Lighting circuits tripping


Postby Lawrence » Thu May 05, 2011 6:36 am

I've moved in to a new house and identified a problem with the lighting circuits. The circuits have 1mm cable but have 16a MCBs fitted i.e. no circuit protection is being provided. I changed to a 6a MCB and the lights keep tripping despite, on the face of it, the load being low.

On one circuit I have -

6x 35W low voltage
3x 7W low energy (230V)
1x 15W low energy (230V)

The low voltage lights are all connected to a single transformer - not seen one like this before... it's a 6" cylindrical shaped transformer and all the light cables connect in to it, presumably fitted when the house was built about 20 years ago

On the other lighting circuit I have similar low voltage transformers.

Can any bright sparks give me pointers as to what may be causing these problems? Clearly I need to get a correctly rated MCB for these circuits, but I am not sure why there appears to be such a heavy load from so few lights. Is it likely that these older transformers are the issue and should be changed?

thanks
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Postby ericmark » Thu May 05, 2011 10:09 pm

Transformers may have a high in rush when switched on and this can trip a MCB although I have a transformer like you describe and it has not tripped my B type MCB.

Of course the transformer could be faulty.

MCB's come as three types B, C, and D. The letters refers to the instant tripping current which acts on the magnetic part of the trip. B means 5 times, C means 10 times and D means 20 times rated voltage. So a B6 MCB will need 30A to trip the magnetic part. A B16 would need 80A where a D6 needs 120A so for in rush a D6 will take more than a B16 MCB.

What matters is the earth loop impedance. The idea is that a short circuit will always trip the magnetic part of the MCB. So 230/120 = 1.92 ohms. We would test the furthest part of the circuit with a loop impedance meter to ensure the impedance is below 1.92 before we would change the MCB.

Problem is the meters cost around £75 to hire the complete set so most DIY people are unwilling to pay out this to hire the meter. Especially when likely they can get an electrician to test it for them for less than hire charge.

Having said that if it were my house then I think I would change the transformer for an inverter. Today the reducers work on the switch mode system which means they turn AC to DC then charge a capacitor then back to high frequency AC which then uses a much smaller transformer to transform it down at which point it is sampled and parameters changed to keep it at 12V even though the input may vary from 200 to 250 volt.

This means the lamps run at exactly the right temperature and active material is reflected off the very hot quartz back onto the tungsten element. This means lamps last longer and give out more light. This is why you should not use dimmers with quartz lamps.
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Postby Lawrence » Fri May 06, 2011 6:35 am

thanks for the reply ericmark. would there be any issues from changing the B type MCB to a C or D type?

can you give me some examples of the inverter you refer to.
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Postby gw139xl » Fri May 06, 2011 10:13 am

I would check that there is nothing else on the radial - you don't know what the previous occupant did with the circuit.
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Postby ericmark » Sat May 07, 2011 6:04 am

[quote="gw139xl"]I would check that there is nothing else on the radial - you don't know what the previous occupant did with the circuit.[/quote]

You make a good point. It is common to connect TV pre-amplifiers, bathroom fans, and shaver sockets to the lighting circuit.

What I have seen in the past is where a 13A socket was used to plug the TV amp into which had then been extended to include other items.

However it is hard to trace without meters. I find my clamp on ammeter is a real boom to tracing. Originally I bought it after getting a nasty shock from a shared neutral and I check the current in every wire before disconnection now.

But even with a clamp on meter I am nervous telling anyone to test live cables. We have no idea how good people are.

Extra low voltage lamps are the most likely problem. Please remember 230 volt is classed as low voltage, under 50 volt is classed as extra low voltage. One can get quite a belt from low voltage. And if someone has tapped into the lighting circuit then borrowed neutrals are a real danger.

With a borrowed neutral wires will test dead with a volt meter. But as disconnected they become live. This is why officially a neutral wire is considered as live. The Phase wire is referred to as the Line conductor and often has L for Line marked on Plug etc. But many think the L stands for Live.

In a perfect world there would never be shared neutrals but you know something is wrong so you have to be super careful.
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