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# Calculating Zs

## Calculating Zs

(OP)
Hi,
We have a 3ph&n(415v) installation protected by a bs88 160A fuse. Bs7671 states that Zs maximum is 0.25 ohms for a 5s disconnection time. The swa is being used as the protective conductor.

To estimate our value of Zs I have assumed Ze is 0.35 ohms as we have no other information.
I am struggling to find formula for estimating r1+r2 again as we have no other information
So I can check the actual Zs based on Zs=Ze+(r1 +r2)

Can anyone please point me in the direction of estimating r1+r2 using the tables in bs7671.

Thanks

### RE: Calculating Zs

According to Regulation 411.4.5 Zs=impedance in ohm of the fault loop it
is the following impedances series:
The source
the line conductor up to the point of default
the protective conductor between point of default and the source
Let’s say the source it is a transformer of 11/0.4 kV 400 kVA 4% impedance
then Zsource=0.4^2/0.4*4/100=0.016 ohm
Let’s say the conductor it is 100 m of 3*70+35 mm^2 copper conductor XLPE[90oC] insulation.
According to Table 4E4B BS7671/2008 z=0.45/1.73=0.26 ohm/km
Zcable=0.026 ohm per phase.
Protective conductor SWA 18 steel wire of 2.5 mm diameter 1.2 ohm/km
[Since the SWA is in contact with the earth then the impedance with respect to Earth
it could be 0.089 ohm]
Zloop=0.016+0.026+0.12=0.162 ohm

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