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Thread: Extra low voltage lighting question

  1. #1
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    Extra low voltage lighting question

    So I have a dude who has bare 10mm copper conductors strung across various parts of his shop in pairs, no more than 5 meters long, with 12 volt halogens connected to them fed from a various size safety isolating transformers contained in metal ventilated enclosures in the roof. The transformers range from 200 VA to 350 VA, depending on the number of halogens he has on each pair of conductors. From the the transformer, which either are fused with fast blow glass type, or are "short circuit proof" a 2.5mm cable or a 10mm (depending on the size of the transformer) cable, insulated, is crimped onto the bare 10mm cables, the longest run of this cable is about 2 meters. If I read the bible correctly, does it require that every secondary circuit, the low voltage side, need to have an overcurrent protective device as it say in 7.9.3.2.2? I don't really see a problem with it, if the transformers become overloaded they are covered the fast blow fuse in the transformer on the primary or a thermal cut out or are "short circuit proof". The transformers are designed for this kind of thing so why do i need to fuse every secondary circuit. I could understand if it was just a standard transformer but these are designed specifically for lighting and if the regs required you to have extra over current protection on the secondary side the surely they woudl be sold with that built in.
    The circuits feeding the primary have been wired in 1.5 single and the max load on any circuit is 6 amps, cable protected by 10 amp breaker. The bible says about (7.9.2.4.2) that 200VA shall not be exceeded if a converter is used when supplying bare conductors, I presume is ok to exceed 200 if using safety isloating transformers.
    I can't find any info on sans 60590-2-23 or 60998-2-1 or any of the other sans numbers mentioned in this section of the bible(7.9)
    Originally before i got there there was 65 x 50 watt halogens on one circuit and its been sitting like that for the last 8 years with no problems, apart from and insane volt drop (170 Volts at the furthest point) and the fact it was covered by a 20 amp fuse.
    Thoughts? Does it comply? I don't see a problem as the transformers are safety isolating, designed for low voltage lighting and have built in protection and the circuit are no where near overloaded now(volt drop is a consideration of course on the secondary but changing the 2.5 to a larger cable will solve that).
    There are a lot of transformers(about 35 ish i think) and installing a circuit breaker in everyone will be a pain.
    Thanks

    Cheers

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by skatingsparks View Post
    If I read the bible correctly, does it require that every secondary circuit, the low voltage side, need to have an overcurrent protective device as it say in 7.9.3.2.2? I don't really see a problem with it, if the transformers become overloaded they are covered the fast blow fuse in the transformer on the primary or a thermal cut out or are "short circuit proof". The transformers are designed for this kind of thing so why do i need to fuse every secondary circuit. I could understand if it was just a standard transformer but these are designed specifically for lighting and if the regs required you to have extra over current protection on the secondary side the surely they woudl be sold with that built in.
    With a transformer, the fuse on the primary will not effectively protect against an overload condition on the secondary side.

    The primary-secondary ratio is about 240:12 or 20:1. You need to load the secondary by 20Amps to increase the primary load by 1 Amp. With this ratio it's impossible to protect the secondary current accurately by a fuse on the primary side. Hence the secondary fusing is important to prevent overload and possibly a fire on the low voltage side.

    Short circuit proof just means that the device developes sufficient internal impedence that it shouldn't burst into flames or burn a winding if the secondary is overloaded. It doesn't mean it offers overload protection for the supply wiring to the lamps.

    Quote Originally Posted by skatingsparks View Post
    The circuits feeding the primary have been wired in 1.5 single and the max load on any circuit is 6 amps, cable protected by 10 amp breaker. The bible says about (7.9.2.4.2) that 200VA shall not be exceeded if a converter is used when supplying bare conductors, I presume is ok to exceed 200 if using safety isloating transformers.
    200VA is the max on any 12volt lighting system that has bare 12volt conductors under any circumstances as far as I know.

    Quote Originally Posted by skatingsparks View Post
    I can't find any info on sans 60590-2-23 or 60998-2-1 or any of the other sans numbers mentioned in this section of the bible(7.9)
    Originally before i got there there was 65 x 50 watt halogens on one circuit and its been sitting like that for the last 8 years with no problems, apart from and insane volt drop (170 Volts at the furthest point) and the fact it was covered by a 20 amp fuse.
    65x50w=3250watts of lighting
    3250/12=271Amps @ 12volts.
    271/20=approx14Amps @230volts

    Depending on the length of the supply and ambient temperature I don't think 14Amps is sufficient current to cause such a 50-60volt drop on a 1.5mm cable although it would certainly be overloaded.

    Quote Originally Posted by skatingsparks View Post
    Thoughts? Does it comply? I don't see a problem as the transformers are safety isolating, designed for low voltage lighting and have built in protection and the circuit are no where near overloaded now(volt drop is a consideration of course on the secondary but changing the 2.5 to a larger cable will solve that).
    There are a lot of transformers(about 35 ish i think) and installing a circuit breaker in everyone will be a pain.
    Thanks
    I don't think safety isolation or short circuit proof is compliant when it comes to secondary circuit protection. You can just use an in-line fuse holder and a standard 20mm fuse for secondary protection on the 12volt side. I'm not sure why you want to fit a breaker at each transformer?
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    I kind of knew that it would be the case. I'm going to put a 20 amp fast blow fuse in line with the outgoing 12+ on the secondary side. The place is a complete mess, the massive volt drop is because he has loaded up one of the 2 phases with all the lights and it appears to be only a 30 amps per phase supply(OLD system) so the supply voltage is quite low already(165 x 50 watt halogens). Also the length of run is in excess of 80 meters(!). The lights he has put in are the bare wire type which he made himself, the lamp holders are proper and connect to the 10mm copper fine. I don't think the fittings are to SABS standards though. I'm just going to break the lighting down into smaller loads, max 30 lamps per circuit (5 lamps on each of the 6 transformers) so the volt drop isn't insane, spread it the load over 2 phases, redo the switching so that you only have 220 present in each switch box(prefer doing that over have 2 phases and labeling).
    I made him aware that I didn't think that the fittings, the bare wire, are compliant so I'm just going to put in a COC just that, that the supply up to the transformer and the transformer is compliant but what is connected to it is not part of the report. It is reasonably safe if used correctly, the fast blow fuse will protect it from overload/short circuit and if he signs the COC to say he accepts that he is aware that the home made 10mm light fittings are not part of the report then I think I'm ok. If he doesn't want to buy SABS 60598 light fittings then that is his choice. I can sleep at night knowing that its better than it was(seriously you couldn't see the ratings on the circuit breakers for the birds nest of cable no matter put the cover on the DB). I'm redoing the distribution board, all the sockets, the switching of the lights and what he connects to the transformers is his business. Basically its a full rewire, just not going to say his home made light fitting are SABS approved. Sound reasonable?

    Oh, and where is the 200VA on bare wire from, I saw that 7.9.2.4.2 says about the power of a converter which is different from a transformer. I have seen you can buy 300VA rated kits with bare wire with 6 x 50 watt halogens on them.

    I know its not ideal but as long as i make clear what is covered and what is not, i don't see a problem. As long as i state in the COC what is covered and what isn't I'm legally in the clear and if I put those fast blow fuses in the secondary then my conscience is clear knowing its protected. Its a lot better than it was believe me.

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    Good day, I am a new member, my name is Lee, and I have become very interested in low voltage lighting since I heard it described by more than one electrician as a minefield. I have some thoughts on the subject and would like to gather all your thoughts on these.

    In table 7.3 I refer to the first column "single circuit lighting load".
    On a transformer which has multiple secondary circuits, eg. one x 200 Va transformer supplies 4 individual cables which, in turn, each cable feeds one 50Va light. My question is, is this column referring to each circuit individually (which is what I think) or the combined secondary circuit as a whole?

    7.9.3.2.2 Each secondary circuit of a safety extra low voltage (SELV) supply source (transformer or convertor) shall have over-current protection (see 6.7.1 and 6.7.2). The over-current protection may be either by a common protective device, or a protective device for each SELV circuit.
    I understand this to mean, in the case of multiple secondary circuits on one 300Va transformer for example, if the cable supplying each individual light is 1.5mm then each cable must be protected by a 10 amp protective device or the entire secondary output must be protected by a 10 amp protective device.(which would not be feasible in the case of multiple secondary circuits)

    I look forward to seeing your reactions to this.

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