How do loads affect a PSCC reading?

[mfarrow]

Both inside the installation, and outside the installation, and to what order of magnitude?

I'm just curious.

Discuss.

 

[JohnW2]

Both inside the installation, and outside the installation, and to what order of magnitude? I'm just curious. Discuss.

I'm not sure what you're thinking of here. Loads will surely only affect PSCC if they result in a change of voltage as seen at the installation, since the only other factor is the measured L-N loop impedance, which I wouldn't expect to be influenced (significantly, if at all) by loading. What are you thinking of?

Kind Regards, John

 

[mfarrow]

the only other factor is the measured L-N loop impedance, which I wouldn't expect to be influenced (significantly, if at all) by loading. What are you thinking of?

That's exactly what I was thinking of

You are part of an LV network fed by a single MV:LV transformer. If you have a combined resistive load of other consumers of 144A, that represents an impedance on the network of 1.6Ω. If the feed transformer also has an impedance of 1.6Ω, simplistically this means you will see 0.8Ω at your service head.

This would be a false reading, and double your recorded PSCC vs reality. If you are on TN-C-S, I would have thought this also affects your Ze.

No? Yes? Or have I missed something fundamental?

 

[JohnW2]

That's exactly what I was thinking of ... You are part of an LV network fed by a single MV:LV transformer. If you have a combined resistive load of other consumers of 144A, that represents an impedance on the network of 1.6Ω. If the feed transformer also has an impedance of 1.6Ω, simplistically this means you will see 0.8Ω at your service head. ... This would be a false reading, and double your recorded PSCC vs reality. If you are on TN-C-S, I would have thought this also affects your Ze. ... No? Yes? Or have I missed something fundamental?

I would say the latter

If you isolated the primary of the MV:LV tranny and then ('passively', with power source in your meter) measured the impedance (directly) between L and N at the installation then, yes, things would be as you say. the 1.6Ω of the network would be in parallel with the 1.6Ω worth of loads, so, as you say, you'd measure 0.8Ω

However, that's not how loop impedance is measured. It's measured in an 'active' fashion with the network live with the tranny being a power source. Think of an 'extreme' situation in which one measured PSCC directly by applying a dead short between L and N and measured the consequent current which flowed (until some protective device operated). The path through which that current flowed would involve only the L&N conductors of the network, and would not flow at all through other consumer's installations or loads (which would therefore not affect the current). A loop tester essentially does the same thing (in a more controlled fashion!).

Kind Regards, John

 

[plugwash]

You are part of an LV network fed by a single MV:LV transformer. If you have a combined resistive load of other consumers of 144A, that represents an impedance on the network of 1.6Ω. If the feed transformer also has an impedance of 1.6Ω

If the load had an impedance of 1.6 ohm and the source also had an impedance of 1.6 ohm then the loaded voltage would be half the open circuit voltage. I don't think anyone would consider that acceptable.

To keep voltage reasonablly stable the source impedance needs to be much lower than the likely load impedance.

 

[JohnW2]

You are part of an LV network fed by a single MV:LV transformer. If you have a combined resistive load of other consumers of 144A, that represents an impedance on the network of 1.6Ω. If the feed transformer also has an impedance of 1.6Ω

If the load had an impedance of 1.6 ohm and the source also had an impedance of 1.6 ohm then the loaded voltage would be half the open circuit voltage. I don't think anyone would consider that acceptable. ... To keep voltage reasonablly stable the source impedance needs to be much lower than the likely load impedance.

That's obviously true, but it doesn't alter what I said about the measurements.

With mfarrow's (hypothetical and, as you say, impractical) scenario, a conventional live L-N loop impedance measurement, using the supply tranny as the source, would still give the correct answer of 1.6Ω for the supply/source impedance, since none of the test current being observed would flow through the load. On the other hand, if one did a 'dead' measurement on the isolated system (primary of tranny isolated), using a meter containing the test voltage source, then he would get the 0.8Ω answer. since the test meter would then be seeing two 1.6Ω impedances in parallel, and would send the same test current through each of them.

Kind Regards, John

 

[plugwash]

With mfarrow's (hypothetical and, as you say, impractical) scenario, a conventional live L-N loop impedance measurement, using the supply tranny as the source, would still give the correct answer of 1.6Ω for the supply/source impedance, since none of the test current being observed would flow through the load.

Wrong.

Lets suppose we have a supply with an open circuit voltage of 240V, and a purely resistive impedance of 1.6 ohm. Initially we don't connect any load to this source.

Now lets suppose we have a loop impedance tester that measures loop impedance by measuring open circuit voltage and short circuit current. We get an open circuit voltage of 240V and a short circuit current of 150A and our meter calculates an impedance of 1.6 ohm.

Now suppose we add a 1.6 ohm load and use our tester again. This time the tester measures an "open circuit"* voltage of 120V and a short circuit current of 150A and calculates an impedance of 0.8 ohm.

The PFC on the other hand wont change between the two tests, in either case it remains at 150A.

* That is the tester is presenting an open circuit.

 

[mfarrow]

Good spot re. the potential split on a 1.6 ohm transformer!

I'm still unconvinced how the meter actually works though? How does it do a controlled short circuit? Surely everyone's lights must dim?

 

[flameport]

Modern test devices connect a certain known load to the supply, so not actually a short circuit. It is only applied for a very short time, so any change in lighting brightness is tiny, and not generally noticed.

Unlike old meters where a substantial current of many tens of amps was allowed to flow for a significant number of seconds.

As for other loads on the supply - not particularly relevant, although certain appliances such as fridges can cause issues with interference / noise particularly when using the fairly unhelpful 'no trip' settings.

What can give all kinds of defective readings are solar panel inverters - try measuring the L-N impedance at an installation with an inverter powered on, and then again with it disconnected.

 

[JohnW2]

With mfarrow's (hypothetical and, as you say, impractical) scenario, a conventional live L-N loop impedance measurement, using the supply tranny as the source, would still give the correct answer of 1.6Ω for the supply/source impedance, since none of the test current being observed would flow through the load.

Wrong. ... Now suppose we add a 1.6 ohm load and use our tester again. This time the tester measures an "open circuit"* voltage of 120V and a short circuit current of 150A and calculates an impedance of 0.8 ohm. ... The PFC on the other hand wont change between the two tests, in either case it remains at 150A.

Yes, you're right. I let myself get confused my mfarrow's talk about resistances. The point in my mind, in response to what he actually asked about in his OP, was the one you've just made (that the PSCC would be measured correctly, deven in the face of massive loads on the network) - but, as you say, a ('pre-test') reduction in supply voltage (at installation) due to such massive loads on the network would result in an incorrect calculation of supply impedance.

Kind Regards, John

 

[JohnW2]

I'm still unconvinced how the meter actually works though? How does it do a controlled short circuit? Surely everyone's lights must dim?

Needless to say, they don't use (negligible impedance) short circuits! They introduce a non-negligible-impedance 'fault' for a very short period of time and measure the consequential brief drop in measured voltage. At 230V, my Fluke uses a maximum of 12A for 10msec - i.e. it introduces a 'fault' (L-N or L-E) of about 19Ω for 10msec.

Edit: too slow again!

Kind Regards, John

 

[JohnW2]

If the load had an impedance of 1.6 ohm and the source also had an impedance of 1.6 ohm then the loaded voltage would be half the open circuit voltage. I don't think anyone would consider that acceptable. To keep voltage reasonablly stable the source impedance needs to be much lower than the likely load impedance.

All this talk about VDs in supply cables has got me thinking (always dangerous!) ... something doesn't seem to add up.

Consider a not-atypical PSCC (measured at origin of installation) of 460A with 230V (no load) delivered to consumer - i.e. a L-N loop impedance (as seen from installation) of 0.5Ω. Turn on a 45A shower and, say, 20A worth of cooker/kettles/whatever and the installation's voltage should fall to below 200V. Add in some loads from adjacent properties (which are sharing much of the L&N path back to tranny) and the supply voltage (at installations) might well drop to a ridiculously low level. This surely doesn't happen, and wouldn't be acceptable. What am I missing?

Kind Regards, John

 

[EFLImpudence]

Surely the PSCC test at source will be carried out when no loads are being used (or more correctly vice versa).
After all, Ze is taken with the earthing conductor disconnected and therefore the installation must be isolated.
Would Zn not be measured at the same time?

If the measurements are being taken at appliances or accessories and the meter indicated a very low voltage would the tester not realise?
In any case, for the values at these points, Ze + R1+R2 is more accurate as a determination of the circuit but a measurement of the actual situation may be more indicative of what will really be the case in normal usage.
I don't know how other properties' loads can be avoided but, presumably they affect the situation most of the time.


Furthermore the meter does not actually measure the PSCC but merely calculates it from the voltage and resistance.
I recall, I think, that you, John, have previously stated that the meter uses a standard nominal voltage, not the actual voltage, for this calculation in which case it would not matter what the actual voltage was.
I am not sure that this is the case judging by the following from the Fluke manual -

PFC. PSC Test
Accuracy Determined by accuracy of loop resistance and mains voltage measurements

 

[Spark123]

Megger MFTs use nominal voltages in their calcs.

 

[EFLImpudence]

Right, thanks, maybe it wasn't John who said it.

So the actual voltage doesn't matter with Meggers but not sure about Flukes.