Calculating resistance under fault conditions

[A22TC]

I know i should know this but if i have 2.5/1.5 twin & earth with a BS1361 15 amp fuse what calculation do i need to do to work out the R1 + R2 value for each metre of cable.
If anyone can help i would be very grateful.

 

[EddieCurrent]

Hello,

R1+R2 is a measurement (of resistance in ohms) between 'live' and 'earth' , it should be measured using a calibrated test equipment with the supply 'off', (and from each point). The point furthest from the CU should have the highest value, and this is the figure that you need to put on your test sheets.
It has to be physically measured to ensure correct values.

ps You do mean R1+r2, not r1+r2 I take it.

I do hope that this is of some help.

 

[davelx]

You could look up the voltage drop for each cable size in mV/A/m and apply Ohm's Law to find out the resistance of 1M of a single core of each, and add them together.

But that will give a different (higher) result from using the tabulated resistance/metre values due to temperature effects.

You could look up the resistivity of copper in a physics text book and work it out from first principles.

 

[dingbat]

Look up the voltage drop for each cable size in mV/A/m and apply Ohm's Law to find out the resistance of 1M of each, then add them together.

Or use the table designed expressly for the purpose, Table 9a of the Onsite Guide.

 

[davelx]

Look up the voltage drop for each cable size in mV/A/m and apply Ohm's Law to find out the resistance of 1M of each, then add them together.

Or use the table designed expressly for the purpose, Table 9a of the Onsite Guide.

Yes - but that's a trivial calculation, isn't it? If he's going to do it he might as well gain some understanding out of it

 

[sparkyspike]

19.51 mOhms/m