What am I missing?

[Canny]

I need an electrician to put me right on this.

I am helping organise a small outdoor private event soon. We are going to need lighting over an area and have a selection of floods to do the job. Unfortunately the best position for the lights is approximately 60-70m from the nearest mains outlet and the longest extension reel I have is 50m.

Every fibre of common sense is telling me not to "chain" 2 extensions together but I'm struggling to figure out why. I know there will be voltage drop over that distance but unless I'm misunderstanding the way the lights work, they just won't be quite as bright rather than drawing more current to compensate for the lower voltage.

There definately wouldn't be anything else plugged in to the set up, so am I missing a reason for it being unsafe (if so, could someone explain please) or am I wrong and putting the 30m and 50m reels together wouldn't cause a safety issue if its just lights on the end?

Any advice/explanation welcome.

 

[tim west]

its to do with heating the cable up, volt drop across the cable is to be minimised as it means energy is dissipated within the cable as heat with every volt dropped instead of heating the lights(filaments) up instead.

 

[ban-all-sheds]

1) You might be surprised how poorly the lights work, and if they are halogens the lamp life will be shortened by under-voltage.

2) There could be a safety issue of the fault-loop impedance being too high. See Why does this matter?

 

[hairyben]

1) You might be surprised how poorly the lights work, and if they are halogens the lamp life will be shortened by under-voltage.

Think you're back to front on that, voltage reduction increases lamp life.

2) There could be a safety issue of the fault-loop impedance being too high. See Why does this matter?

True, but the extension really should be on an RCD, and I doubt his leads will exceed... what is it 1,667 ohms?

~
technically poor practice, but in reality nothings gonna go wrong, I wouldn't go too silly on the power rating though.

 

[ban-all-sheds]

Think you're back to front on that, voltage reduction increases lamp life.

No it doesn't. //www.diynot.com/forums/viewtopic.php?p=154058#154058

True, but the extension really should be on an RCD, and I doubt his leads will exceed... what is it 1,667 ohms?

True.

 

[Canny]

Thanks for the responses.

There will be an RCD. Lamp life isn't an issue, we have spare lamps and they only have to last 6-8 hours twice so are almost disposable. My main concern is safety, effectivenss (can test before they're needed to make sure its bright enough) and not tripping the supply for the whole site. The extension lead(s) will be in open air which should help cooling as its likely to be cold, although they will be sheltered from precipitation .

What would people suggest as a maximum power rating? I was looking at our options with totals of between 2 and 3kW (we have 1x1kW flood and 5x500W). Would this suggestion change if the distance was limited to just the 50m extension?

 

[tim west]

which should help cooling as its likely to be cold.................................with totals of between 2 and 3kW (we have 1x1kW flood and 5x500W). Would this suggestion change if the distance was limited to just the 50m extension?

The extension should be nicely warm by the time you wrap it up so it'll stop your hands getting frostbite
Seriously I think thats too much load for a single extension reel over such distance

Is the event paying for the lights? if so can you not hire suitable sized cables and distribution to go with them?

 

[flameport]

Typical extension lead will be 1.5mm - with a 3kW load over 70m distance, the volt drop would be over 26V - far too much, and this will be VERY noticeable with the lamps being much dimmer than usual.

The other issue is the RCD - although in theory this would operate if the loop impedance were under 1666 ohms, at 70m away from the source, there is no guarantee that it would be under that value.
(If using the earth conductor in the extension lead, the value would of course be far less than 1666, however consider a person standing on the ground in contact with a live conductor and the loop now has to include the 70+ meters of ground back to wherever the source is).
A local earth rod would probably help, however the real answer is not to use such a silly length of extension lead, but to hire a suitable lighting setup with generator included.

 

[JonoPashley]

could you not get hold of a diesel genny and run your lights off that? would probably be the safest option here with a potential 70M run.

Edit: doh missed last sentence of previous post...my bad

 

[ban-all-sheds]

consider a person standing on the ground in contact with a live conductor and the loop now has to include the 70+ meters of ground back to wherever the source is.

If you carried out an EFLI test with the earth connection being to the surface of the ground, what sort of value do you think you'd see?

 

[Lectrician]

Run a few extensions. You dont want 3kw on one.

You say this is the closest socket. Is there any other form electrickery closer than you could use?

You can hire fairly cheaply long extension leads of a decent CSA.

 

[hairyben]

Your extension leads can handle 3 Kw, in the same way that your car can handle for example 7,000 RPM because thats where the red line is- you wouldn't drive for 8 hours like that though...

 

[bernardgreen]

If the lighting is essential then at least use two supplies through two extension leads so that if one fails for any reason( damaged lamp, fuse blows etc ) then at least half the lights should remain on

 

[tim west]

you could go to a lighting hire company ask for a couple of 18kw HMI's and stands, no need for extensions then

http://www.amlighting.com/hmifre.html

seriously though what about helium balloon lights?

http://www.bakerlite.co.uk/Helium_balloons.htm
plus genny

 

[Spark123]

I'll admit I like the generator idea better than running long extension leads, depending on application more than one generator may be required.
Weighing up the risks, what is the likelyhood of an extension lead becoming damaged? - a 1.5mm lead doesn't have much mech protection. If they are all fed from a single point then a single trip can plunge the whole lot into darkness.
What is the supply type?