Zs calcs using OSG.

[richardindorset]

Our level 3 lecturer seems to be confused on how to use the tables in appendix I, for calculating Zs.

They said that you look up the resistance in Table I1 and if the ambient temp is below 25°C, use the factors in Table I2. If over 25°c, use table I3.

I don't think this is right. Table I2 is a correction for temperature and table I3 is a correction for cable insulation type.

I think that table I2 is only used for testing not for doing Zs calcs when checking circuit protection design.

Any ideas?

Cheers!

 

[EFLImpudence]

They said that you look up the resistance in Table I1 and if the ambient temp is below 25°C, use the factors in Table I2. If over 25°c, use table I3.

Ok.
I agree with you that Table I3 is nothing to do with ambient temperature.

I don't think this is right. Table I2 is a correction for temperature

Yes.

and table I3 is a correction for cable insulation type.

Well not really.
I3 is for the temperature of the conductor at maximum operating temperature; i.e. 70º for T&E etc..

I think that table I2 is only used for testing not for doing Zs calcs when checking circuit protection design.

Yes, and then the correction factors are used for the subsequent calculations of Volt drop etc.; i.e. multiply by 1.2 for the change of resistance from 20º to 70º if required.

 

[EFLImpudence]

I am not sure what Table I3 is.

It seems to include two different things - bunching and maximum operating temperature.

The bunching might cause the conductor temperature to rise above 70º (T&E) so you can't just increase the resistance value.


The On Site Guide is not a very good publication and I would advise throwing it away - or rather, I suppose, keep it for the annual assessment but never use it..

 

[richardindorset]

I am not sure what Table I3 is.

It seems to include two different things - bunching and maximum operating temperature.

The bunching might cause the conductor temperature to rise above 70º (T&E) so you can't just increase the resistance value.


The On Site Guide is not a very good publication and I would advise throwing it away - or rather, I suppose, keep it for the annual assessment but never use it..

good to see I'm not the only one confused...