Discuss Supplier's fuse, disconnection time and max Ze in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

ringer

-
Reaction score
116
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?
 
souds logical to me. however, can you be sure that your meter is accurate to 0.01 ohms?
 
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.
 
it is a bit high fot TNC-S though usually get a reading of 0.2 or less. possible slack connection on tails?
 
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.
 
silly me i didnt remember but thanks for the correction spinlondon. i need to look and keep quiet me thinks
 

Reply to Supplier's fuse, disconnection time and max Ze in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock