Question regarding 400V three phase lighting connected in Delta, without Neutral.

[westward10]

I think this hole thread is a fishing exercise from previous posts. It is not uncommon in large commercial or industrial installations to have three phase lighting circuits which incorporate a single neutral conductor provided you have a linked triple pole protective device.

 

[LankyWill]

Enlighten me fellas, how does it work without a neutral? i thought a neutral was required to create a potential difference thus allow the current to flow. Be gentle...

 

[static zap]

Enlighten me fellas, how does it work without a neutral?. Be gentle...

The whole basis of the three opposing phases is they always add to zero , making their own neutral when stared , saving on 3x Line currents flowing up return !

Hence the preoccupation with balanced loads so neutral , doesnt get cooked up.

 

[AReynolds]

Thanks for the replies, although I'm not sure why people are saying that this thread is an attempt at a 'wind up' or 'Fishing'- I am simply asking a question in good faith, as an electrician who has worked overwhelmingly in domestic installations and is trying to progress to commercial and industrial installations as well as design. Anyway, I realise that 400V, 3 Phase LED lighting is probably overkill in this case. Single Phase would indeed do the job well, if it wasn't for the length of the cable run, which would be about 50m from the Consumer Unit to the last light fitting, which may entail unacceptable voltage drop, and in turn require the use of 2.5 or 4mm conductors, which could be rather difficult to terminate in lighting accessories. As for the existence of 400V LED Drivers, they do exist (Although someone made a good point about them likely requiring neutral connection)- try Google if you don't believe me! Your responses have made me rethink the 400V three phase option- especially the fact that a malfunction of one or more of the lights could unbalance the loading of the circuit.

 

[static zap]

"cable run, which would be about 50m"
Tighter specs on lighting volt drops ,doesn't help !

 

[marconi]

'As for the existence of 400V LED Drivers, they do exist (Although someone made a good point about them likely requiring neutral connection)- try Google if you don't believe me! '

Out of curiosity I did a google search and found this example:

LPFH-60-54 - MEAN WELL - TRC Electronics - http://www.trcelectronics.com/View/Mean-Well/LPFH-60-54.shtml

I note though that the input voltage specification is 200 - 400V ac to 'Meet 277Vac and 347Vac input requirements of North America'. At first glance then, the manufacturer has in mind two operating voltages lower than the upper figure of 400V. The three phase supply in my road in London is 242 x square root of 3 = 420V.

 

[Andy-1960]

As was stated earlier, if you are planning to switch them all on at the same time, beware of the inrush current, as this may cause nuisance tripping of the protective device. I recently looked at some 140w led fittings .. inrush current was 10.2A per fitting!

 

[The Ghost]

potential difference

Think water heater 3 phase, or motor. P.D. between the phase angles. So Phase 1 peaks, phase 2 rising P.D. between the two etc. Ph. 3 as well. Thats how I (mis?)understand it.

 

[westward10]

Think water heater 3 phase, or motor. P.D. between the phase angles. So Phase 1 peaks, phase 2 rising P.D. between the two etc. Ph. 3 as well. Thats how I (mis?)understand it.

No you nailed it in #20, sorcery.

 

[The Ghost]

I don't know quite why, but every time I get an alert from @westward10 I get worried.

 

[Lucien Nunes]

I regularly used to demonstrate this by having three 60w lamps connected across three phases

This works in star with a floating star point using filament lamps, because the loads, despite their large temperature coefficient, are approximately resistive for a small change in temperature, and at least are always electrically continuous. Electronic drivers are switched-mode power supplies that work at approximately constant power input so they can have negative dynamic resistance, i.e. the current falls as the voltage rises. A unit that, through minor differences in load or construction tolerance, is using slightly less current, will receive slightly higher voltage just as though they were in series on a single pahse supply. This will further reduce the current and the system will experience positive feedback, almost inevitably swinging to extreme unbalance until one unit receives too low a voltage and shuts down. At that time almost the full 400V will be across the unit that is still operating, until it decides not to operate any more! The same will occur if one unit starts up slightly quicker than another.

In summary - identical single-phase loads in star with floating star point, OK with constant resistance loads like heating elements, no good with electronic loads even if apparently perfectly balanced.

 

[westward10]

Agree and it would be foolhardy to install any lighting circuits across a three phase supply with no neutral.

 

[HandySparks]

Single Phase would indeed do the job well, if it wasn't for the length of the cable run, which would be about 50m from the Consumer Unit to the last light fitting, which may entail unacceptable voltage drop, and in turn require the use of 2.5 or 4mm conductors, which could be rather difficult to terminate in lighting accessories.

So make the main run in 4mm², or whatever, then into a junction box and drop down to 1mm² or 1.5mm² for the final link to the fittings.

 

[HandySparks]

Single Phase would indeed do the job well, if it wasn't for the length of the cable run, which would be about 50m from the Consumer Unit to the last light fitting, which may entail unacceptable voltage drop, and in turn require the use of 2.5 or 4mm conductors, which could be rather difficult to terminate in lighting accessories.

So make the main run in 4mm², or whatever, then into a junction box and drop down to 1mm² or 1.5mm² for the final link to the fittings.

 

[davesparks]

Of course perfectly balanced single phase loads will function perfectly well without a neutral until something fails and then things don't function so well.

In star yes, but he's talking about delta, balancing of the loads won't be necessary.

 

[The Ghost]

You know @westward10 that sorcery you do with the lights you ought to set up a rig and demonstrate that here on the forum. It would be great to demonstrate graphically to all the theory of circuits and 3 phase etc. Of course you would have to warn us not to do it at home.

 

[westward10]

It will work in star with three tungsten lamps.

 

[davesparks]

Anyway, I realise that 400V, 3 Phase LED lighting is probably overkill in this case. Single Phase would indeed do the job well, if it wasn't for the length of the cable run, which would be about 50m from the Consumer Unit to the last light fitting, which may entail unacceptable voltage drop, and in turn require the use of 2.5 or 4mm conductors, which could be rather difficult to terminate in lighting accessories.

It would be an unconventional, and in my opinion design which will lead to maintanence issues in the future as the drivers will be non-standard and hard to replace. You will also be putting 400V where nobody normally expects to find it.
You'll also need the neutral anyway as you almost certainly won't get emergency lighting that'll work on 400V.


2.5mm is pretty common as a minimum size of conductor for lighting circuits in commercial jobs. It's not too bad to terminate into the usual fittings (rock rose etc) if you're careful.

Have you actually calculated the voltage drop or are you just guessing?
Even if you were daft enough to put the whole lot on to one single phase circuit a quick calculation suggests that 1.5mm would be fine if you connected the whole load at the very end of a 60m cable run. So in a real world scenario where the load would be distributed along the length of the circuit, and you'd almost certainly spread them across at least two circuits for a bit of redundancy should one circuit trip, I can't see any issue.

 

[marconi]

(Posh) Wet pants explains 3 phase electricity. Enjoy

 

[MJPD29]

Surely the driver has to have a neutral?