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Thread: If there is an isolating transformer, COC apply?

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    Diamond Member Justloadit's Avatar
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    If there is an isolating transformer, COC apply?

    A question to my learned friends.

    We wish to place some lights on a very long fence, the connection between lights is made using silicon cabtyre wire, to handle the elements. The lights are connected to the secondary of an isolating transformer, and the isolating transformer is connected to the DB box, does the light circuit require to have a COC?

    Under the same scenario as above, the isolating transformer is swopped out to a low voltage AC transformer, what are the requirements for the secondary light circuit?
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Diamond Member AndyD's Avatar
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    Unless the system is on a plug and socket I'd say it would require a certificate.
    Why are you using an isolating transformer? Are the lights installed in a zone that requires it?
    I'd also look at other options rather than silicon cabtyre. There's hybrid PVC cables that are UV stable such as 'mastlight', these will be cheaper.
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    Diamond Member Justloadit's Avatar
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    Its on a high fenced sports field, apprximately 250 meters long. Being a very very old school, the complete electrical system would then have to be evaluated and possibly upgraded to get the COC. All that we would be contracted to do is the lighting the sports field. If we have to contract out the COC process, the upgrade of the current system would force the school to abandon the project due to the financial implication, and we will lose a very lucrative order.

    So I am looking for acceptable safe ways to do the lighting circuit. Our original idea was to use 12V LED flood lights. Some clever wiring must be done, as the total current from the transformer on the 12V side will be 183Amps. By spliting the cables to the respective lights, they could switch banks of lights from a control panel depending on what they were doing that specific night.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
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    Diamond Member AndyD's Avatar
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    With lighting systems of this size you need to steer clear of ELV distribution. When you're talking about a total load of nearly 200Amps the cable sizes become immense and that's just the start of it. The size of any switchgear (contactors, isolators, relays etc) also becomes very large, the transformer to produce a secondary current of 200A will also be enormous, the distances involved of 250 meters also mean you're going to have to upsize cabling to reduce volt drop, the cost will be astronomical. My advice is to keep as much of the distribution network as possible in 230 or even 380v. If the fittings are 12 volt then install a transformer or driver in each fitting.

    With this kind of installation I don't see how you're going to get away without issuing a COC, I think you'll be taking a large risk if you don't.
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    Diamond Member Justloadit's Avatar
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    The total consumption is just over 2Kwatts when all the lights are on, but at 12V it is 183 Amps, or lets say 24v will be 90 odd amps. Each light only draws 50 watts or 4.2amps at 12v but will draw 2.1Amps at 24V. The LEDs use switch mode supplies which ensure that only 50watts of power are consumed, irrespective of the supply voltage, so if I go for 24V, there can be a 12V drop in the cable with out affecting the LED output performance. If I bundle 9 cables so that each cable carries 10 amps, then there is no real issue with the cable required. The silicon cable can carry a far higher current than normal cable, due to the higher temperature rating of the silicon insulation with out degradation, plus it is UV resistant and weather resistant.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Site Caretaker Dave A's Avatar
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    Quote Originally Posted by Justloadit View Post
    Being a very very old school, the complete electrical system would then have to be evaluated and possibly upgraded to get the COC. All that we would be contracted to do is the lighting the sports field. If we have to contract out the COC process, the upgrade of the current system would force the school to abandon the project due to the financial implication, and we will lose a very lucrative order.
    I suggest the professional thing to do is point out this inconvenient fact of life anyway.

    Who knows, maybe they have already got a COC on the rest of the installation. And if they haven't:
    • They can't even add a plug or light point without going through the rigmarole
    • The user or lessor is responsible for ensuring a COC is available when legally required to have one
    • A school should be very concerned about safety issues anyway (especially old ones).



    What are the odds that they've added to the installation since 1994 already? There's every chance they're already on the hook and might not even know it.

    Watch CYA kick in and be the hero that saved the day.
    The trouble with opportunity is it normally comes dressed up as work.

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    Diamond Member Justloadit's Avatar
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    Hi Dave I hear you.
    An interesting question, you have R150K. Do you spend it to get a COC on the school electrical system, or do you help underpreviliged kids to have a place train soccer, rugby , tennis and cricket in the evenings under lights, who will not be there if there is noone. The reason for this project is to give the kids something to aspire too.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Diamond Member AndyD's Avatar
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    Forget 12v distribution and even forget 24v distribution; do the design, work out the cable sizes and lengths then find a comfy chair, take a deep breath and work out the price. Then when you've recovered sufficiently phone for a price for a transformer that can acommodate 183A at 12v or 90+ Amps at 24v PLUS INRUSH CURRENT, PLUS POOR POWER FACTOR (add approx 30% to the expected running load depending on efficiency of PSU's).

    Quote Originally Posted by Justloadit View Post
    The total consumption is just over 2Kwatts when all the lights are on, but at 12V it is 183 Amps, or lets say 24v will be 90 odd amps.
    You can't take the lamp wattage and work backward to size your supply, it doesn't work that way when there's a transformer or constant current driver (SMPS) involved. Use the stated current on the driver/transformer because this will allow for PF and efficiency losses. See SANS 0142 for correction factors according to presence of 3rd harmonic. With switch mode supplies you should accomodate all harmonics in the triplen range.

    Quote Originally Posted by Justloadit View Post
    Each light only draws 50 watts or 4.2amps at 12v but will draw 2.1Amps at 24V. The LEDs use switch mode supplies which ensure that only 50watts of power are consumed, irrespective of the supply voltage, so if I go for 24V, there can be a 12V drop in the cable with out affecting the LED output performance.
    This is not legal, not safe and not compliant. The short answer is refer to the tables in SANS0142 and the clauses stating the volt drop across the cable will be maximun 5%. Don't forget the wire length of the supply wiring is effectively double the distance between the supply point and the load, this is because the live wire and the neutral wire have resistance (ZL AND ZN).

    Quote Originally Posted by Justloadit View Post
    If I bundle 9 cables so that each cable carries 10 amps, then there is no real issue with the cable required. The silicon cable can carry a far higher current than normal cable, due to the higher temperature rating of the silicon insulation with out degradation, plus it is UV resistant and weather resistant.
    Yes silicon cable can carry a higher current, refer to the tables again in 0142. Problem with silicon (apart from expense) is it's lack of durability, I would never use silicon cable for general installation especially if it's accessible by hand or open to the kind of impact it might receive on a sports field fence. Running cables in general on fences in public or semi-public areas isa bad idea because of the theft issue.

    As for the COC, I would say you don't have a choice.
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    Diamond Member Justloadit's Avatar
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    Quote Originally Posted by AndyD View Post
    Forget 12v distribution and even forget 24v distribution; do the design, work out the cable sizes and lengths then find a comfy chair, take a deep breath and work out the price. Then when you've recovered sufficiently phone for a price for a transformer that can acommodate 183A at 12v or 90+ Amps at 24v PLUS INRUSH CURRENT, PLUS POOR POWER FACTOR (add approx 30% to the expected running load depending on efficiency of PSU's).
    There is a lot of merit in 12v and 24V systems. Battery backup is one advantage. The other advantage is the safety issue against shock or faults is negligible, a simple fuse will suffice for protection. Humidity does not influence the supply voltage.
    The transformer cost is about R4000.00. Whilst 183 amps sounds fantastically large, it is really not a n issue. The starter in your car draws a 1000 amps when initially cranked and then drops down to a couple of hundred amps when turning. A slow rise in voltage is not an issue for the LED supplies as the internal electronics has a built in low voltage detector which waits for the supply to be above 12V.


    Quote Originally Posted by AndyD View Post
    You can't take the lamp wattage and work backward to size your supply, it doesn't work that way when there's a transformer or constant current driver (SMPS) involved. Use the stated current on the driver/transformer because this will allow for PF and efficiency losses. See SANS 0142 for correction factors according to presence of 3rd harmonic. With switch mode supplies you should accomodate all harmonics in the triplen range.
    The lamps are 12V DC with an internal DC constant current SMPS, running at over 100Khz, the power factor is not even measureable because the SMPS act both as buck and boost supply, ensuring a constant current draw from the supply irrespectivce of where the sine wave is. It does not behave as in the old days with an inductive dimmer. The power factor is negligible because the total amount of power draw from the mains system is 2Kw. Far less than a unloaded squirel cage motor. Adding a few micro farads of power factor capacitor on the primary side of the transformer would correct that at minimal cost of a couple of hundered Rands.


    Quote Originally Posted by AndyD View Post
    This is not legal, not safe and not compliant. The short answer is refer to the tables in SANS0142 and the clauses stating the volt drop across the cable will be maximun 5%. Don't forget the wire length of the supply wiring is effectively double the distance between the supply point and the load, this is because the live wire and the neutral wire have resistance (ZL AND ZN).
    I would agree if the system is on a 220V system connected to the grid, it will then fall under the SANS0142, but if the system does not require a COC because it is low voltage then one practically ignore the SANS0142.

    Quote Originally Posted by AndyD View Post
    Yes silicon cable can carry a higher current, refer to the tables again in 0142. Problem with silicon (apart from expense) is it's lack of durability, I would never use silicon cable for general installation especially if it's accessible by hand or open to the kind of impact it might receive on a sports field fence. Running cables in general on fences in public or semi-public areas isa bad idea because of the theft issue.
    Point noted, but the fence is approximately 6 meters high.

    Quote Originally Posted by AndyD View Post
    As for the COC, I would say you don't have a choice.
    The situation that has a arisen is that there is limited donor money available to install the lights for the benefits of underprivilidged kids, who do not attend the particular school, but have been offered the use of the fields and sports facilities. If the money is to be routed to do the COC, and no lights installed or inadaquatly installed lights, there will be no more future donour money to help the underprivilidged kids, as the money will have been deeemed to have been used for othere uses than what it was intended for. A no win situation for all. We are simply trying to find a solution that will accomodate everyone.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    +1 for andy. listen to his advice. long 12 volt runs are bad news .

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