DIY Doctor

Outdoor Light Wiring Getting 13 Watts but no Volts and not Working?

Postby christoph.lawrence » Sun Oct 04, 2015 1:18 pm

Hi guys,

I have just recently moved into a new house and the yard has no light. There is a wire there, which i tested with my multi-meter and got a reading of 13 watts, yet virtually no voltage. I hooked up a light to it today and i got zilch. Using the multi-meter i can see the light is receiving the 13 watts, and almost no voltage. Any idea what's going on?

The wire is direct from the mains, and i presume designed for an outside light originally. It's simply a switch on the kitchen wall right now.

Any help solving this problem will be great!

Many thanks!
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Postby ericmark » Wed Oct 07, 2015 1:21 am

I am intrigued as to how you measured the 13 watts? I am assuming one of the plug in power meters? Yet you say simple switch which would not really lend its self to using a plug in power meter.

To use power there has to be voltage it is simple ohms law. In physics a watt is a measurement of energy and time, for that energy to exist there needs to be a pressure in electric the volts and a flow in electric the amps. With AC there is also power factor so watts = volts x amps x power factor correction.

The plug in power meters do measure all three and give in the main a reading in volts, amps, PF and the resulting watts. Although I suppose in essence it is a multi-meter it is not what most people consider as being a multi-meter.

We refer to an electrical circuit and for anything to work we need a return path, with single phase that return path should be the neutral. For power to be used and no work to be seen to be done the power must be going else where. Most likely is to earth rather than neutral.

It is common for people to measure against earth using neon testers, the users body is used like a aerial and connects to earth with various paths allowing enough current to flow to light the neon, but not enough to do the user any harm. But because it is not using the neutral as return it is not testing the neutral and often it causes errors as a result.

With outside lights we tend to switch the neutral and line as neutral to earth faults can trip the RCD protection device. But in the house we only switch the line. We consider both neutral and line as being live so we call the phase wire line not live to distinguish it from neutral.

Before 2008 when we fitted RCD protection to near everything it was common only to have line wires plus earth going to the switch. Since the colour change from red and black to brown and blue twin and earth cable with two browns seems to have vanished. There was a lot of twin and earth cable with twin reds used for light switches, but now it relies on the electrician to over sleeve the blue wire with brown sleeving. This is often not done with light switches so often the blue wire in a light switch is line not neutral.

I have seen many times where some one has not realised this and has tried to wire a new light from the switch rather than from the ceiling rose. This causes some odd things like switching on one light makes another go off. With tungsten bulbs the two lights affected would glow. But with CFL and LED you can get all sorts of odd results.

To my mind there is some thing wrong with your measuring methods, it does not make sense to use watts and have no volts. I hope my reply will prompt you to detail better what and how you have taken the readings.

I have a feeling you have not measured watts? But not really a clue as to what you have measured?
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