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Issues With Fitting Low Voltage Spotlight in Bathroom

Postby vincenti » Tue Jan 24, 2017 10:49 pm

I am a fairly competent DIYer but am struggling to fix a Low Voltage Spotlight in my Bathroom. It was installed by an electrician but stopped working after a while. It has it's own transformer and I have replaced this, as well as the actual connector in the light unit but with no success. I've also checked the connections with an electrical test screwdriver and this is where my knowledge fails me. There is proper polarity in the mains connection with the transformer, ie., one neutral cable and one live. However at the output side of the transformer both the wires are showing as 'live'. This occurred at the original transformer and hence I changed it but the new transformer is the same. I have now ended up replacing the bulb, connector and transformer but still the downlight won't work.
Can someone throw some light on to my problem please?
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Postby ericmark » Thu Jan 26, 2017 4:39 pm

I will guess you have lost the neutral connection on the low voltage (230vac) side.

So first let me remind you 230 Vac is "Low Voltage" and 12 volt is "Extra Low Voltage".

Most so called "Electronic Transformers" are not simple transformers, they have electronics which change both voltage and frequency and regulate the output.

In theory the output should be separated from input with call it Separated Extra Low Voltage (SELV) but in practice the output is measured and the information used to adjust the mark/space ratio of the input, so although a very small current, there is likely enough between input and output to cause a neon to light.

The neon screwdriver is very good at alerting one when an error has been made, testing for dead may show a wire is dead, but errors like borrowed neutrals can make that wire live as it's disconnected. However the neon screwdriver is useless as a test instrument.

You need a tester with two connections.
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Postby vincenti » Fri Jan 27, 2017 3:53 pm

Thanks Eric.

If I read this correctly then I may have lost the Neutral connection somewhere prior to my connection with this transformer, perhaps at a previous connection. I'll trace back the cables to the previous light and see if there's anything suspicious there. Unfortunately this will probably involve a large skip as it's in the loft, probably well buried.

Thanks for taking the time to respond

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