If there is an isolating transformer, COC apply?

[AndyD]

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.

The safety issues for ELV systems are as critical if not more so than 230v systems. ELV systems are highly prone to fires because of the high currents involved. Most of the vehicals on the road that catch fire are due to 12vdc faults. It's a common misconception that with ELV it's much safer but in the days when 12v halogen lighting was becoming fashionable there were many fires caused by these systems which lead to several changes to the installation codes.

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.

True but look at the size of the wires that supply a starter motor on a car. This is going to be the size of wiring you'll need around your transformer and the reason you'll need 10 or more circuits of 2.5mmsq cable to carry the power to the lights whereas if it's 220v distribution a single cable would suffice.

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.

SMPS's in their basic form have notoriously poor PF. Depending on the individual item and the mode of its operation it may have internal correction installed so whilst PF is always a consideration it may not be an issue.

[Justloadit]

PF's are only issue on large currents, a single circuit drawing 5 amps and over at the 230/400V level, not when you are pulling 220mA from the mains. A simple 1uF 400V capacitor on the DC side of the bridge rectifier eliminates the PF caused by the SMPS, which costs around R3.00. Typical high PFs in this case would be speed drives, and banks and banks of flourescent lighting, where power factors are in the magnitude of 0.82, attempting to compensate for this yields capacitors costing more than the drive itself.

This is where power electronics are comming into play where you boost the voltage during the rising and falling parts of the rectified sine wave, and you buck on the peaks, attempting to maintain a constant current draw from the supply. I have been succesfull with a prototype driving a 1.1Kwatt motor achieving PFs over 0.92 at full load, and unloaded, which is when the PF really is bad.The inductor required is hefty as it has to content with a constant DC current flow in the same direction into the load. If you have an understanding of electronics, here is a typical IC from MOTOROLA MC33262 which will do the task. If you open the data sheet, on page 12 they print a table of volatges in which the application design was tested, yielding efficiencies above 99% which is close to a power factor of 1.

I have succesfully replaced flourescent fittings with equivalent brighter LED retrofits, in which the power consumed has reduce by more than 65%, and simultaneously improving the power factor. There is much to be said about proper LED circuits, mind you not the cheap rubbish many stores are selling.

Your explanation of the halogen lamps is very true, the installers were using incorrectly sized wire for one, and bad joints for the other part due to incorrectly crimped cheap rubbish connectors, causing hot joints and burning of cables as you have suggested. The silicon cable does not burn for one, and if you place a suitably rated fuse on each of the cables as protection, any shorts that may occur will be protected by the fuse. There is also no requirement of an earth conductor for the detection of ground faults.

I understand where you are comming from with the high standards required in the 230/400V arena, but on the LV side, simple common sense is more than sufficient to ensure that the sytem works.

[AndyD]

I agree PF may or may not be an issue depending on the driver design but like I say it's always a consideration that needs quantifying when designing the supply network even if it's found that no improvement or derating of cable sizes is necessary.

The cause of fires with ELV is often undersized cabling or inadequate connections. Silicon insulation doesn't support combustion admittedly but it's the terminations and connections come under far more strain because of the high currents involved and many connections that do give problems at 12 volts wouldn't give problems with the same power at 230v. The terminations tend to overheat and arc causing combustion at or around the lug, connector block or whatever the connection is and fuses aren't protection against this as an arcing connection will actually increase the circuit resistance so reduce the current flow.

More for the benefit of others reading I'd say it's a common trap thinking that ELV, in this case 12 or 24V distribution is safer. In some respects maybe but in other respects definately not, they may be safer with regard to the user receiving a fatal shock but they're far more of a fire hazard. I would also expect the overall cost of an ELV system to be several times higher than the cost of the same system design in 230v.

Rereading the thread from the start I realise we're arguing the same technicalities from two completely different viewpoints. I'm basing my recommendations primarily on standard system design considerations and you appear to be basing yours primarily on avoiding the necessity of issuing a compliance certificate. As such we're always going to be short on common ground or agreement.

As for the COC problem, to this end I would suggest you investigate going with a 230v distributed system using 3 OR 4 circuits of floodlights with staggered switching to lessen inrush current on startup. From the figures you've given (circa 9Amps @ 230v) you could run the entire system on a 2.5mm trailing lead (or even 2 x 2.5mm leads) plugged into a standard EXISTING wall socket. Even if you upsize the long distribution cables to reduce voltdrop this will still be a fraction of the cost of your 9 or ten ELV circuits in silicon and an additional transformer. Maybe also investigate using black surfix which is UV stable and can be clipped direct to the top of the fence.

I assume your prospective installation is in the JHB area. If on the off chance it's in Cape Town you can give me a shout and I'll do a survey and your design for gratis if you're also working on a not-for-profit basis and it's for a good cause.

[Justloadit]

AndyD, I thank you for your invaluable information that you have provided in your posts.
The installation if it materializes will be in the Johannesburg area.

[skatingsparks]

If its Joburg, you're working on a not-for-profit basis, it's for a good cause and if you need help - private message me. All for the pay it forward thing.

[AndyD]

Did you reach a final design spec for this installation? I'm interested to here the outcome.

[Justloadit]

Hi Andy,
The project got canned, once I mentioned the problems that could arise in the future. They simply said that they could not take a chance with our proposal, and what if they were forced to do the COC, it could just break them to conform.

[AndyD]

Sorry to hear that

[Sparks]

I see its all over now but you might want to consider just running your cable along the perimeter and installing 220V LEDs. Quicker easier and I am sure can be worked out cheaper. I agree all the way with what Andy said. I would have said 220V LEDs from the start.

[Dave A]

Could we revisit this particular issue for a moment.

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.

The harsh reality is despite the requirements of the regulations that Justloadit quite rightly points to, additions to installations are happening
a. without a COC being issued on the additions, and
b. without there being a COC in place on the existing installation.

My question to all and sundry is what would you prefer to see going forward -

1. An amendment to the regulations that accommodates this situation (and stops penalising the honest contractor when it comes to competing for these jobs), or
2. Raise the general awareness of the industry and the public to try to bring industry practices into line with the regulations as they currently stand?