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Thread: Isolators

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    Isolators

    I have a 3 phase armoured cable circuit feeding 220V lights. The cable will run to 4 way junction boxes with built in terminals and then on to the next junction box. The two remaining ways on each box will then be used to feed individual lights. Each phase will be used and the neutral will be continuous through the terminal box.

    The regs deal with lights in a multiphase circuit but it is a bit confusing. It mentions multiphase disconnectors but isolating the neutral as well so that work can be performed on the circuit without switching all the lights off.

    I am correct in assuming that a D/P isolator will have to be used at all Tee off's.

    Anyone else have any views on the interpretation of the regs

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    Gold Member Sparks's Avatar
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    The regs require that the weakest cable in the circuit be protected. You must protect your circuit at the point of origin. If your 3ph cable is of a bigger gauge then protection must be of the correct rating for the weakest portion of your circuit UNLESS you provide protection at the origin of the weaker circuit for that portion. With a lower rated 4pole isolating circuitbreaker at the DB it is not necessary to have isolators at the tee-offs. Before deciding however I suggest you weigh up the cost now against the convenience when trouble shooting /servicing the end circuitry. Remember too that the terminal boxes must be labelled "multi phase/multiple" supply within.

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    Quote Originally Posted by RegElec View Post
    I have a 3 phase armoured cable circuit feeding 220V lights. The cable will run to 4 way junction boxes with built in terminals and then on to the next junction box. The two remaining ways on each box will then be used to feed individual lights. Each phase will be used and the neutral will be continuous through the terminal box.

    The regs deal with lights in a multiphase circuit but it is a bit confusing. It mentions multiphase disconnectors but isolating the neutral as well so that work can be performed on the circuit without switching all the lights off.

    I am correct in assuming that a D/P isolator will have to be used at all Tee off's.

    Anyone else have any views on the interpretation of the regs
    Could you please direct me to which regs you are referring to here so that I can try and investigate them?
    I dont seem to be able to find anything
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    The regs that I am looking at is 6.14.1.2

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    Question

    Quote Originally Posted by RegElec View Post
    The regs that I am looking at is 6.14.1.2
    That is very interesting, I have never come across that before and I would also appreciate it if someone who has a more clear explanation than what is in the regs will comment.

    Lets see if I am anywhere close.
    My understanding is that there is one 3 phase supply cable running in the roof from the DB to an end point, possibly at the end of the building. Each office is fed from a a cable that is tied into the 3 phase supply cable via a joint box in the roof. The branch cables may be either single or multi phase but not a combination of the two.

    Scenario1: The cable feeding the lights is a 3 phase cable running from the joint box into a sub DB in the office and through a 4 pole disconnecter. Light circuits are then distributed equally over the 3 phases.

    Scenario 2: The cable feeding the lights is a single phase cable running from the joint box and directly into the office lighting circuit. All lights are on the same phase, obviously, and there is no need for any DB or isoator.

    This is purely hypothetical as i have never come across this before but would, however, like to stimulate some debate around this
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    Gold Member Sparks's Avatar
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    The way I understand the query is that RegElec has a 3ph cable from the DB to wherever he has his lights, outbuilding or wherever. The 3ph cable enters a junction box in the roof where the 3 phases are split into 3 seperate single feeds for 3 different areas. Being in the roof there is no sub-DB. In this case the 4-pole isolator at the DB must be of the correct rating to protect the single phase conductors. The main problem with this is that if one SP circuit has a fault all 3 will be dead, that is why it would be more convenient in the long run to consider a sub DB at the split.

    Unless 3 breakers are used in the Main DB after a 4pole isolator.

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    Quote Originally Posted by Sparks View Post
    The way I understand the query is that RegElec has a 3ph cable from the DB to wherever he has his lights, outbuilding or wherever. The 3ph cable enters a junction box in the roof where the 3 phases are split into 3 seperate single feeds for 3 different areas. Being in the roof there is no sub-DB. In this case the 4-pole isolator at the DB must be of the correct rating to protect the single phase conductors. The main problem with this is that if one SP circuit has a fault all 3 will be dead, that is why it would be more convenient in the long run to consider a sub DB at the split.

    Unless 3 breakers are used in the Main DB after a 4pole isolator.
    Thank you Sparks, thats a scenario I never thought of.
    Lets just insert the reg in question here

    6.14.1.2 Each identified group of single-phase luminaires supplied from a multiphase supply that also feeds other luminaires, shall be controlled by a local multiphase disconnecting device.
    NOTE The disconnecting device should disconnect all live conductors that feed the group of luminaires, including the neutral, in order that maintenance work can be carried out without switching off all the lights.

    What do you make of the requirement of a "local" multiphase disconnecting device which I assume needs to be in the room with the lights?

    This is an interesting thread.
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    Quote Originally Posted by Leecatt View Post
    Thank you Sparks, thats a scenario I never thought of.
    Lets just insert the reg in question here

    6.14.1.2 Each identified group of single-phase luminaires supplied from a multiphase supply that also feeds other luminaires, shall be controlled by a local multiphase disconnecting device.
    NOTE The disconnecting device should disconnect all live conductors that feed the group of luminaires, including the neutral, in order that maintenance work can be carried out without switching off all the lights.

    What do you make of the requirement of a "local" multiphase disconnecting device which I assume needs to be in the room with the lights?

    This is an interesting thread.
    Hmmm, thanks for posting the reg. my disk was not with me. Local in this instance would mean the same building(a DB in a comunal shower block?), that would mean that if the DB is in the same building or on the same floor, it would be acceptable, Having the DB in the same room would be a perfectly "normal" scenario of a multiphase supplied DB with standard circuits. The requirement for isolators would then not be necessary.

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    Gold Member Sparks's Avatar
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    I just reread it again. They do say "in order that work may be carried out without switching all the lights off". I reckon that would have to mean that each fitting or group must have it's own isolator. Damn why they always have to play around with words. It would be so much easier if it read "every fitting must have it's own isolator in the same room or every fitting must have it's own isolator enclosed in the fitting".

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    Thanks for the comments.
    This is a job that I am currently working on. The 3 phase cable will be run from a DB for the complete area where lights will be (large factory installation). The cable will run into 4 way CCG boxes and out the other side to the next CCG box and so on. Out of each remaining two ports on the CCG box will be single phase cabling running to 220V lights. The load will be balanced across the phases. The cable according to the regs must be supplied from a multipole CB. That part is fine. Then we run into the regs as above which to me means that I must install a D/P isolator at each 'T' of the CCG box so that each phase including neutral can be isolated allowing the other two phases to continue to work. This cannot be done at the point of supply (DB) because I will have to isolate the neutral in the cable so all lights will go off anyway.

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