Tek-Tips is the largest IT community on the Internet today!

Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!

  • Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Calculating Zs

Status
Not open for further replies.

Indy

Industrial
Joined
Dec 14, 2012
Messages
172
Location
GB
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
 
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

 
Status
Not open for further replies.

Part and Inventory Search

Sponsor

Back
Top