How do loads affect a PSCC reading?

[JohnW2]

I'm still ahead of you on Riesling.

I'm primarily a red man

In practice it makes no real difference. My son would express 'pulling reasonable-sounding numbers out of the air', slightly differently. I imagine it was an off-the-cuff design decision and in practice seems to work.

That's all true. Provided the voltages one is dealing with are even half-near their nominal voltages, it doesn't make a blind bit of difference where one puts the dividing lines.

Having said that, if I were doing it, whilst I would probably use numbers pulled out of the air, I think I would be inclined to have appreciably 'tighter' ranges with 'gaps' between them ("I refuse to calculate, because I'm not too sure what the nominal voltage is meant to be") - just in case someone used the device on some non-standard supply e.g. perhaps 190V-270V for 230V.

Nevertheless the logarithmic approach seems more logical to me in general. In the specific case the nominal voltages are not equally spaced but approximately double each time.

I suppose that's not a bad argument.

Happy drinking!

and you!

Cheers, John

 

[AdrianUK]

In practice, the load makes no difference to the PSCC (since when an actual fault hits, the voltage at the short circuit point must be zero ... only the impedances of the network determine fault current).

When doing calculations for fault currents on real networks the load current is neglected (it would be unknown/unpredictable anyway). This can be proved using any decent simulation software or on a system simulator.

Adrian

 

[JohnW2]

In practice, the load makes no difference to the PSCC (since when an actual fault hits, the voltage at the short circuit point must be zero ... only the impedances of the network determine fault current).

Indeed - I think I said that way back - that if one were to measure PSCC by brute force (actually introduce a zero/negligible impedance L-N fault and measure the current for the few milliseconds it persisted until some protective device operated!), then the (correctly measured) PSCC would clearly be the same regardless of any other loads (which, as you say, would be 'shorted out' by the fault).

However, as also discussed, that is (thankfully, not the least for them!) not how electricians measure PSCC, which is what the OP was (I presume) asking about. Electricians measure loop impedance, and then use either actual or nominal voltage to calculate PSCC from it - and that introduces all sorts of other factors/variables.

Kind Regards, John