern was "on" at the time). There would have been skin to Parcan casing contact.

 

So apologies for long winded post but any advice, exclamations of shock horror gladly received.

[Hilary Watts]

This thread may be of help, although no firm conclusion was reached. I seem to remember a similar thread about 2 years ago although again I think opinions varied.

[Brian]

It is my contention that all the new exposed metalwork must be earth bonded everywhere, from boom to stanchion (to which it is attached) and metal trunking (for lantern supply cables) both tray and cover. These bonds to be 10mm cable back to "known" earth points in auditorium.

Probably a bit over the top. I know the 'regs' talk about exposed conductive extraneous metalwork and other such but step back a mo' and consider the fault mechanism. A lantern with a faulty earth suffers an insulation breakdown and the metalwork gets connected to live. So we already have a double fault scenario. Via the stirrup and hookclamp the mains tries to raise the scaff bar to mains potential; which is a bad thing ©

 

The bar *might* be earthed by other lanterns but you can't rely on this.

 

So you might want to earth the bar.

 

But the rest of the brackets? Probably not. Just consider how they might become live if the scaff is earthed.

 

 

One of our volunteers, a qualified pro sparks as it happens, says the RCDs, supplying each dimmer pack, should be protection enough for earth leakage and the dimmer fuses (10A) for over current situations.

I'd probably agree with that. If your earth protection is up to muster then your fuses will only ever blow on true L-N overcurrent.

 

A local contractor, doing other works on site, advised that earth spikes should be used, as near as practical to the booms, obviously to ensure a very, very low resistance earth path in the event of an over current situation thus ensuring the fuses blow.

And this same person installs earth spikes next to every 13A outlet he fits does he?

[ramdram]

Thanks Hilary, I got the impression "if in doubt bond the metalwork".

 

Thanks Brian, I must be using the wrong terms, sorry, but I used boom where bar is the correct label.

 

I suppose cost of bonding is so cheap in relative terms it made sense to me to bond the lot.

 

As for the earth spike for every 13A skt, I could not say... Perhaps I should have mentioned the 3 phase supply enters the auditorium around 60ft as the cable runs from the bar/boom, whereas the earth spike would be less than 6 feet.

 

Regarding the RCD, I am still unsure of how to read the H&S leaflet ref the point it makes about RCDs not being totally reliable given the stray DC.

[Jivemaster]

Premises should have only ONE earth potential. This is created by one or more electrodes sunk into the mass of the earth then brought together to form the circuit protective conductor (or earth wire) It's this CPC that should be connected in appropriate gauge wire to everything in the building that is earthable aparatus or extraneous metalwork.

[timtheenchanteruk]

Hmm, might have to look in the red book for this one,

 

I was under the impression, as long as you are meeting the trip times required for the installation, extra bonding isnt required.

 

Id look at having the system checked, and if DC is a problem, having appropriate RCD protection in place, ie type B

[ramdram]

Premises should have only ONE earth potential. This is created by one or more electrodes sunk into the mass of the earth then brought together to form the circuit protective conductor (or earth wire) It's this CPC that should be connected in appropriate gauge wire to everything in the building that is earthable aparatus or extraneous metalwork.

 

Thanks for that Jivemaster, looks like more discussions with our sparks...

[sleah]

Premises should have only ONE earth potential. This is created by one or more electrodes sunk into the mass of the earth then brought together to form the circuit protective conductor (or earth wire) It's this CPC that should be connected in appropriate gauge wire to everything in the building that is earthable aparatus or extraneous metalwork.

 

That's exactly what I was thinking. I'm no expert and am unfamiliar with regs, but to have more than one true earth could in fact be more dangerous, as there is potential (pun intended :) ) for there to be a potential difference between two or more metal objects, such as two bars or the bars and the dimmers.

It could also play havoc with RCD's....

[ramdram]

Hmm, might have to look in the red book for this one,

 

I was under the impression, as long as you are meeting the trip times required for the installation, extra bonding isnt required.

 

Id look at having the system checked, and if DC is a problem, having appropriate RCD protection in place, ie type B

 

 

Now that you mention it the RCDs were checked by our volunteer sparks and were found to be working as per spec. However I conjecture the RCDs were tested on their own, not through the dimmer packs in a "working" situation.

 

Thanks very much for this gen on a type B RCD. At least I am a little better informed.

 

By process of elimination and deduction from the various responses I believe my next task is to simply ring up the dimmer manufacturers, Zero 88, and discover exactly what they specify in the way of RCD type...which (blush), in hindsight was probably the very first thing I should have done.

 

A concurrent post has been automatically merged from this point on.

 

Premises should have only ONE earth potential. This is created by one or more electrodes sunk into the mass of the earth then brought together to form the circuit protective conductor (or earth wire) It's this CPC that should be connected in appropriate gauge wire to everything in the building that is earthable aparatus or extraneous metalwork.

 

That's exactly what I was thinking. I'm no expert and am unfamiliar with regs, but to have more than one true earth could in fact be more dangerous, as there is potential (pun intended :) ) for there to be a potential difference between two or more metal objects, such as two bars or the bars and the dimmers.

It could also play havoc with RCD's....

 

Good point, thanks. Now, with reference to the one or more electrodes sunk into the ground, IF the new ground spike was connected to a known earth on a nearby (<2ft) 13A socket box with metal backplate, would this count as the one or more earth electrodes and provide a lower resistance to earth overall?

 

I had originally contemplated earthing the bar/boom/metalwork to this earth point but was under the impression all earth cables were supposed to be continuous, as in unbroken, back to the earth block provided by the supplier.

 

Naturally I stand to be corrected on that one.

[Tomo]

Good point, thanks. Now, with reference to the one or more electrodes sunk into the ground, IF the new ground spike was connected to a known earth on a nearby (<2ft) 13A socket box with metal backplate, would this count as the one or more earth electrodes and provide a lower resistance to earth overall?

No, that's not a good idea.

The fault current going back to that spike could easily be a lot higher than the rating of the earth wire in the 13A ring main.

 

If you want to bond these bars, the bonding point to choose is the Earth at the dimmer supply.

- Many installations have a big exposed Earth busbar in the dimmer room with several threaded holes or bolts for this purpose. If yours doesn't, there will be an earth point inside the DB that feeds the dimmers.

 

There is however an argument that bonding the bars isn't necessary at all due to the double-fault scenario required for a bar to become live as mentioned above.

[Brian]

Have a read of this.

[ramdram]

Thanks for last two replies. All grist to the mill.

 

I have made further enquiries from both the maints electrical guys and the stage electrician at another place I do volunteer stuff.

 

Both strongly advised earthing the bars/booms back to the earth terminal block where the supply enters the theatre in 10mmsq, which should have sufficient capacity to handle the current before the 10A fuse goes on the dimmer pack. (I believe 10mmsq will carry around 120A in free air which would be the case in our theatre.)

 

Further, the maints electrical guys were in favour of multi interconnected earth points owing to the overall reduction in earth resistance.

 

I learned from them that a new directive/piece of legislation is in the process of "introduction" especially for theatres/stage which advises the earthing of all metalwork throughout.

 

On the RCD/dimmer issue the dimmer manufacturers were very helpful/patient (** laughs out loud **) and suggested it might be a prudent notion for our entire system to be tested, not just the RCDs themselves. They too advised earthing all the metalwork back to the earth block.

 

Anyway thanks for all replies so far.

[Brian]

I learned from them that a new directive/piece of legislation is in the process of "introduction" especially for theatres/stage which advises the earthing of all metalwork throughout.

Really!?

[PoppaDom]

Do elaborate.....

[Jivemaster]

If you find any reason to adjust your electrical installation's connection with the body of the earth then you definitely get an approved electrician to review the entire design, even though this will NOT be cheap! Then you connect everything back to the earth bus in the riser room or main dis board. Firstly you should read and thoroughly understand the link Brian put above from theiet