When I look at line 10 (B40 mcb on a 6/2.5 twin and earth,TNS system the max circuit length due to zs is given as 23m(no RCD).Yet when I calc R1 +R2 x 23m I get 10.49x23/1000 = 0.24 ohms.Add ze for TNS to this(0.8) gives 1.04 ohms,whilst table B6 gives a max zs of 0.88 ohms for a B40.Wondered where I,m going wrong.
Regards,
Hz
hertzal123:
When I look at line 10 (B40 mcb on a 6/2.5 twin and earth,TNS system the max circuit length due to zs is given as 23m(no RCD).Yet when I calc R1 +R2 x 23m I get 10.49x23/1000 = 0.24 ohms.Add ze for TNS to this(0.8) gives 1.04 ohms,whilst table B6 gives a max zs of 0.88 ohms for a B40.Wondered where I,m going wrong.
Regards,
Hz
Table B6 in OSG is the maximum measured earth fault loop earth impedance. There is no way of getting this to reconcile with the circuit calculations at all. To properly do the calculation, you should use the Zs values from BS 7671.
The calculation method that is recommended, and used for Table 7.1(i) in OSG, is discussed in the IET's Electrical Installation Design Guide (EIDG). Standard circuits are covered in Appendix B, but reference is made to other sections of the book so you can see where it's coming from if you need further explanation on any particular point.
The calculation used is roughly as follows (in LH column, tables in Red are from BS 7671, black the EIDG):
hertzal123:
When I look at line 10 (B40 mcb on a 6/2.5 twin and earth,TNS system the max circuit length due to zs is given as 23m(no RCD).Yet when I calc R1 +R2 x 23m I get 10.49x23/1000 = 0.24 ohms.Add ze for TNS to this(0.8) gives 1.04 ohms,whilst table B6 gives a max zs of 0.88 ohms for a B40.Wondered where I,m going wrong.
Regards,
Hz
Table B6 in OSG is the maximum measured earth fault loop earth impedance. There is no way of getting this to reconcile with the circuit calculations at all. To properly do the calculation, you should use the Zs values from BS 7671.
The calculation method that is recommended, and used for Table 7.1(i) in OSG, is discussed in the IET's Electrical Installation Design Guide (EIDG). Standard circuits are covered in Appendix B, but reference is made to other sections of the book so you can see where it's coming from if you need further explanation on any particular point.
The calculation used is roughly as follows (in LH column, tables in Red are from BS 7671, black the EIDG):
