Separate Neutral

[RegElec]

Hi. Client wants to install fixed appliance and to save time and money they want to use two redundant motor cables - cable one for the three phase supply and the other cable to carry the neutral (both fed from the same DB). I cant seem to find any code against this. Anyone have any comments?

[Dylboy]

Intresting... I can't think at the moment if there is anything wrong with it, it does not feel right I agree. Will have a read and see if I can find a reg.

My thinking is that GP in conduit is separate conductors so perhaps separate cables may then be ok as well.

Edit: also the phase and Neutral being same CSA. I believe may be 50% CSA the neutral for 3 phase loads but the book will have more detailed info.

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[GCE]

Hi
I see no regulation that prevents the neutral being in a separate cable if it originates from the same D/Board, but it does definitely not feel right.

The closest reference to your scenario would be 6.2.1.3 but that is for parallel cables. I would ensure that the cables are like for like.
I would label the core ends and cables to ensure that nobody makes a mistake and disconnects the neutral by accident or even better would be to use a 4 pole CB to feed the circuit , at least that way there is less change of a mistake happening with the neutral

You could use 5.3.4 to advise against the neutral in the 2nd cable - If there is a risk of the cable being damaged - You could have the neutral damaged and loose the neutral to the fixed appliance whereas if the neutral was in the same cable it would more than likely damage a phase conductor as well and trip the CB preventing a floating neutral.

At the end of the day the client needs to understand the risk and balance against the cost of cable or the potential damage to his fixed appliance.

Make sure you that if you advise against it that you have it in writing and maybe attached to the COC and keep copies - These things have a terrible habit of coming back to bite and then nobody remembers that you advised against it

5.3.4 Type of wiring and methods of installation
The type of wiring and methods of installation shall be determined after
consideration of the following:
a) the location (also consider intentional or inadvertent damage);
b) the nature of the building elements for supporting the wiring;
c) the accessibility of the wiring to persons and livestock;
d) the voltage;
e) the electromechanical stresses and thermal effects likely to occur as a
result of short-circuits; and
f) stresses imposed on the wiring during installation and in service

6.2.1.3 Except in the case of a ring circuit, there shall be no branches, teeoffs
or socket-outlets along parallel conductors and, where two or more
conductors are connected in parallel, the conductors shall
a) have the same characteristics in respect of conductor material, nominal
cross-sectional area, length and, where applicable, insulation and cable
construction, and
b) follow the same route and have the same installation method(s).

[GCE]

Hi

This question has been nagging me in the background

I have been thinking regulations and not practical .

With the neutral in a separate cable you are going to have a Hysteresis effect especially on the gland plates and the effect will increase with higher loads

This reason should be sufficient to convince the client and I have pasted link below to help you convince him.

Alternating current flowing in a conductor sets up an alternating magnetic field which is much stronger if the conductor is surrounded by an iron-rich material, for example if it is steel wire armoured or if it is installed in a steel conduit. The currents in a twin cable, or in two single core cables feeding a single load, will be the same. They will exert opposite magnetic effects which will almost cancel, so that virtually no magnetic flux is produced if they are both enclosed in the same conduit or armouring. The same is true of three-phase balanced or unbalanced circuits provided that all three (or four, where there is a neutral) cores are within the same steel armouring or steel conduit.

Hence, it is important that all conductors of a circuit are contained within the same cable, or are in the same conduit if they are single-core types

For this reason, single-core armoured cables should not be used. If the single core cable has a metal sheath which is non-magnetic, less magnetic flux will be produced. However, there will still be induced EMF in the sheath, which can give rise to a circulating current and sheath heating.

http://www.gohz.com/what-means-by-circulating-current-in-single-core-cable

[RegElec]

Thanks.
I am also of the opinion that this is not best practice and the client is aware of this and agrees. Due to time constraints and the need to erect scaffolding to install a new cable it was decided to use the two redundant cables. Fortunately, this is a temporary installation.
The cables are fed from a 4 pole C/B in the sub and have been terminated into a CCG box about 3 metres from the installation and from there to the machine they have used a single 4 core trailing cable via angle iron droppers.

[GCE]

I would be interested to know the amp rating and cable size and once it is running on load if there is a heat increase on the cables

I once put up 2 pieces of wire mesh ,side by side, and ran singles from the transformer onto the tray and terminated on busbars in the main LV panel ( 1250KVA ) . 2 phases on one tray and a phase and neutral on the 2nd
I found that the wire mesh tray heated up that it was hot to the touch. Had to cut the sides of the tray and join across to stop the effect of Hysteresis.

[AndyD]

I've seen it several times where neutrals and lives have been installed in separate holes through a steel gland plate but I've never come across any signs of excessive heat. I usually just cut a slot between the glands if possible. I'd guess the current would need to be pretty high to create thermal issues, probably far higher than you get in a normal domestic installation.