Low IR with Multimeter.

[EFLImpudence]

Ah, I think I have misled you; it was the storage heater element that kept increasing.

However, another query:
When IR-ing the oven element the 500V and 1000V tests stop at 320V, beep and give a reading today of 0.21MΩ

From the manual:
If insulation resistance is bad, the test voltage is
automatically reduced to limit the test current to
safe ranges.

I.e. 1.5mA

 

[JohnW2]

Ah, I think I have misled you; it was the storage heater element that kept increasing.

Fair enough.

However, another query:
When IR-ing the oven element the 500V and 1000V tests stop at 320V, beep and give a reading today of 0.21MΩ
From the manual: If insulation resistance is bad, the test voltage is automatically reduced to limit the test current to safe ranges. I.e. 1.5mA

OK - so the 'limit' is 1.5 mA, and not 1 mA as I suggested. I've just done a series of tests with a range of resistors right down to 10 kΩ. Regardless of whether I set the meter to 250V, 500V or 100V, it gives the correct resistance reading but stops at the voltage which corresponds to ~1.5 mA through the resistance (all in the range 1.5 mA to 1.6 mA).

But you said it was a 'query' - what is the question?

Kind Regards, John

 

[EFLImpudence]

Bearing in mind the manual stated that the test was at 1mA and, because of poor IR in this case, the test voltage was reduced to limit the test current -

how does it rise to 1.5mA?

Does it increase the voltage? - cannot if limited to 320V or

reduce the internal resistance while carrying out the test?

 

[JohnW2]

Bearing in mind the manual stated that the test was at 1mA ...

Not really - as I said, the manual was really only telling you about Ohm's law - namely that the test current would be 1mA with an IR of 0.25MΩ with a 250V test voltage, with an IR of 0.5MΩ with a 500V test voltage and with an IR of 1MΩ with a test voltage of 1000V. If the IR were above or below those 'example' figures, the current would be different from 1mA (just per Ohm's law) - although we now know that, in the 'downward IR' direction, it will only allow current to rise to 1.5mA, decreasing test voltage if necessary in order to stay below that current limit.

... and, because of poor IR in this case, the test voltage was reduced to limit the test current - how does it rise to 1.5mA?

As above, as the IR falls below the "Ohms Law example" in the manual, current will increase above the (1mA) 'example' figure (with the voltage remaining at the full test voltage) until it gets to the 1.5mA limit. If R falls further, voltage is 'reduced' so as to achieve that 'limit value' of 1.5mA.

Does it increase the voltage? - cannot if limited to 320V or ... reduce the internal resistance while carrying out the test?

Again, as above, it does not have to 'do anything' to get the current to rise from 1mA to 1.5mA - Mr Ohm will do that automatically as IR reduces.

We (and the manual) talk about 'reducing the voltage' if the specified test voltage (250V, 500V or 1000V) would result in more than 1.5mA flowing. However, in practice, it can't ever even 'try' the full voltage initially - since the IR (which it doesn't yet know) could be low enough to result in a (potentially damaging - particularly to meter) current much greater than 1.5mA. I therefore can but presume that it undertakes what you would probably call a 'ramp test', gradually increasing voltage from zero until EITHER the specified test voltage (250V, 500V or 1000V) has been reached OR the current has risen to 1.5mA. That's presumably why it takes a second or three to do an IR test - straight measurement of resistance (as in multimeter) would be 'instantaneous'.

Does that help?

Kind Regards, John

 

[tommo2]

Never rely on low-voltage meters for IR values.
When testing IR you are measuring leakage current
Think of it like this:

You have a bicycle tube that you need to test for leakage. The normal pressure is 30psi. You immerse the tube in water and pump it to 1psi. You dont see any bubbles so there are no leaks right??? WRONG. You then pump it up to 30psi and see a few small bubbles. Then you give it 60 psi and see air ****ing out.

The reason for testing at higher voltages (ie500V) is that some leaks only appear at higher pressures. Think of a tap that was fine until the supply pressure was increased and then a small dripleak was spotted. Reduce the pressure againand the leak disappears. It does not mean that it is fixed.

 

[JohnW2]

Never rely on low-voltage meters for IR values.

Indeed - that's what most of this discussion (and the included illustrations) has been emphasising. The voltage used for testing is only really an issue if one is measuring something which is not a 'simple/straightforward resistance' - but the sort of causes of measurable IR in electrical installations are often not 'simple/straightforward resistance' - so you comment is clearly true in terms of that sort of measurement

When testing IR you are measuring leakage currentThink of it like this: ... You have a bicycle tube that you need to test for leakage. The normal pressure is 30psi. You immerse the tube in water and pump it to 1psi. You dont see any bubbles so there are no leaks right??? WRONG. You then pump it up to 30psi and see a few small bubbles. Then you give it 60 psi and see air p******g out.

You make it sound as if 'the problem' arises because what one actually measures is leak leakage current, which obviously varies with testing voltage, but that is not really a problem. With your analogy, if it were a rigid container with a small hole in it, then (ignoring some minor 'clever complications'), then the rate of flow through the hole would be directly proportional to the pressure within the container - so, provided only that one has a sufficiently accurate means of measuring flow (aka current) one will get exactly the same answer (for 'resistance of the hole') whether one uses 1psi, 30psi or 60psi.

Returning to electricity, if one has a 'simple straightforward resistance' (e.g. a carbon resistor), which is analogous to the 'rigid container with a hole', then (again, provided only that you can accurately measure the current) you will get exactly the same answer for the resistance whether you measure with 1000V, 100V, or 1V. ... anyone can confirm that for themselves - measure a 1MΩ resistor and they should get the same 1MΩ answer with their 1000V IR meter/MFT, a 9V (or 3V or 1.5V!) multimeter, or any testing voltage in between.

What your analogy helps to illustrate is that resistances may not be 'simple and straightforward'. So long as the pressure in it is high enough to more-or-less maintain it's normal inflated shape/size, then, as above, you should get exactly the same answer for 'resistance of the hole' more-or-less regardless of what pressure you use. However, at very low pressures, the tube will start to collapse, ultimately to the extent that flow of air out of the hole will (for a given pressure) be reduced, possibly even to zero - so the measurement of 'resistance of the hole' (by dividing pressure by flow through the hole) will no longer be a correct reflection of what the resistance would be at higher pressures. The measured resistance is not 'incorrect - it's correct for the pressure at which it was measured (since the resistance has actually been increased by the partial occlusion of the hole at low pressure), but not the same as it would be if measured at higher pressures.

This is analogous to the deviation from a linear voltage/current relationship (i.e. deviation from Ohm's Law) which can occur when one is talking about something more complicated that a 'simple resistance' - e.g. when electro-chemical processes are at work due to fluids, electrolytes or corrosion, or when materials (e.g. insulation) undergo voltage-related changes, such as (temporary or permanent) dielectric breakdown. However, again, the 'high' result one might get by measuring with a low test voltage is not 'incorrect' - it is merely lower than it would be in the face of higher voltages (which is what one usually wants to know about!).

The reason for testing at higher voltages (ie500V) is that some leaks only appear at higher pressures. Think of a tap that was fine until the supply pressure was increased and then a small dripleak was spotted. Reduce the pressure again and the leak disappears. It does not mean that it is fixed.

Indeed - that's another example of a 'non-simple' resistance - the washer may 'stick' to it's seating until a certain pressure is present, thereby again creating a non-linear pressure/flow relationship. However, if the leak/drip were through a pinhole in the pipe, we would be back to a more-or-less linear situation, such that the rate of flow would be more-or-less directly proportional to pressure - so that, again, 'resistance of the pinhole' calculated by dividing pressure by flow ought to be much the same for any pressure (within reason!).

However, all that aside, I think we are all agreed that one cannot trust 'IR' measurements of electrical circuits or components that are undertaken using test voltages below their normal (peak) operating voltages - hence, even 250V DC is not really high enough for circuits/components designed to work at 230V (RMS) AC ... hence most routine IR tests on 230V circuits/components being undertaken at 500V.

Kind Regards, John