Checking the physical length of a circuit?

Postby CarrotTop » Wed Feb 06, 2008 12:17 pm

Does anybody out there know how you can measure the actual length of a domestic light / power circuit without physically looking? Thanks in advance.
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Simply Build It

Postby ericmark » Wed Feb 06, 2008 5:38 pm

Simple answer yes. If you look in BS7671 any version it give you for each size of cable the volt drop per meter per amp this can easy be converted into ohms per meter so connecting both ends together at one end and measuring ohms at other end will give ohms of cable etc. Of course not very precise as meters not too good at low ohms. If already connected then you can use earth loop impedance but you have to work out allowing for earth wire being smaller that feed. I find excel really good for this once the sheet was set up I just bang in the readings.

Postby ericmark » Thu Feb 07, 2008 10:17 am

First answer was a little on quick side so here it is in more detail.
Take for example 2.5mm² cable with a 1.5mm² earth. Shown as 18mV/A/m and 29mV/A/m if you work this out you will find it is the same as ohms per meter i.e. Volts/Amps = ohms. So for earth loop impedance = 23.5mΩ/m. So if on a radial at the consumer unit the ELI was 0.5Ω and at a socket was 0.9Ω then 0.4/0.0235 = 17 meters. This of course assumes no parallel routes. If you measured between conductors you should get 0.3Ω of course if it were a ring main it would be 34 meters if socket was in centre of ring. But if for example the ELI was 0.54Ω and 0.8.6Ω i.e. then we get 13.6 meters instead of 17 meters which points out how actuate the reading need to be. If you measure the ELI three times and see how the readings change, you will realise how inaccurate this is. All depends how accurate you need the results?
I use same idea in reverse to work out what reading should be expected at mid point on a ring main, if results match I trust the reading if they don’t I would test house myself. Most electricians who guess their results don’t seem to guess close enough for me not to catch them out. Hope their not reading this they thought I was psychic the way I always seemed to know when they were cheating. Of course if I were to cheat I would calculate the results which of course I don’t.

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