DIY Doctor

Lighting circuit

Postby alan7 » Tue Mar 30, 2010 3:57 pm

Trying to help my son by doing some measurements on lighting circuits.

We found that a voltage reading of 240v was seen between live (switch) and neutral and between live and earth.

Is this right?
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Postby ericmark » Tue Mar 30, 2010 6:31 pm

In the UK the supply voltage is 230vac +6% -10% so your readings do fall into that value. I further 3% volt drop is allowed for lighting circuits.

Earth and Neutral are at some point bonded together so no load the same voltage is likely Line - Earth and Line - Neutral.

Both Line and Neutral are considered as "Live" as under fault conditions the neutral can have same potential as Line to earth. Line is also sometimes called phase.

The problem with the low voltage supplies to British homes is to be safe careful measurements need to be made to ensure that under fault conditions the supply will auto disconnect within a safe time.

To ensure this electricians use a set of meters often referred to as 17th edition test set which cost around the £750 and most DIY people do not have access to these meters which are a very fast computer and meter built into one and can calculate how much current will flow under fault conditions.

I would consider if you are helping or if you are likely to hid faults which could put your son in danger?
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Postby jackhawk » Tue Mar 30, 2010 7:03 pm

Hi alan,

You will get 230-240V at a 'light fitting' as there (normally) isn't a N in a switch. However, if in the switch there is a LNE in and LNE out with the two Ns joined in a block and the two Ls in common and L1 then this tells me that a light fitting is down stream and at the end of that switch instead of using the loop method. You find this in lofts alot to save running wire to and from a single light. So when you test between L & N in the switch you get 230/240. If you test between L & E at the switch you will still get 230/240V. If the circuit under test is switched off at CU and LNE removed safely for that circuit then you should get no volts.


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Postby sparx » Tue Mar 30, 2010 8:44 pm

yes, what would you expect it to be?

be very carefull testing on live circuits,

Neutral and earth are connected together at substations and other points outside the consumers wiring so would expect similar readings.

Lighting circuits can kill just as easily as power ones!

Average non-sweating persons resistance hand to hand around
2000 Ohms, 30 mAmps max current considered non lethal from ohms law- 2000 X 0.03 = 60volts required to drive deadly current across your heart.
This is reason most older sparkies will use one clip on type lead to earth or neutral and one probe used with one hand & other hand behind our backs!!

regards Sparx, Circa 1947 and still working-carefully!
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