Fluctuations in reading ZLine Ipscc

"Shopkeeper Syndrome" .

Back in the 5os, 60s and 70s, most folk who worked behind a shop counter were pretty well clued up, they had to be to get employed there, they did most of the calcs in their head (they learned at school up to 12 x 12 =) or sometimes with a pencil and a scrap of paper.

Roll forward to today and you get "I can`t work out 7 x 8 cos I not got mee calculator" or they turn around to get you something off the shelf (1 item) and they cant remember what you just asked for one second ago. or they work out you shopping bill comes to £1,765.76 when you only asked for two bags o` crisps and a chocolate bar.

Just think of your meter reading of being a bit better than a modern day shop assistant. Sort of are you in the ball park or way off? sort of thing

they work out you (sic) shopping bill comes to £1,765.76 when you only asked for two bags o` crisps and a chocolate bar.

The bane of the computer age.

Instant high precision beautifully presented *&*^%$ rubbish, un-moderated by any common sense that a human would apply.

Also seen with word processors and power point style presentations, where no facts at all can be spread over several sides with impressive clip art and special effects to rob folk of time they could more usefully spend drinking beer and setting the world to rights.

The 3 line sum on the paper bag was never that bad, because a sense of 'that can't be right, I'll just check that before I make a fool of myself  ' would have kicked in.

Back to the OP.

In the days of the moving meter needles it would have been near the 'here be dragons' end of the scale where infinity, 5kA and 10kA all bunch up together,  and in the other coloured ink 0 ohms was full scale the next division down was 0.1 or 0.2 ohms or something while 250A 500A and 1kA and the higher resistances were wider spaced. If the needle  wobbled a bit with local loads coming on and off you'd have probably gone 'mmm' and written down a single value - the flickering digits hide an internal  PSSC calculation that approaches a knife edge as the loop resistance approaches being too small to measure for that sort of tester.

Mike.

Can the PFC be calculated with the information of the supply transformer, length and size of service cable?

Mike's approach will give you a decent idea of the cable's resistance, but larger cables also have significant reactance, which means their overall impedance can be somewhat higher. Unfortunately BS 7671 tables for cables (r,x,y) don't include the concentric cables that DNOs usually use, so you might have to source that data from elsewhere (having different shapes and copper rather than steel armour may make a difference to the reactive components especially).

Paul Cook's Commentary on the Wiring Regs has a section on this very subject (my copy is pretty old now, but I guess the modern versions have something similar). He does caution against making assumptions about the DNO's mains (e.g. running along the road) as these can change significantly over time, and suggests only considering the service cable (between the main and the consumer's cut-out) to see how much the DNO's declared PFC might be attenuated, and gives quite a few tables for different cable types and lengths (rather than a calculation).

There's also a mention of PF for the fault current - which initially seems odd for a conventional fault 'of negligible impedance' as (unlike say a motor) there's nothing about the 'load' that would influence the power factor - so I'm guessing that's down to the inductive effect of the supply transformer itself ... perhaps some here could confirm/deny that. Certainly his tables list PF as being pretty poor (e.g. 0.55) close to the main, and improve (get closer to 1.0) as the cable lengthens.

   - Andy.

It is exactly that- the transformer may drop 2% to  3 % of the volts at full load, but it sure has heck does not lose it resistivity - if it did the transformer oil would be making chip-pan frying noises and the old Bucholtz trips would be working overtime. 

Even 2 % of a 500KVA load is about 10kW - something more like 3 kW is reasonable - and allows a few fans to significantly up the rating of a transformer that is struggling thermally, assuming volt drop does not get you.

This one loses 500 watts (0.1%) unloaded, just magnetizing and reversing the core 100 times a second, and losses rise to 3.3kW (0.7% say) at full load with the oil around the windings at 75C . The ~ 3kW part is the heating of the copper due to I²R losses  Further down it gives the 'impedance voltage' as 4.75% - that is the output terminal voltage loss at full load..).

It is the lines of flux that thread one winding and not the other, that appear as an unavoidable inductance in series with the winding (if you like the inductance you see on one winding with the other shorted.) that causes more than half of  the voltage droop.

Indeed as you get further away, the cable (which really is more resistive than inductive) dominates and lowers the Q (ratio of X to R).

All of this subtlety is ignored by the inaccuracy of using a DC-like resistance calculation.

Tx inductance  is also the reason for a couple of other items of notes

1  Why the maths in Lees original arc-flash paper is so peculiar - he treats the arc as a resistor and the supply as a perfect voltage and a series inductor) If instead you assume both are resistors then the max power arc is when the arc voltage is half the supply open circuit value, and then the upper limit on total arc blast energy is just ADS opening time times the half the PSSC squared  times the loop impedance. But that would not have covered 4 sides of vector algebra.

2 Why fault current waveforms are far from rectangular at the instance of contact, the inductance  helps slow the rate of rise of fault current, allowing fuses to operate before the peak in the fault current would have been reached further  limiting of energy to do thermal damage.

Mike