Supplier's fuse, disconnection time and max Ze

[ringer]

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?

 

[telectrix]

souds logical to me. however, can you be sure that your meter is accurate to 0.01 ohms?

 

[ringer]

That works both ways Tel, so if I had measured 0.35 ohms (the supposed top limit) could I then be sure that it wasn't really 0.36 ohms? As it is, if the max Ze in this case is in fact 0.5 ohms then I am comfortably inside that. But it is good to know that I am not barking up the wrong tree.

 

[telectrix]

it is a bit high fot TNC-S though usually get a reading of 0.2 or less. possible slack connection on tails?

 

[netblindpaul]

It seems correct, however, please remember BS7671 does not apply to the DNO equipment and installations.
The fundamental science however does, so your example is correct in that aspect.

 

[Deleted member 26818]

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?

Yes it is correct.If the supply was 100A the max would be about 4Ω .
This is why often the DNO, will refuse to allow TN-S supplies above 60A, or will stipulate that an upfront RCD is required, or even that the supply must be changed to TN-C-S.

 

[leesturgess]

100A max 230/630 0.36

 

[Deleted member 26818]

100A max 230/630 0.36

Not quite.
Remember ESQCR allows a voltage fluctuation, between + 10% and - 6% of 230V.
As such the volatage, could be 253V.
253/630=0.4.

 

[leesturgess]

silly me i didnt remember but thanks for the correction spinlondon. i need to look and keep quiet me thinks

 

[Deleted member 26818]

silly me i didnt remember but thanks for the correction spinlondon. i need to look and keep quiet me thinks

No not really.
To my mind, if you don't question, you don't learn.
Also, anyone who makes any statement, should be able to explain their reasoning behind the statement.