Earth Faulth Loop Impedance HELP!!!

[stevemu7]

[FONT=Arial, Helvetica, sans-serif]The 80% rule of thumb is only intended to be applied to a test result taken from an impedance test. You have to ensure that the tested value does not exceed 80% of the tabulated value.[/FONT]

[FONT=Arial, Helvetica, sans-serif]If the value is above 80% of the figure then improve the result this can be done by improving or providing additional earthing. [/FONT]

 

[Deleted member 72666]

I may be wrong but I was under the impression that the cables could reach 70 degrees under fault conditions (short cct) where the ocpd was to operate and disconnect within the specified time, (0.4s 5 s,etc) thus causing an increase in cable resistance and that's why we de-rate the zs tables by the 'rule of thumb' 0.8. Well it sounds logical to me anyway!

 

[Richard Burns]

See table 43.1 in BS7671 for the limiting temperature of cables under fault conditions, up to 250ºC for MICC.

 

[Engineer54]

I may be wrong but I was under the impression that the cables could reach 70 degrees under fault conditions (short cct) where the ocpd was to operate and disconnect within the specified time, (0.4s 5 s,etc) thus causing an increase in cable resistance and that's why we de-rate the zs tables by the 'rule of thumb' 0.8. Well it sounds logical to me anyway!

Cable operating temperatures and limiting temperatures under fault conditions are two entirely different scenarios...

 

[Deleted member 72666]

See table 43.1 in BS7671 for the limiting temperature of cables under fault conditions, up to 250ºC for MICC.

Thanks for that. From that table it's interesting to see that thermosetting insulated cables are on par with unsheathed mi cables.

 

[Deleted member 72666]

Cable operating temperatures and limiting temperatures under fault conditions are two entirely different scenarios...

Yes you're right....I'm easily confused these days!

 

[Engineer54]

The 80% rule of thumb is only intended to be applied to a test result taken from an impedance test. You have to ensure that the tested value does not exceed 80% of the tabulated value.

If the value is above 80% of the figure then improve the result this can be done by improving or providing additional earthing.

As i've stated elsewhere i rarely calculate impedance values they are always tested!! lol!!

As for improving the tested/calculated result, i think you'll find the modern formula for that scenario is bung a RCD in!! Or where one is already provided, who care's, the limit is 1667 ohms, so it's protected!! ...And there lies the far more important question than the 80% rule, when does additional protection become your main protection, by ignoring ADS requirements (other than on most TT systems that is) ??