Cable Calc ?

moose · Jul 1, 2013

Hi guys im having a brain storm of absolute brain death.

Fire alarm Loops of around 600m and 500m Give or take FP 200 1.5mm (1100m Total cable not accounting for Cutting to much cable etc)

My problem is this, the 2 Loops of cable gave me a resistance Value of about 3.07ohms cpc to cpc and 2.93ohms cpc to cpc respective, but i would expect a larger Figure than this as the total cable use would be 1000m ish of cable, BGB 12 for it FP200 cable mv/per/m etc as im sure im not thinking straight.

Im getting like 2/3 hundred meters lmao so help defo would be very helpfull...

No other connections to earth are there only two loops as yet, but will be adding to 230 Earthing due to tranformer/contactor, cables etc...

Thanks guys catch you L8r

Richard Burns · Jul 1, 2013

If you are measuring the cpc on FP200 cable the cpc is in contact with the aluminium screen (which is a minimum of 0.075mm thick) the combined resistance of the cpc and the screen would therefore be about 2/3 of the value for just the cpc, so from OSG 12.1mohm/m is now 7.75mohm/m.
I would get 4.65 ohm and 3.87 ohm for the 600m and 500m cables.
But if the screen is say 0.12mm (me attempting to measure it with a very poor caliper!) the resistance would be about half the expected value, which is what you got.
Compare with your line and neutral end to ends and see if they about twice as high.

guest119 · Jul 1, 2013

So what you are saying is that the end to end on the earth core is reading lower than you would expect for its csa, 1.5 or whatever?
Are you not then getting assistance in the ohms department from the ally sheath?
It may be touching/making contact with the bare earth at some/many points.
Also if you are using metal boxes fixed to steelwork the ally sheath may then also be touching the boxes in places?

Boydy

moose · Jul 2, 2013

Thanks guys this is what i was missing the foil the reading is defo on about par with the readings you have mentioned, the reason for not mentioning the Phase and Neut was because they are not in a loop like a ring only the Earth was a Loop, being a Analogue System (Addressable..(NOt continues through the devices))..

Yes boydy the reading was lower than i thought, but now with the guidance makes a lot more scence..