Volt drop, PSCC and loop impedance with DIY tools?

[ericmark]

OK I have a box full of three test instruments to test all of the above plus the time taken for a RCD to trip but these are not going to be available to most DIY people.

Some of the stuff a DIY meter will measure is however surprising for example power factor with a simple portable plug-in power meter. So this got me thinking how much he could really measure with such a simple meter.

After getting one of these simple plug-in meters I was under the impression they were nothing more than a toy. I worked out the kWh/Annum my fridge freezer used then cost of replacement and quickly realised even if I could show it used 4 times what it should it still would not be economical to borrow the money to replace it. So why bother measuring it? Power factor was another I may be able to calculate I need a 2μF capacitor on my CFL but I’m still not going to fit one so why bother measuring?

So then it can measure without having to use GN38 leads and hold open socket shutters voltage, current and power combine this with a simple load in my case tried my wife’s smoothing iron and one can get on and off load volts and the load amps so one should be able to work out volt drop. Then hang on it was not so long ago I was trying to work out volt drop from prospective short circuit current and loop impedance so could the reverse be done?

Well first was a look at the readings and volts are only given in whole numbers 3 s.g. so loop impedance 0.1 Ω maximum accuracy not really good enough but better than nothing. The PSCC was silly one volt difference could change reading from 1515 amps to 3042 amps so really not accurate enough but B32 we are looking at 160A and that equals an 18 volt drop with 13 amp load.

So where the question is “Can I extend my ring final” would a test with a kettle and portable plug-in power meter be good enough to give an answer?

Or are the testers a complete waste of space? Also any other ideas of how cheap DIY test equipment can be used in a safe manor to emulate our expensive kit.

 

[JohnW2]

It is perfectly valid to measure L-N loop impedance (hence PSCC) by observing the voltage drop resulting from application of a load (as large as possible) of known size. If the device you’re talking about gave adequately accurate measurements of voltage and current (which you seem to be saying it doesn't, at least for voltage), then it should give reasonably accurate results for L-N loop impedance/PSCC. However, if one were prepared to get a bit more 'invasive' (hence invoking more potential 'risks'), measurement of the voltage drop with even a cheap multimeter would give fairly accurate results (if the available current measurement was reasonably accurate!).

However, the main interest/concern usually relates to L-E loop impedance (i.e. EFLI/Zs) and PEFC (rather than PSCC), and that obviously cannot be measured directly in the manner you describe.

If the entirety of the loop impedance were due to the wiring within the installation, then one could obviously (since one knows the relative CSAs of CPCs and live conductors) estimate the L-E loop impedance from the measured L-N loop impedance, but that’s not how it is - one has the complication that Ze is a component of the total EFLI.

In the case of a TN-C-S supply, I suppose one could get an estimate of Ze (which, with TN-C-S would be the same as the external component of the L-N loop impedance) by measuring the voltage drop (in response to a known load) very close to the origin of the installation - but you couldn't do that with TN-S (and certainly not TT!).

However, all I've said there about estimation of EFLI/Zs/PEFC requires a fair degree of understanding and abilities to undertake the necessary calculations (as well as the knowledge/ability to undertake live tests safely, if one had to use a multimeter to get adequately accurate voltage measurements), so we're probably back to wondering whether it would be suitable for the average DIYer.

Kind Regards, John

 

[ban-all-sheds]

OK I have a box full of three test instruments to test all of the above plus the time taken for a RCD to trip but these are not going to be available to most DIY people.

A lot of people will cheerfully pay the cost of a set of testers to upgrade their seats on one holiday flight.

 

[ericmark]

Thank you JohnW2 I agree calculations could be a problem but I can easy enough use Java Script to make a calculator given on load volts and off load volts and size of load to work that out for any DIY person.

I had considered the earth question and with TT clearly any measurement line to earth without a very limited time as used with our meters would be dangerous.

But with a TT supply the ELI has only to trip a RCD not open a MCB so really there is little need to measure a simple resistor line to earth and a test push button can give a go no go test very easy with a 1.5kΩ resistor i.e. 5 times 30 mA a rather quick and easy test and should it go wrong a maximum of 150 mA so if required to test no real point in re-wiring any extension lead to do the test.

TN-S I agree is more of a problem but again how worried are we about earth loop impedance when 7666Ω will trip the RCD? With both TT and TN the earth loop impedance is really not a major issue.

The line - neutral impedance is to my mind far more likely to be exceeded that the line - earth impedance with any addition to a circuit when we accept it must have RCD protection and for those not following the rules it hardly matters anyway.

However more to the point is would telling some one how to test with a simple volt and amp meter decrease or increase risk? So yes we are "back to wondering whether it would be suitable for the average DIYer."

Once the question is asked by a DIYer and we give an answer the Rubáiyát of Omar Khayyám quote

The Moving Finger writes; and, having writ,
Moves on: nor all thy Piety nor Wit
Shall lure it back to cancel half a Line,
Nor all thy Tears wash out a Word of it.

comes in so before jumping in with both feet thought it better to test the water.

So given that using volt drop PSCC could be worked out should we tell DIYers or should we say:-

OK I have a box full of three test instruments to test all of the above plus the time taken for a RCD to trip but these are not going to be available to most DIY people.

A lot of people will cheerfully pay the cost of a set of testers to upgrade their seats on one holiday flight.

Which is of course true. I would prefer a debate in general terms than wait until some one asks the question yet again.

 

[JohnW2]

Thank you JohnW2 I agree calculations could be a problem but I can easy enough use Java Script to make a calculator given on load volts and off load volts and size of load to work that out for any DIY person.

Well, yes (although calculation of EFLI in a TN-C-S installation, which is what I was talking about, would be a fair bit more complicated than that). However, I thought you were talking about how DIYers, in general, might be able to undertake some of these measurements without expensive equipment, not just those who had access to whatever software calculators you may have created for them.

I had considered the earth question and with TT clearly any measurement line to earth without a very limited time as used with our meters would be dangerous.

Defintely not to be advised!!! Although none of the installation would come to any harm if one used a load that the CPC could safely carry, the big danger would be the potential to which exposed-c-ps throughout the installation could rise should it transpire that the Ze was high. Furthermore, one obviously could not do it on an RCD-protected circuit.

But with a TT supply the ELI has only to trip a RCD not open a MCB so really there is little need to measure a simple resistor line to earth and a test push button can give a go no go test very easy with a 1.5kΩ resistor i.e. 5 times 30 mA a rather quick and easy test and should it go wrong a maximum of 150 mA so if required to test no real point in re-wiring any extension lead to do the test.

All true - but a different matter, and rather peripheral to, the issue you raised.

TN-S I agree is more of a problem but again how worried are we about earth loop impedance when 7666Ω will trip the RCD? With both TT and TN the earth loop impedance is really not a major issue.

Well, for a start, even if it's TN, then it might not be an RCD-protected circuit. However, even with TN, as the regs say, one should not normally rely on an RCD to provide the required fault protection (i.e. disconnection times in response to L-E faults) - so one really does need to ensure that the Zs (i.e EFLI) is acceptable. Given that CPCs are nearly always smaller in CSA than live conductors, the PSCC can be adequate to meet disconnection times for L-N faults, yet the Zs may be too high to ensure that required disconnection times are achieved (by the OPD) for L-E faults.

Kind Regards, John