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Patrick Griffin

Hi all,

I am hoping someone can clarify if my understanding on earth fault loop impedance testing is correct.

A low efli is needed so a protective decive can operate in the required time, the lower the z the quicker it will operate.

ZE= External resistance from borad to supply, board isoltated, main earth disconnected, test between incoming phase on main fuse and disconnected main earth.

ZS= Board live, main earth connected, test between out going live from main fuse and earth terminal. Zs must also be carried out on the furthest point on each circuit.

R1+R2= The maximum measured resistance of phase and earth on any circuit, lighing or sockets etc

ZS= ZE (R1 + R2)

What i would like to know is firstly is the above correct and if not where am i going wrong?

Thanks very much
 
Zs = Ze + (R1+R2). so your Zs is your measured Ze, then add the ( R1 + R2) from your dead tests. the actual measured value may be less than calculated, due to parallelm paths through bonding.
 
Hi telectrix,

Thank you for the reply.

Ok so we can calculate zs via the calculation above, however to measure zs is it the way i mentioned in my first post?

Also is the way i described the R1+R2 test correct?
 
yes you're correct. however, Zs is generally done at points in the circuit. the furthest point being the highest reading, which is what you put on your sotr.
 
I would like to ask one more question on this topic.

In my regs book here in ireland it states a zs test has to be done both at the output side of the main fuse and at the furthest point on each circuit. I know this is a uk forum with different regs but could anyone explain why 2 readings of zs are needed and how would the 2 readings differ??

Thanks lads!!
 
For the UK the Zs test taken at the output side of the main fuse would not just give you the Zs at origin but would also give you the prospective earth fault current.
This would be required to ensure that the protective devices are suitably rated to be able to handle the current generated by a fault of negligible impedance to earth.
As far as I can tell there would be little point in determining the Zs, rather than the PEFC, at origin.
It would tell you if the main fuse would disconnect in the required time, but if it did not there would be little you could do about it.

The Zs at the furthest pint of a circuit would be higher than the value measured at the origin because it would be increased by the resistance of the final circuit (and any distribution circuit) cabling.
The value obtained will be the worst case scenario for disconnection of the protective device, whilst in use.
(Obviously the calculated value from the R1+R2 dead tests and adding Ze will be the worst case but not when in use)
 
Thanks richard, The guide book i have here becomes vague with efli for some reason. I didnt see the point of a zs at the board either but it says it. For the test record sheets what value of zs should you use, the measured, the calculated or the highest of the two?
 
For Zs for a final circuit I always tend to use the calculated value (this will be the highest) but I think the normal approach is to use the measured value as being the most representative.
 

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