Earth Fault Loop Impendance

[Litttle Shonet]

Hi there.

I'm new to this forums lark, so pleasde bear with me. here goes!

My question is: In chap 4, table 41.2 to 41.4. of the 7671 it gives a list of pre determined figures for Max Zs, but in appendix 14 it gives a formula to work it out. The formula gives completely different measurements. What is this formula used for:

Many thanks

L S

 

[a1spark]

Hi, The formulae in Appendix 14 is used for calculating the acceptable in-service

value where the circuit will be under load and therefore its resistance will be higher , thus the nominal 0.8 factor has to be applied to the max values of those given in tables

412-41.4

 

[Litttle Shonet]

Thanks for that. I thought it was something like that, but if you use the formula for circuits under load, won' all circuits use it, thus making the tables redundant.

 

[a1spark]

Fair question maybe someone can expand on this. I see it as the tables are a base level from which to start and the 0.8 figure is an averaged figure applicable for all cable type..70 degree - 90 degree , protective conductor combined ----protective conductor seperate etc. If a more scientific / exact figure is required in advanced design situations then the Tabulated values wopuld be required for each specific calculation

Note the second part of the Appendix 14 allows a bit of a get out clause if the adjusted figure is too high

 

[Litttle Shonet]

Thanks for that. Thought I was losing it for a moment. Ambiguous or what?

 

[sparky b]

If you have the small red on-site guide, check out page 103, it gives you the ratings already worked out.

SB

 

[a1spark]

If you have the small red on-site guide, check out page 103, it gives you the ratings already worked out.SB

Yers Sparky B. your right there , but what the OP was enquiring over was why bother to give the non adjusted figures in the first place why not just give the on-site adjusted figures .I was trying to give my thoughts as to the requiremets for both sets...obviously the non adjusted figures are also needed for the schedules of test results on the Certs, but I was giving my thoughts as to reasons. in terms of circuit design.....What do you reckon on it ?

 

[sparky b]

Hi A1, i don't really have a take on it as such. But I do remember my instructor saying it had to do with ambient temperatures. I spose the on-site guide is already done for you as it's the most used when on the job so it saves you having to whip out the calculator, and it was written after the BRB was published.....

Sorry, but probably not much help..... ?:|

 

[a1spark]

You are right about the temperatures [ of the cables ] ,I touched on that above... but thankls for your input mate !

a1 spark