Supplementary Bonding

[cydodimead]

I'm just coming to the end of my 2330 level 3 and have been asked to teach some students Basic Electrical Theory. It is University level and the course is based on lighting for theatre.

I want to explain about bonding of metallic set pieces that have lighting on them and are on stage (i.e actors are either standing on them or could touch them). In the past I have used a 10mm earth clamped to the set piece and taken back to the MET or distribution board (usually a rubber box with commando sockets that has a special earth terminal. Something similar to this http://www.rubberbox.co.uk/on-the-shelf/large-boxes/RUB4201.php

Question is with the 17th Edition assuming that RCD fault protection is in place do I need additional bonding if all the cicuits on the metal set have CPC? Personally I would say it wouldn't hurt to have and extra earth bond but do I really need it? Could I just use 4mm just like supplementary for pipe work, taps, baths etc?

My experience is in the Theatre and live entertainment world so I'm struggling between the domestic/commercial training and my world.

Any comments are gratefully appreciated

 

[RF Lighting]

A lot of my work is in theatres and similar venues.

RCD protection is irrelevant here, and the levels of RCD protection varies greatly between venues, but the majority of installs will not be RCD protected. Even the new installations we are doing now tend to be specced without RCD protection.

From what you describe, there is no need for any additional earthing at all to anything.

All your lanterns should be well maintained (I know!) so they will have a CPC to them. This is all you need.

The only requirements of BS7671 are to bond all extraneous metalwork, and to earth all exposed conductive parts. I wouldn't class anything on stage not directly forming part of the electrical installation as either of these.

You would earth an IWB as it directly forms part of the electrical installation and could come live as a result of basic insulation failing. You would not earth a normal flying bar as even though you might have lanterns hanging on it, the actual bar does NOT directly form part of the electrical installation and could not come live as a result of basic insulation failing.

Earthing metallic set peices could actually increase the risk of shock on stage, as you may be exposing another potential for someone to be shocked between.

 

[cydodimead]

Thanks RF

So are you saying that in both an RCD protected circuit and one without there is no reason to bond the set?

The lanterns attached to it may only be attached via a hook clamp http://www.doughty-engineering.co.uk/shop/84/87/index.htm so there may be a high resistance between that and the metal of the set. If a cable gets trapped and the set piece becomes live the only CPC is the lantern(s) connection to it's cover and ultimately to the set via the clamp.

I had always assumed (probably by following by example) that providing a shorter route to earth would be better.

Just checking as I don't want to telling a whole bunch of students wrong method before they enter the industry.

 

[JohnD]

RF, can you help with my //www.diynot.com/forums/viewtopic.php?p=1619008#1619008 ?

ta

 

[Adam_151]

Even the new installations we are doing now tend to be specced without RCD protection.

I've always wonder what would happen in the event of a fault between the output of a dimmer and earth if the dimmer was at a low level or pre-warm, would it flash and burn a bit without tripping the breaker due to the limited energy being allowed through?

Not that an AC type RCD would be foolproof either on a dimmer installation!

 

[RF Lighting]

Good question. Perhaps that ought to be my next experiment

 

[Spark123]

I don't think it would be much of a problem - as long as the fuse goes pop before the touch voltage exceeds 50v. Ours use 10A fast blow fuses in an attempt to protect the triacs.