Accuracy of test instruments for NIC

[JohnW2]

As I said, all that proves is that the slope of the true/displayed value has not changed. Although it's probably not a very likely scenario, the finding that addition of a particular resistance results in the same change in indicated reading as it 'always has' does not preclude the possibility that the meter is giving incorrect absolute readings.

Extraordinarily unlikely, I would have thought.

I'm not so sure. The problem with loop testing is that the nature of it is such there is no way of 'checking the zero'. It is therefore far from impossible that everything could have shifted up or down (which would not take much of an electronic malfunction or maladjustment), such that a zero loop impedance, if one could emulate it, would be measured/displayed as non-zero, and all readings would be XΩ above or below the true value.

However, having said that, it would not be such a bad idea to use two or more different added resistances, particularly given that a malfunction would not inevitably be linear in relation to incremental resistance.

Two would give you 3 different ΔΩ. Three would give you 7, which I submit would be both unnecessary and tedious to work through.

As I said (in the bit before my "However..." that you have quoted), I would personally not normally do it with more than one value of resistance (although I guess that I would probably try a different resistance if the first one did not give the expected answer - but that hasn't happened yet). I don't know how 'properly' or exhaustively the calibration people check loop impedance measurement (they may do no more than we are discussing!) but one might like to think that 'proper checking' would involve more than one loop impedance (or incremental loop impedance).

Kind Regards, John

 

[westie101]

Make sure your voltage reading is accurate as well.
We're getting fed up of reports of high volts, that ae reports of inaccurate voltmeters in actual fact!

 

[JohnW2]

Isn't 50M a bit low for the maximum on the IR side?

It's certainly 10 times lower than the top one in my box. However, if you took the same view of IR measurement as what you are saying in other posts about loop impedance measurements, you ought to be happy with a check with just one resistance - somewhere (anywhere) in the range of credible IR measurements, oughtn't you?

Kind Regards, John

 

[JohnW2]

Which one would you buy?

Frankly, I wouldn't (and didn't) buy any of them, although not really for money reasons. I wanted my checkbox to do a lot more things than just contain a few resistors so I built accordingly! I do have 'resistance boxes' (and capacitance boxes), for use in relation to electronic activities - but, again, they are home-brewed.

... but still only 9 resistors in a box!

True, but for £15... It is the one I was using last night and on the rear it states: "Only to be used for checking calibration of insulation resistance or continuity testers"

At least that's realistic. The £34.98 similarly states that it's only for checking IR and 'continuity', but what it goes on to say about loop impedance and RCD tests (and, indeed, the requirement for regualar on-site testing of IR and 'continuity' meters) is all rather fascinating:

Simply plug your meter (digital or analogue) into the Calibration Checkbox to test for continuity (0.5 - 10 ohms) and insulation (0.5 - 50 Mohms). You can test for RCD and earth loop impedance back in your office on a known socket - leaving the Calibration Checkbox as an essential on site tool for all electricians. The requirements as laid out in the NICEIC handbook makes it self-evident that electricians need only do two tests on site (insulation and continuity) in order to meet the Part P regulations; the other earth fault loop impedance and RCD tests were allowed to be done in their office using a known socket! This means that the calibration checkbox complies with the requirements to regularly check the accuracy of your instruments at regular intervals. 34.98

... which makes a lot more sense that the (identical) one electrospark found for £29.99, which said

With direct access to a socket, this calibration checkbox can also be used to test RCD and earth loop impedance

!!!

BTW, the one you posted when it was at its 'starting bid' of £9.99 sold for £17.77.

Kind Regards, John

 

[JohnW2]

Make sure your voltage reading is accurate as well. We're getting fed up of reports of high volts, that ae reports of inaccurate voltmeters in actual fact!

My 'box' includes a pretty well-regulated voltage source!

Kind Regards, John

 

[EFLImpudence]

BTW, the one you posted when it was at its 'starting bid' of £9.99 sold for £17.77.

That's people for you.

 

[electrospark]

Well this certainly started a debate...

I suppose at least the 9 resisters I found do come in a box, I'm not sure what they'd look like without it.

Ok so maybe I'll speak with the NIC when they carry out my assessment. I think as long as I get a new calabration certificate for then it'll be fine.

Thanks for the feedback.

 

[electrospark]

Make sure your voltage reading is accurate as well. We're getting fed up of reports of high volts, that ae reports of inaccurate voltmeters in actual fact!

My 'box' includes a pretty well-regulated voltage source!

Kind Regards, John

Can I see your 'box'

 

[JohnW2]

I suppose at least the 9 resisters I found do come in a box, I'm not sure what they'd look like without it.

Sure, they obviously need to be in something and, so long as one understands what one can (and cannot) reasonably do with them, that's fine.

Ok so maybe I'll speak with the NIC when they carry out my assessment. I think as long as I get a new calabration certificate for then it'll be fine.

I would imagine that a recent calibration certificate would have to satisfy them at the time. I don't know what requirements, if any, they have in relation to 'checking between calibrations' (the statement from an eBay listing above seems rather fascinating!) but one would hope that, if there are any such requirements, they would not require you to have them in place at this stage - but who knows?!

Kind Regards, John

 

[ban-all-sheds]

Can I see your 'box'

Fnarr fnarr

 

[Adam_151]

]I would imagine that a recent calibration certificate would have to satisfy them at the time. I don't know what requirements, if any, they have in relation to 'checking between calibrations' (the statement from an eBay listing above seems rather fascinating!) but one would hope that, if there are any such requirements, they would not require you to have them in place at this stage - but who knows?!

My experience of the NIC is that, yes you can use a check-box on a monthly basis as a way to need require yearly calibration on your test kit... however they will then require calibration of the check-box yearly (and it'll have to be transmille or similar I dont think anyone would be willing to try and calibrate a home-made effort?). Clearly for a single test set there is no point, but if you had 20 electricians with twenty sets of test gear it becomes worthwhile. This also covers against the error with proving the loop test is correct as john points out, the check-boxes generally have a socket for the loop tester and then a switch which adds in 1 ohm and you record the two results and the difference between them. If you are check boxing a load of testers you soon start to know if these values should be around 0.4 ohm and 1.4 ohm and if one tester starts reporting 0.8 and 1.8 you will suspect something, try a different loop tester on it to rule out supply authority network changes and you'll know when to send instrument for repair.

 

[JohnW2]

This also covers against the error with proving the loop test is correct as john points out, the check-boxes generally have a socket for the loop tester and then a switch which adds in 1 ohm and you record the two results and the difference between them.

Indeed. That's what BAS and I have been discussing, and it's what my home-made one does - not the least because there's not much more one can do to check loop impedance without getting very much more complicated/expensive.

If you are check boxing a load of testers you soon start to know if these values should be around 0.4 ohm and 1.4 ohm and if one tester starts reporting 0.8 and 1.8 you will suspect something, try a different loop tester on it to rule out supply authority network changes and you'll know when to send instrument for repair.

Sure, if one is testing more than one (using the same circuit), it will become apparent if the absolute readings being given by one of them is suspect, even if it increases by 1Ω with added resistor. However, if (like me) one is usually only checking one meter, and particularly if the error is fairly small (but still 'important'), it will be difficult to pick up the error - e.g. if one is 'expecting' and increase from 0.6Ω to 1.6Ω, but sees 0.7Ω and 1.7Ω, or 0.8Ω and 1.8Ω.

Kind Regards, John

 

[ban-all-sheds]

Buy another tester then

 

[JohnW2]

Buy another tester then

If I understand you correctly, one would have to buy at least two more. To have two meters/testers giving different results is no real help, since one doesn't know which (if either!) is correct. If one has three or more, one can at least work on the principle of 'majority voting'.

If one has several meters/testers, one hardly needs a 'check box' - one can just compare the results of all of them on some (any) real-world circuit/installation, in which case one producing 'rogue' results would stand out.

Kind Regards, John

 

[EFLImpudence]

True - but you wouldn't have to buy the third until the first two differed.