Christmas tree lights

[barrhump]

This thread is not about Christmas trees and their combustibility; rather the potential of ignition given that a tree will be decorated with electrical tree lights.
To prevent ignition, would you consider it to be a suitable control measure for the lights (240v with no transformer) to be connected to the mains via a 30m/s circuit breaker? Any thoughts?
Regards
Barry

[Wiz]

Surely it is good practise to protect any electrical equipment with an RCD or similar device. This is mainly to protect against the chance of an electric shock and would definitely be important for 240V xmas tree lights. Whether this would protect against 'ignition' is a question. If it is right that it takes more than 30mA to start 'ignition', then, of course, it would seem it would be useful.

[John Webb]

I presume you mean a '30milliAmp Residual Current Device' or earth leakage circuit-breaker? These will be of little use, I think. Christmas Tree lights are usually without an earth connection and the insulating nature of the tree (and it would probably be standing in a plastic bucket on a wooden floor or on a carpet) means that it would be unlikely to pass sufficient current to trip an RCD.

The principle risks with lights on a Christmas tree are:
(a) bulb overheats because it is faulty or possibly because it is caught up in the foliage;
(b) bulb is broken and the naked filament is touching the foliage when the lights are turned on
(the filament will rapidly burn out in the open air once the lights are turned on);
(c) the cable covering is damaged allowing a short-circuit to take place within the set.

The best precautions are to see that:
1) the set is in good order before placing on the tree - all bulbs intact, no damage to the wires or bulb holders and a 3A fuse in the plug;
2) bulbs are clear of foliage as much as possible.

I would prefer to use lights supplied through a transformer, myself. This keeps the voltage low and isolated from the mains minimising the potential shock factor, and the transformer will limit the power that can be drawn, I hope, to rather less than that available from mains voltage through a 3A fuse.

[jayjay]

Oh if every thing was so simple,
I have a couple of photographs of a low voltage transformer "mobile phone" with the front blow off after a internal failure, fortunately it was not near anything combustible like a christmas tree !
and talking about christmas trees I have a nice short video of a real Scotch Pine christmas tree fire. The video last less than a minute and and shows a christmas tree catching fire and spreading to the whole room in less than a minute.
The video can be freely downloaded from NIST in the USA,
I use the video in fire safety training and it really frightens people with the speed and extent.

If any one can't find the link let me know, I will try to post it here.

[Wiz]

I presume you mean a '30milliAmp Residual Current Device' or earth leakage circuit-breaker? These will be of little use, I think. Christmas Tree lights are usually without an earth connection and the insulating nature of the tree (and it would probably be standing in a plastic bucket on a wooden floor or on a carpet) means that it would be unlikely to pass sufficient current to trip an RCD

John, whilst understanding your advice about the likely risk of ignition/combustion, the point I made about the importance of electric shock protection is surely still valid even though there is normally no earth connection to christmas tree lights. Someone putting their hand into a xmas tree with 240V lights, and, say, touching a radiator with their other hand is asking for trouble.  Surely a RCD should trip when there is sufficient differential between the current flowing in the Live and that flowing in the Neutral conductors. It doesn't need the short-circuit current to only flow back along any earth wire connected to the lights for the RCD to operate.
In respect of transformers, they should be the model supplied with the lights in question, which will almost certainly be a double-insulated safety transformer. Any old transformer won't do. And even then it is sensible to keep it away from any combustible material to try and avoid the sort of problem that jayjay mentions.

[kurnal]

In terms of electric shock protection I agree with you wiz and one very common problem with the lights is that the wires are often cromped or soldered into the bulb holders and after a few years can come adrift and exposed- especially those using the threaded bulbs.

Now a question for John. Since the mains lights are series wired each bulb will act as a fuse with a very low current rating indeed- in the order of milliamps. So surely in the event of a short circuit (c in your examples) the mains series lamps will be very safe indeed? Whereas the low voltage lamp sets  will have at least some bulbs wired in parallel and therefore higher currents will flow with more potential to cause overheating should a short circuit arise.

Perhaps you were referring to a double wound mains isolation transformer with a 240 volt series set running from this- in my opinion the safest solution possible? No need for an RCD then.

[John Webb]

Wiz, kernal,
I accept your comments re shock.

Regarding the type of holder I switched over some years ago to 'push-in' bulbs because they seemed better than the screw-in (LES?) both in construction, lower shock hazard and quick bulb replacement.

I'd forgotten too that the mains sets often have a 'Fuse Bulb' somewhere along the chain to blow if for some reason an excessive current flows through the chain.

A number of churches in my area mount displays of christmas trees around this time. I'm looking forward to seeing their FRAs for these events!

[Ken Taylor]

A number of the churches around here have Christmas Tree Festivals with numbers of trees of various types (real, articial and DIY) with almost every type of  decoration imaginable (including bits of paper) and various types of lights - all donated by local organisations or individuals in order to raise funds for charities by donations from hundreds of visitors touring the buildings. The fire risk controls seem to be: electrics plugged in by church 'officers'; having staff on duty; extinguishers; and switching the things off at 'close of play' each day. Electrical safety will be a matter of visual inspection (hopefully) Time will tell whether this proves adequate.