Have a job where Ze is 0.36 ohms on a TNC-S with 80A BS1361 suppliers fuse. The general rule, as per OSG (page 11), is if Ze is below 0.35 ohms then this is acceptable, but that relates to a 100A fuse (OSG fig 2.1 on page 15 refers). Now I believe that this installation should comply, so have been looking in to the theory behind max Ze to confirm this. The OSG is, after all, only a guide. Please check my thinking below....
This is not a final circuit, therefore 411.3.2.3 should apply, so the allowable disconnection time must not exceed 5 seconds
Appendix 3 Figure 3.1 'Fuses to BS 1361' shows that a current of 460A is required to disconnect a 80A fuse in 5 seconds.
Ohm's law V=IR can be rearranged as R = V/I
R = 230 / 460 = 0.5
So for a 80A BS1361 fuse to disconnect in a time not exceeding 5 seconds then the Ze needs to be 0.5 ohms or less
Therefore my measured Ze of 0.36 ohms complies
Is this correct?
This is not a final circuit, therefore 411.3.2.3 should apply, so the allowable disconnection time must not exceed 5 seconds
Appendix 3 Figure 3.1 'Fuses to BS 1361' shows that a current of 460A is required to disconnect a 80A fuse in 5 seconds.
Ohm's law V=IR can be rearranged as R = V/I
R = 230 / 460 = 0.5
So for a 80A BS1361 fuse to disconnect in a time not exceeding 5 seconds then the Ze needs to be 0.5 ohms or less
Therefore my measured Ze of 0.36 ohms complies
Is this correct?