Any Sparks able to help ??

[SS2.]

Came into work this morning, flicked on the light switch and BOOM :yikes: One blown light bulb smoking away and destroyed 5A fuse wire in the distribution box.

Fitted a new bulb, replaced fuse wire, switched on and another BOOM..OK, the problem is not the bulb :scratchchin:

Its a one way lighting cicuit and a single triple switch feeds three wall light fittings.

Remove bulb, replace fuse wire and switch on an its OK.

So, the fuse only blows when a new bulb is in place.

Removed the offending wall light fitting completely and there is a complete short circuit between earth and neutral within the cable exiting the wall. I'm guessing this 'aint right :( and when a bulb is in place and the switch flicked this causes live to be connected to earth via neutral, hence the bang...

Pinpointing a fault over an internet forum is difficult, if not impossible, but does anyone have any suggestions where to start looking ? Mains electrical circuits are not my forte !!

TIA :)

[ultimasimon]

I would still think its the lamp Check that the lamps are actually 240volt ones and not inspections lamps which can be 12/24/48/110etc.

If a lamp is blows when you switch it on it is for one of two reasons:

a) The lamp is not rated at 240 volt (its way under)

b) The circuit is supplying MORE than 240 volt

If the lamp is rated at 240v it is impossible for it to blow a 5 amp fuse unless the lamps are defective.

A 100 watt lamp draws less than an amp so for a 5 amp fuse to blow, the problem lies elsewhere.

To fault find I would first find as much information from people who work there, ie recent problems, has anyone tampered with the switch etc.

If you are at work and the fault is not immediately obvious, then call in a fully qualified 16th edition sparks in to find the fault, as you are putting other employees at risk if you don't know what your doing.

[SS2.]

The fittings take standard 240v bulbs and just get replaced like for like..

It was all working OK last night when I left the office and I was the first in this morning so nobody has been fiddling.

The fitting / bulb was my first thought. But no short exists within the fitting and a sacrificial new bulb suggests it wasn't the old bulb either.

Its the short circuit between neutral and earth in the cable (with thwe fitting disconnected) that has me stumped and why this suddenly appeared overnight...Don't think we have mice

I recognise the importance of not putting employees (or the building !!) at risk and if its not something straightforward then I feel a call to a qualified spark will happen sooner rather than later..

[parrot of doom]

Possibly a damaged light fixture.

Get an electrician to sort it. If you don't know what you're doing, electricity will kill you.

You could also invalidate your insurance if there were to be a fire.

[BliarOut]

Short is unlikely, but not impossible to occur in the cable. Have you looked at the switch to make sure the wires haven't come adrift there and shorted?

[SS2.]

parrot of doom said:

Possibly a damaged light fixture. Get an electrician to sort it. If you don't know what you're doing, electricity will kill you. You could also invalidate your insurance if there were to be a fire.

No shorts exist in the fitting so don't think its that.

Its just disconnected for now so at least we have some light in the office

Well aware of the dangers of mains leccy - had a few jolts in my time

[ultimasimon]

SS2. said:

The fitting / bulb was my first thought. But no short exists within the fitting and a sacrificial new bulb suggests it wasn't the old bulb either.

It has to be a bad batch of lamps or the cable from the lamp is damaged/shorted - bulbs are what you plant in the garden btw

As stated before, if the lamp is rated at 240v, you would need a lot more than 240v to blow the lamp. It would not blow the fuse however, especially if it were a 5 amp fuse as for one lamp thats overkill but I accept that it is probably a communal lighting circuit.

SS2. said:

Its the short circuit between neutral and earth in the cable (with thwe fitting disconnected) that has me stumped and why this suddenly appeared overnight...Don't think we have mice

In the circuit for the lamp to light using an earth instead of a nuetral, it would still allow the lamp to light - the earth still provides a potential difference in voltage of 240v between the live conductor and the neutral or earth wire. The only difference is if you have an RCD (residual current device), which disconnects on earth fault, the RCD would trip out before the fuse would blow as they take on average around 30 milliseconds which is far faster than it takes for a fuse wire to burn through. Your mice theory is a very plausable one

SS2. said:

I recognise the importance of not putting employees (or the building !!) at risk and if its not something straightforward then I feel a call to a qualified spark will happen sooner rather than later..

I would call in the pros, it may be a simple fault but if the place did burn down it would not be nice to be on the receiving end or allogations that you were at fault. As others have said your company would not be insured also.

[Pigeon]

When you say there's a short circuit between earth and neutral, I take it you mean that you've tested for continuity between earth and neutral with a meter, and got a low resistance? That's OK. Neutral is connected to earth at some point - precisely where depends on the installation - to allow the earth protection to function. It's if you don't measure a low resistance between neutral and earth you should worry.

Does it go BOOM when you put a dud bulb back in? As has been said there's no way a 240V supply will make a 240V bulb explode. But equally there's no way for a 240V bulb that worked last night to suddenly not be a 240V bulb in the morning, and if you were getting more than 240V you'd be getting it everywhere and other things would be blowing up. I'm thinking that there's an insulation fault in the fitting, and when a bulb is inserted it moves the wires enough to touch. That could suddenly become apparent after the insulation has degraded over time.

[SS2.]

So a zero resistance (yup, with a DVM) between earth and neutral is not a bad thing ?? Like I said, mains electrics never was my strong point

I'll have another look at the fitting - it makes sense that a wire could be part dislodged and shorts when a bulb is inserted. Good point

[Pigeon]

Yep, to actually check for a short between neutral and earth conductors in a cable you need to disconnect the cable at the fuse box.

[SS2.]

Spot on guys Took the fixture to bits and the live and neutral insulation were blackened / part melted and conductors were exposed. Not shorting without a lamp (I stand corrected ultima ) in but screw in a lamp and there was enough free play in the holder to twist the wires together. Replaced with a new cable and job's a good 'un

Cheers all

[dilbert]

How many pistonheaders does it take to change a lightbulb?

Answers on a postcard please!

[parrot of doom]

Earth and Neutral electrically are more or less the same thing. The neutral connection in your house goes down the street, to the substation, and into a big spike in the ground.

This is how people cheat their meters - they just connect the neutral end of the circuit to a spike buried in the ground under their house, and the meter doesn't run. Thats why they have anti-tamper seals on them.