Have I a problem with lighting circuit


Postby tsb » Sun Jun 29, 2008 5:00 pm

I'm having the old fuse box changed for a new consumer unit, to accomodate new circuits from a new extension.No power at the house now for the next week as the electrician is a mate and doing me a favour in his spare time. Also waiting for longer meter tails to be connected to supply. All new circuits (lighting and sockets)have been dead?? tested so far,with no problems. I've been having a play with my multimeter on the old circuits( I don't have the test equipment the electrician has) and seem to have continuity between live and neutral, on the lighting circuit. Is there a problem somewhere. All lighting on the old circuit worked fine before. Will save me alot of time if I locate The problem before the electrician returns. Where do I start looking?
tsb
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Joined: Thu Jul 26, 2007 7:15 pm

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Postby ericmark » Sun Jun 29, 2008 9:17 pm

If you have anything switched on you will get a connection between line and neutral.
ericmark


Postby tsb » Sun Jun 29, 2008 11:03 pm

Thanks for quick reply. I've been round every light switch and made sure everything is switched off and still found to have continuity. I'm pulling my hair out trying to figure it out as I thought any fault would have shown up when the cable was connected up to the old fuse box. Can you point me in another direction to try.
tsb
Posts: 40
Joined: Thu Jul 26, 2007 7:15 pm


Postby ericmark » Mon Jun 30, 2008 9:23 am

How about a bathroom fan? these often have a supply not through light switch to keep them running after switch off. Also any neons in switches. Maybe reading will give a clue is it the same reading you get testing across one bulb? Remember at the very low voltage your meter is using a bulb does not measure as calculations would show. Shaver socket in bathroom is another typical addition to lighting. Are you talking meg ohms or ohms?
ericmark


Postby tsb » Mon Jun 30, 2008 9:06 pm

Thanks again ericmark. There are 2 extractor fans, which get their power from the lighting circuit. These can be isolated by separate FCU's and both are turned off. Would it be better to take all light bulbs out and test again. Don't know if it was meg ohms or ohms(can you explain in more detail why you ask). There is a 2way switch on the circuit(would that make a difference). By the way, have made first available appointment for meter tails connection--- TWO weeks. Its a good job we are not living there yet!!
tsb
Posts: 40
Joined: Thu Jul 26, 2007 7:15 pm


Postby ericmark » Tue Jul 01, 2008 10:32 am

With ohms reading between live and neutral your looking for something left switched on but with meg ohms reading it could be a wiring fault.
Removing bulbs always good idea specially where there is two way switching.
Some times you can work out what is wrong by the reading for example if you measure a bulb and it shows 600 ohms and the neutral live measurement is also 600 ohms then there is a good chance a lamp has been left switched on.
All best Eric
ericmark


Postby tsb » Tue Jul 01, 2008 10:31 pm

One last question Eric, then I promise I'll leave you alone. What do you mean by measure a bulb. Thanks for all your help and will put it into practise this wkend
tsb
Posts: 40
Joined: Thu Jul 26, 2007 7:15 pm


Postby ericmark » Wed Jul 02, 2008 9:08 am

Measuring a 60 watt bulb I get 82Ω Yet ohms = V²/W so it should be 880Ω this is because one the resistance changes as it warms up and two the element is coiled so it’s Impedance (AC) is not the same as it’s Resistance (DC). This is why I said measure the resistance rather than calculate. Different bulbs will give different results checked a 50W GU10 and got 98Ω and an energy saving bulb may show open circuit so if you find line neutral measurement is 72Ω and the stairs bulb is 71Ω then there is a good chance that switch is on. At the time of first writing I did not have meter handy but I knew the ohms on a bulb are lower than calculated so rather than give a guess on resistance I suggested you did the measuring. This is why when people put 10 down lighters in a kitchen the MCB trips even though the calculations show they should be able to use a lot more.
All best Eric
ericmark


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