Just got an IR tester, is off scale normal.

[plugwash]

I just got an insulation resistance tester to pre-check some hardware i'm developing (formal testing will be done elsewhere but we want to make sure it suceeds). Specifically I got http://cpc.farnell.com/1/1/91037-tenma-72-9400-tester-insulation-resistance.html

The tester is supposed to have a range of 2 megohm to 2000 megohm

Anyway having got the tester I tested a few things, stuff i'd built, power bricks (testing both P+N to E and P+N+E to output), various leads, the inputs of a couple of different multimeters.

The multimeter inputs unsurprisingly read around the 10 megohm mark. The multimeters also indicated that the voltate from the IR tester was slightly over nominal.

One old set of test leads gave readings ranging from about 100 megohm to off-scale depending on how I held them.

Everything else I tested gave an off-scale reading. In one sense off-scale is good but in another sense it leaves you worrying if you actually connected the tester properly. Do people find it is common/normal to get off-scale (e.g. greater than 2 gigohm) readings when IR testing stuff?

 

[deadshort]

Easy - create a fault and test it !

DS

 

[JohnW2]

Everything else I tested gave an off-scale reading. In one sense off-scale is good but in another sense it leaves you worrying if you actually connected the tester properly. Do people find it is common/normal to get off-scale (e.g. greater than 2 gigohm) readings when IR testing stuff?

Meters like my Fluke tend to only give answers that high when testing at 1000V. IIRC, mine with have a ceiling of ">200MΩ" for 250V tests and ">500MΩ" for 500V tests. However, that aside, to supplement what DS has said, what you really need is a few resistors (probably in the 0.1MΩ to 20MΩ range) with which to reassure yourself that the machine is giving reasonable answers.

Kind Regards, John

 

[plugwash]

Meters like my Fluke tend to only give answers that high when testing at 1000V. IIRC, mine with have a ceiling of ">200MΩ" for 250V tests and ">500MΩ" for 500V tests.

Do you find it common/normal to get those readings when testing stuff with it?

 

[JohnW2]

Meters like my Fluke tend to only give answers that high when testing at 1000V. IIRC, mine with have a ceiling of ">200MΩ" for 250V tests and ">500MΩ" for 500V tests.

Do you find it common/normal to get those readings when testing stuff with it?

It obviously depends on what one is testing. If it were, say, a brand new sockets circuit, with new cable and accessories, then there would probably be something very wrong if one did not get an 'off the scale' answer. With a long-standing existing circuit, complete with dust, moisture, insects etc. in the accessories, it is (even in my limited experience) less common, so one quite often gets 'finite' answers. In terms of tests on stuff that you've built, it shouldn't be that difficult to achieve near-infinite resistances (at DC). Don't forget that it would be very different if one tested with AC (which is why we don't do it), since filter capacitors etc. could then result in quite low impedance measurements.

Kind Regards, John

 

[Risteard]

If it were, say, a brand new sockets circuit, with new cable and accessories, then there would probably be something very wrong if one did not get an 'off the scale' answer.

Not really. It could just be indicative of long circuit length or resistances in parallel if testing other circuits at the same time (which is what should be done, given that the minimum insulation resistance is stated for the installation - or in the case of a complex installation at least the entire distribution board and distribution circuit).

Otherwise you would have to perform a calculation to demonstrate the effect of resistances in parallel to work out the actual insulation resistance of the installation.

 

[JohnW2]

If it were, say, a brand new sockets circuit, with new cable and accessories, then there would probably be something very wrong if one did not get an 'off the scale' answer.

Not really. It could just be indicative of long circuit length or resistances in parallel if testing other circuits at the same time ...

I thought it was clear that (for the purpose of illustration) I was talking about a brand new circuit (with brand new cable and accessories) being tested in isolation (hence no 'parallel resistances').

I would be worried, at least in a domestic environment, if there was less than an 'off the scale' IR because of 'long circuit length'. As a matter of obsession, I always IR test new reels of cable, and I have never (yet) come across a 100m reel of any T+E which does not give an 'off the scale' IR reading.

... (which is what should be done, given that the minimum insulation resistance is stated for the installation - or in the case of a complex installation at least the entire distribution board and distribution circuit). ... Otherwise you would have to perform a calculation to demonstrate the effect of resistances in parallel to work out the actual insulation resistance of the installation.

I'm not talking about 'requirements' but, rather about good practice and electrical common sense. If I had just installed a complete new circuit, one of the first things I would want to do would be to test that new circuit (including IR testing) in isolation - otherwise I wouldn't have a clue as to whether any 'bad readings' were the consequence of my new work/circuit or something else. Measuring IR of the whole installation, if required, can be done subsequently.

Kind Regards, John

 

[Risteard]

Measuring IR of the whole installation, if required, can be done subsequently.

Kind Regards, John

There is no reason to do it subsequently, because if the entire installation has an adequate insulation resistance then we know that the new wiring is of an even higher value than that measured for the installation.

 

[JohnW2]

Measuring IR of the whole installation, if required, can be done subsequently.

There is no reason to do it subsequently, because if the entire installation has an adequate insulation resistance then we know that the new wiring is of an even higher value than that measured for the installation.

That's true, but it's a gamble. If the IR of the entire installation is not satisfactory, then one has to 'subsequently' check each individual circuit (including the new one) to identify the culprit circuit.

The way I see it, all the other tests (continuity, R1+R2, polarity, Zs etc.) have to be undertaken on the new circuit in isolation, so why not measure the IR the new circuit at the same time? (it would only take a few moments whilst the circuit was disconnected at CU for other tests).

Kind Regards, John

 

[ultomoat]

I just got an insulation resistance tester to pre-check some hardware i'm developing (formal testing will be done elsewhere but we want to make sure it suceeds). Specifically I got http://cpc.farnell.com/1/1/91037-tenma-72-9400-tester-insulation-resistance.html

That is pretty cheap. Posters are continually complaining about DIYers not having the tools to test new installations properly. Does that tester do everything that is necessary to complete a minor works cert?

 

[JohnW2]

That is pretty cheap. Posters are continually complaining about DIYers not having the tools to test new installations properly. Does that tester do everything that is necessary to complete a minor works cert?

No, of course not - that's just a standalone IR tester, which will measure insulation resistance and nothing else. Other kit would be needed for the other required tests.

Kind Regards, jOHN

 

[PrenticeBoyofDerry]

Everything else I tested gave an off-scale reading. In one sense off-scale is good but in another sense it leaves you worrying if you actually connected the tester properly. Do people find it is common/normal to get off-scale (e.g. greater than 2 gigohm) readings when IR testing stuff?

I normally short the leads out before and after testing, to prove connections are sound.

 

[JohnW2]

I normally short the leads out before and after testing, to prove connections are sound.

Fair enough, but that it just testing 'the other extreme' - one has to do a bit more (or, of course, get it calibrated!) to reassure oneself that it is probably giving reasonable readings for real-world IRs.

KInd Regards, John

 

[ban-all-sheds]

No, of course not - that's just a standalone IR tester, which will measure insulation resistance and nothing else. Other kit would be needed for the other required tests.

And a quick trawl of CPC's site shows no matching, equally cheap, loop or RCD tester.

 

[plugwash]

(or, of course, get it calibrated!)

realistically for an instrument like this "calibration" probablly consists of little more than sticking a few resistors on it and checking the results are in-tolerance. It's not exactly a precision instrument in the first place, nor does it need to be.

The tester is only specced down to 3M (not 2M as I said in my original post). When I tried with a 1M resistor it gave a reading arround 1.8,beeped at me (it's supposed to beep below and showed a corresponding "buzzer" inducation on the display. A short circuit did much the same. Slightly more worrying was the 2M2 case, the beep and it's indicator on the display were still there and the decimal point were still there but a large number of digits on the display turned off leaving a display fairly similar to the over-range case (which shows a "1" on it's own with the decimal point and all the other digits turned off), which seems like a potentially confusing software bug, especially if you were using the meter in a noisy environment.

A 10M resistor and a 3M3 resistor read fine.

And a quick trawl of CPC's site shows no matching, equally cheap, loop or RCD tester.

Yeah, if installation testing is your goal then your cheapest option seems to be a multifunction tester, something like
http://www.tester.co.uk/seaward-powertest-1557-installation-tester?gclid=CNC4-NufrcYCFUTKtAodCUsCug at 358.80 (inc vat and delivery)