Dimmers and RCDs and earth bonding of exposed metal

[ramdram]

Brian, ref the "new legislation", well I can only post what I find. I can't conceive of a valid reason why a responsible maints manager would say that and it not be correct. I can't elaborate on this so when this new gen appears I'll be happy to share any details as I get them, but I would presume that you would be aware of this before I am.

 

Ref your link; paraphrasing, the way I read the article was that if you have exposed metalwork, not part of the electrical system, such as a boom/bar (in my particular instance) then it should be bonded with cable (of a sufficient cross section to carry a large current and with as low a resistance as possible in the case of long cable runs.) back to the main earth terminal.

 

I take the meaning of exposed metalwork, with regards to Class 1 gear, as part of the electrical system to be the casing of a Parcan, or dimmer, say, which is earthed, primarily, via the supply cable to the equipment.

 

My take on protective earthing is that IF there is the remotest chance an incident could occur then provision must be made to protect against a fatal shock. That is any electrical return path to earth, because of a fault condition, should not have a person as a resistive element in that circuit, which would delay any disconnection of the supply and cause serious injury or worse to said person.

 

If that is not correct you need only say.

 

The majority of qualified folk I spoke with were of the opinion that earthing all exposed metalwork was a "good" idea. It was the one qualified guy who did not think it was necessary. The price of even 100 mtrs of 10mmsq cable and earth clips is of no significance whatsoever compared to electrocuting somebody so I did not think it was a cost issue, hence me putting the question to the forum, as in what did he know that everyone else didn't.

 

Jivemaster, it won't be me who designs any earthing modifications, owing to my not being qualified to do so, but, it is of concern that what gets done is as per current regs, especially as it will be me doing the technician stuff, ** laughs out loud **.

[dmxtothemax]

I am not too keen on using earth spikes seperate to the main main earthing point.

I am not too sure of the electrical regulations in europe,

But here in Australia they recomend using one main earth,

as seperate earths can cause RCDs to not function reliably.

 

As for what the electricain thinks will work,

I think it is more impotant to consider what the electrical regulations say

in your area, remember when the safety of the general public is at risk

The aurthorities usually take no risks,

So you can be legally liable if something goes wrong and it is not up to code.

 

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.

[ramdram]

Thanks for your comments dmxtothemax.

 

Looks like the protective earths should go back to the main earthing point. It may be that a less than perfect earth R caused the rcds to not work as per spec when protecting the dimmers.

 

With any luck improving the protective earth system will cure the problem with the RCDs not cutting the supply.

 

it just leaves the question of why one qualified sparks thought different to other qualified sparks (and why does he have to be a volunteer at our theatre, ** laughs out loud **).

 

Now, did I mention PAT testing...

[ramdram]

End of the tale; it seems that the powers that be have taken notice of what has been advised by the forum ( I pass it on...). The volunteer has been "outvoted" as it were and the local contractor is to earth the exposed metalwork; cables taken to the main earthing block and do whatever paperwork is nec, under Part P presumably. So we will be "compliant", result!

 

Thanks again everyone who stuck their oar in.

[Brian]

... under Part P presumably. ...

I hope not. Part P is only for domestic premises.

[ramdram]

Curiously enough there are domestic dwellings on site, which were used for visiting performers/residential courses, and there is a very small flat still in occupation. Said flat was purpose built for the caretaker/maintenance bloke.

 

However Part P is probably wrong as you mentioned...I don't suppose the 200A supply goes anywhere near the flat, and it would be stretching it a bit to say the theatre is simply an "outbuilding", but thanks for the correction.

 

I daresay I meant Certificate of Installation or Compliance or similar term; in any event we are kosher (and no we are not a synagogue...I meant this in the colloquial sense, so no puns on whether our TRS connectors appear not to be sleeved...groan).

[ramdram]

"New" regs as mentioned by me earlier.

 

These "new" regs do in fact allude to advice on best practice, if that is the correct term, from the ABTT and "new" is stretching it a bit.

 

I gather that owing to the different interpretations of the 17th ed regs on the subject of earthing, the ABTT has issued their own info on stage electrics including earthing metalwork in the "theatre".

 

It occurs to me that becoming an associate member would be very worthwhile if only to get the info sheets, publications and what have you.

 

Apologies to those who knew this already, but I didn't, and as you might imagine I get a bit twitchy when qualified sparks have different ideas on electrical safety.

[The Yellow Transit]

having appropriate RCD protection in place, ie type B

anyone tell me what a type B rcd is?

[david.elsbury]

10 seconds on Google gave me this link:

http://en.wikipedia.org/wiki/Residual-current_device#Types

Wasn't so hard was it?

[ramdram]

Just search in google, there are quite a few items as PDF files, or, try this wiki:

 

http://en.wikipedia.org/wiki/Residual-current_device

 

The article explains about the different types and why you would choose them for whatever application.

[The Yellow Transit]

Cheers guys you learn something new every day , I had no idea they were designed in South Africa in 1955