confused about lighting cct voltages , advice needed


Postby fusion1296 » Sat Mar 15, 2008 3:25 pm

hi all,
hope someone clever can help me , i thought i understood basic electrics but now i am not so sure ?
i have in the bedroom a ceiling fan/ light , a few days ago it stopped working so with my meter i went in the loft to check i had voltage at the junction box that feeds the fan/light , with the wall switch in the on postion 240 V , good stuff , with the wall switch off , 80V , not so good ??
i have old wiring ( 1960's) so was worried , went round the house removing light bulbs and measured between the live and netural , most read 0V but some read 40 - 60 V with the wall switch off .
went back to the fan/light in the bedroom to further investigate ,when measuring between netural and live on the feed to the wall switch 240V as expected , netural - return from switch 80V with switch off , disconnected return feed at switch end ( to prove if fault was wall switch ) still 80 V on return feed , went out got new cable and replaced cable to wall switch remeasured on new cable now 38 V .
is my meter faulty ? or do i not understand what i am doing ? how can i have a voltage on a wire ( blue return wire in my case) that is not terminated at either end of the cable ?
any help or advice would be appriciated , thanks Andy
fusion1296
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Postby ericmark » Sat Mar 15, 2008 6:34 pm

In the good old days we had 10,000 ohms/volt meters which in real terms mean we knew the load the meter put on a circuit but today we have meters with near infinity impedance which is all very good when measuring the components in a electronics circuit but not so good when the meter is used on cables which can be receiving power through the transformer effects of being close to other cables. This is why one is not suppose to leave unused cores in a cable free but they all should be earthed. And ELV circuits and LV circuits should not be mixed.
So I would not think either the meter is faulty or you have a fault on your cables these readings are normal with no load.
Telisa was able in early 20th century to transmit power to local town from his power station without wires. And hold florescent tubes under power lines can make them glow. And neon test screwdrivers light up even though only one end is connected. Many electricians used LED or neon test lamps for two reasons 1. Less likely to get the results you have got. 2. No batteries to go flat so less likely to show dead if not dead. They should also use proving unit but I have only seen them used in petro-chemical places.
ericmark

Postby fusion1296 » Sat Mar 15, 2008 7:50 pm

thanks ericmark for your quick reply and reassurance.

i was wondering if that was possible as i couldn't work out any other explanation especially after i change the cable out , i will ignore the voltage readings that i am getting and get on with repairing the fan /light it has 240 volts to it ( with the switch on hehe ) the power goes to a remote control box so that the fan and light can be controlled remotely , but it have nothing on the output side so looks like i need a new unit .
thanks again for your reassurance .

regards andy
fusion1296
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Postby fusion1296 » Sat Mar 15, 2008 7:56 pm

thanks ericmark for your quick reply and reassurance.

i was wondering if that was possible as i couldn't work out any other explanation especially after i change the cable out , i will ignore the voltage readings that i am getting and get on with repairing the fan /light it has 240 volts to it ( with the switch on hehe ) the power goes to a remote control box so that the fan and light can be controlled remotely , but it have nothing on the output side so looks like i need a new unit .
thanks again for your reassurance .

regards andy
fusion1296
Rank: Labourer
Progress to next rank:
50%
Posts: 3
Joined: Sat Mar 15, 2008 3:09 pm

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