What If I Need Stage Lighting Circuits With A Shared Neutral?

What If I Need Stage Lighting Circuits With A Shared Neutral?

What If I Need Stage Lighting Circuits With A Shared Neutral

This question came up late last year when one of our lighting geeks was trying to commission a dimmer fitted with channel RCD protection and the RCDs kept on tripping.

Investigations on site soon uncovered some ingenious wiring - 6 x 10A circuits wired back to a tail patch with an 8 x 1.5mm2 multicore cable arranged as 6 actives, 1 neutral & 1 earth.

I have to confess that after asking “could you please repeat that?” a couple of times, my reaction, while not exactly “What If?”, did start with the word “What” (I’m informed by my teenage son that there is an abbreviation for this, and it stands for “Why The Face?”)

The part of the story that was most disturbing was the fact that there was a licensed, qualified electrician on site stating quite emphatically that this arrangement was not only legal but quite safe.

Let’s first examine the question of whether or not this arrangement is “legal”, meaning, does it comply with the requirements of the Wiring Rules (AS/NZS3000:2007 Amendment 1). Clause 2.2.1.2 “Common neutral”, the relevant portions of which state:

“...A common neutral conductor may be used for two or more circuits subject to the following conditions:

a) The continuity of the common neutral conductor shall not depend on connections at the terminals of electrical equipment, including control switches.
b) Final subcircuits shall be controlled and protected by linked circuit breakers or linked switches.

...The current carrying capacity of a common neutral shall be determined from the current carrying capacity of the associated active conductors in accordance with Clause 3.5.2”

The 6 circuits are wired to a tail patch and the neutral conductor is looped from socket to socket at the socket terminal – this fails to comply with a)

Each dimmer channel is protected by a circuit breaker per channel, not linked so that all breakers trip in the event of an overload in any one – this fails to comply with b).

Neutral current carrying capacity - with the circuits wired to a tail patch, all 6 circuits could be patched to different 10A dimmer channels on the same phase, so the maximum neutral current possible is 60A – with a 1.5mm2 conductor rated at about 10A.

Just on the basis of neutral current carrying capacity, this arrangement is unsafe, but you might get lucky and the neutral could melt through (fuse) before the wiring catches fire...

So when would it be appropriate to use a common neutral for two or more circuits?

A common example is office lighting using fluorescent fittings that would add up to a 60A single phase load. It would be perfectly acceptable to wire these to a 20A 3 phase circuit, with 1/3 of the total load connected to each phase and the (common) neutral, with the circuit being protected by a linked 3 phase circuit breaker.

Other legitimate uses of a common neutral arrangement are in water heaters, air conditioners, space heaters etc, but we were talking about dimmed lighting circuits in a theatre...

 

Jands Staging provide articles to the "Staging What If" section in CX Magazine. If you have any questions regarding this article then please comment below or email info@jands.com.au

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Comment

*****

Kelvin Barclay

Another excellent wiring rules explanation I am volunteering you to re-write the AS3000 (So that electricians can understand it)

*****

Kelvin Barclay

Another excellent wiring rules explanation I am volunteering you to re-write the AS3000 (So that electricians can understand it)

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