Accuracy of test instruments for NIC

[JohnW2]

You can buy the bits at Maplin for less than a tenner, and you'll have fun with a soldering iron while you are at it!

Indeed - I've just said the same thing in a slightly different way!

My NAPIT guy was happy to see proof that I was checking the relative consistency of the tester on a regular basis. Can't comment on other scams though.

Interesting. As I've just written although it is fine for checking the resistance ranges of your kit ("continuity" and IR), a 'resistance box' is somewhat limited in what it can 'check' in relation to loop impedance measurements and I can't for the life of me see how it can help with checking RCD tests (which are probably the two 'most important' types of test).

Kind Regards, John

 

[Taylortwocities]

Loop measurements I run on a non-RCD protected socket. Loop tests on this are ±0.02Ω month to month.
I keep a separate RCD in my test bag for cal checking of the meter and the measured trip times at 0° and 180° are exactly the same both at IΔn and 5xIΔn every time.
That's good enough for me.

 

[ban-all-sheds]

In terms of loop impedance, AFAICS, the only thing one could do would be to insert small resistances (from the box) in series with the fault loop path and see if the loop impedance indicated by you meter increased by the same amount

A socket with a switchable 1Ω resistor in the E connection to check your loop tester.

 

[ban-all-sheds]

I keep a separate RCD in my test bag for cal checking of the meter

Optionally, an RCD.

 

[JohnW2]

Loop measurements I run on a non-RCD protected socket. Loop tests on this are ±0.02Ω month to month. ... I keep a separate RCD in my test bag for cal checking of the meter and the measured trip times at 0° and 180° are exactly the same both at IΔn and 5xIΔn every time. ... That's good enough for me.

Fair enough and, yes, that probably more than reassuring enough. However, it doesn't explain how the resistance box the OP found can check RCD testing, or properly check loop impedance measurements (as claimed).

Serial tests on a 'known circuit' and 'known RCD' obviously assume that the circuit or RCD never change. If, as in your case, you are seeing very consistent results, then that's fair enough - since it is vanishingly improbable that changes in the circuit/RCD and in your meter would 'cancel out'. However, if you saw changes in the results, you would then obviously have to 'investigate further' (rather than assume that your meter had 'drifted'), since it could have been the circuit/RCD that changed.

In passing, I'm a little intrigued by some of the 'highly repeatable' results I see mentioned here. You say you get loop impedances ±0.02Ω month to month, and identical RCD trip times (again, I assume, month-to-month) - but I would not usually see that degree of consistency/ repeatability from minute to minute! With loop (or 'continuity') measurements, it's the connection between probes and circuits which is the weak point - at least in my hands, just 'fiddling' with that connection (or 'removing and re-connecting) can often result in a change appreciably greater than 0.02Ω !

Kind Regards, John

 

[JohnW2]

In terms of loop impedance, AFAICS, the only thing one could do would be to insert small resistances (from the box) in series with the fault loop path and see if the loop impedance indicated by you meter increased by the same amount

A socket with a switchable 1Ω resistor in the E connection to check your loop tester.

Indeed - but, as I said in the rest of what I wrote (but which you didn't quote) "...- but that's not quite a full test of the functionality.". It is far from beyond the realm of possibilities that the slope of the true/indicated result curve (as tested by the 'insert a resistance' method) could remain correct, but with the absolute results being displayed being incorrect.

Kind Regards, John

 

[ban-all-sheds]

The principle is that the delta between Rextra in & out remains constant. As you start with a calibrated instrument, it matters not if your 1Ω, for example, resistor shows as a 0.8Ω or 1.2Ω change to the loop resistance you measure, as long as it is always 0.8Ω or 1.2Ω.

OOI, why did you suggest inserting a series of different values?

 

[JohnW2]

The principle is that the delta between Rextra in & out remains constant. As you start with a calibrated instrument, it matters not if your 1Ω, for example, resistor shows as a 0.8Ω or 1.2Ω change to the loop resistance you measure, as long as it is always 0.8Ω or 1.2Ω.

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.

OOI, why did you suggest inserting a series of different values?

Well, although I confess that I wrote in plural, I wasn't necessarily thinking of that - and I certainly only test with one added resistance myself. 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.

Kind Regards, John

 

[EFLImpudence]

For price comparison.

http://www.ebay.co.uk/itm/NEW-Calib...044?pt=LH_DefaultDomain_3&hash=item5678a120b4

 

[JohnW2]

For price comparison. http://www.ebay.co.uk/itm/NEW-Calib...044?pt=LH_DefaultDomain_3&hash=item5678a120b4

To be fair, that is a secondhand one, and the price currently displayed is the latest bid in an unfinished auction!

Kind Regards, John

 

[EFLImpudence]

OK.

http://www.ebay.co.uk/itm/Calibrati...t=LH_DefaultDomain_3&var=&hash=item53fafb60cf

 

[JohnW2]

OK.
EFLI's one here

Fair enough - but the price range is very wide - look at THIS one

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

Kind Regards, John

 

[EFLImpudence]

OK.
EFLI's one here

Fair enough - but the price range is very wide - look at THIS one

Which one would you buy?

... 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"

 

[ban-all-sheds]

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

 

[ban-all-sheds]

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.

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.