Zs and Ze - both called 'Earth Fault Loop Impedence'?

[swaRRR]

Can someone explain this to me?

The regs book calls Zs 'Earth Fault Loop Impedence' and the On-Site Guide calls Ze 'Earth Fault Loop Impedence'.

 

[westward10]

Zs is the earth fault loop impedance of a circuit, Ze is the earth fault loop impedance external to the installation which makes up part of Zs. So Zs=Ze+(R1+R2).

 

[James]

They are both earth fault loop impedance

Ze is the loop external to the installation.

Zs is the loop to the accessory, including any parallel paths.

edit,

@westward beat me to it.

 

[swaRRR]

Zs is the earth fault loop impedance of a circuit, Ze is the earth fault loop impedance external to the installation which makes up part of Zs. So Zs=Ze+(R1+R2).

It would be nice if this was explained in the book instead of them just using the exact same terminology for both.

 

[westward10]

Pages 46 and 47 of BS7671, brown book shows the definition of symbols.

 

[Rockingit]

In simple terms..


Z= loop,
e = external
s = sum (total at end of circuit)
db = sum of a distribution circuit so far to a sub distribution board

 

[pc1966]

The external value Ze is your starting point for any design, then you can compute a circuit's expected Zs from Ze + R1 + R2 as @westward10 said and you know it should not be more than that, though it might well be less if there are parallel paths (for example, lowering the effective Ze via bonding to shared metallic service pipes). From the circuit (which could be a sub-main) you can then check your disconnection times are being met.

As @Rockingit points out, you also see Zdb which is "similar" to Ze but for circuits coming off a board/sub-board. From a design point of view you would compute Zdb as above (i.e. Zdb = Ze + R1sub-main + R2sub-main), and then for all circuits off it do the same again.

In practice for larger existing systems any new work is likely to use measured values as it might be too disruptive to isolate the whole installation to check Ze (if not well documented). Then parallel-paths would be included which is not always a good idea as they might be removed without notification (e.g. metal water pipes replaced by plastic).