I have bought a new ceiling light for my wifes latest project, an Ar
Deco living room ! On removing the existing fitting I found that ther are only two cables both of which are coloured red! As the new fitting has a step down transformer I assume it will b necessary to wire it correctly i.e. Live to live and neutral to neutra and somehow fit an earth (the two red cables are contained in an iro tube conduit). How do I find out which is live and which is neutral without any tes equipment?
I have used a couple of ways of doing this but neither are suitable for a public newsgroup since they could both be dangerous. Suggest buying a neon srewdriver.
If so, make sure it's the old-fashioned sort, not one of those that lights up anywhere near a live supply and make sure you don't have rubber shoes and aren't standing on an insulated stepladder.
However, since Maplin do perfectly serviceable multimeters in a neoprene hoster for prices which vary from £2.95 to £5, everyone can have a meter.
I have learned to remove my neon screwdriver from my top pocket and hide it when working with real electricians. They are not excatly regarded as de rigueur.
Come on guys - not very intelligent answers. Try thinking first.
The answer is that a transformer is totally insensitive to which way it is connected - it is after all just a floating coil, (or strictly a pair of coils - primary and secondary). So it doesn't matter which way it is wired.
As far as the earthing is concerned, if there is no earth tag on the fitting, then it is safe to assume that any metal parts are isolated from the fitting and that a double fault would be required to make the fitting live.
If on the other hand there is an expectation that the fitting should be earthed, then the OP does have a problem as it is not acceptable to consider the conduit as a an earth as it will have mechanical joints in it that may have suffered corrosion over the years The solution is I'm afraid to pull the two cables back through, pulling in new cables plus an earth cable.
I would suggest that the OP gets professional advise as his original question does suggest that his knowledge of electrics is beyond such a rewiring activity.
On Fri, 18 Jan 2008 17:50:44 +0000 someone who may be Unclebill wrote this:-
If the fitting is double insulated then there is no need for an earth. If you don't know what the symbol for double insulation is then you don't have the necessary skills to do the work.
On Fri, 18 Jan 2008 14:02:20 -0800 (PST) someone who may be robgraham wrote this:-
Pull them back to where? All the way to the consumer unit? That might be difficult in many conduit systems, as there is unlikely to be a wire going all the way even if there is space in the conduit.
An alternative is to measure the impedance of the conduit. Despite the statements of some, conduit can make a perfectly good protective conductor provided it was installed and maintained properly. A short flying lead connected to a bolt tapped into the box is then all that is needed. However, it is vital to measure the PSC at the box and check that a small copper conductor can cope with this. It may be necessary to use a larger size than would be standard in a T&E cable.
On Sat, 19 Jan 2008 09:14:51 +0000 someone who may be Si wrote this:-
Prospective short-circuit current. The hundreds or thousands of amps that will flow at that point if there is a short-circuit.
It should only flow for a short period before the protective device operates, but the wiring has to be able to take this current while it flows if there is not to be further damage to the wiring.
One for the legal groups - but if this poster actually did this, I wonder if you could be implicated in the legal consequences that might follow. Mmm...
OK everyone settle down ! I bought a multimeter, checked that the conduit really is a good eart and found out which cable was which. The new light fitting is up an running. Thanks to all who made practical suggestions. Bil
You can't do that properly with a multimeter. It doesn't take many ohms to stop high enough fault currents from flowing and thus any overload/fault protection from operating within the time it should.
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