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HappyHippyDad

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This is one of those slightly embarrassing questions that I feel I should know the answer to. I can feel the answer lurking under my skin but it's just not surfacing and needs a push!

Pre-work visit for a CU change. There was 65K Ω resistance between N-E on the continuity setting, however when tested on the IR setting at 500v it was 1.1M Ω.

They are both measuring the resistance between N-E so why the big difference? With the IR test you are actually stressing the insulation so I would have thought if anything the reading would be lower!!?
 
Don’t understand why the different reading,but why would you use 2 different methods,surely continuity require a low resistance test,IR requires a high voltage test
The IR is the one to note.
 
Don’t understand why the different reading,but why would you use 2 different methods,surely continuity require a low resistance test,IR requires a high voltage test
The IR is the one to note.
Thanks for the reply Cliff. The reason why I did a continuity test as well is just purely down to enjoying playing around with the new Megger and just generally trying different things.... hence getting these results which you have been unable to explain ;)
 
Hi - only value I can add is my tester only goes up to 1999 Ohms on continuity (clearly not a fancy new Megger ...). But then I wouldn't have questioned the IR reading of 1.1 MOhms :) .
 
Hi - only value I can add is my tester only goes to 1999 Ohms on continuity. And then I wouldn't have questioned the IR reading of 1.1MOhms :) .
Nice one! Yeah, perhaps I shouldn't have continuity tested it then I wouldn't be questioning it :)
 
Don’t understand why the different reading,but why would you use 2 different methods,surely continuity require a low resistance test,IR requires a high voltage test
The IR is the one to note.
What he said^^^^^^^^^^^^^^^^^^^^^^^^^
 
Actually come to think of it, I have come across this a few times!

I remember I couldn't my MFT and I needed to do an IR test. I had my fluke multimeter that measured continuity up to 40M Ω so I tried that. I then found the MFT and did the IR tests which were very different to the continuity results on the MFT?
 
As you say at 500V the insulation resistance should be lower than on the continuity test if the insulation is failing anywhere, the higher voltage should be able to bridge gaps that the lower test voltage of continuity cannot.
If there were a capacitor (or significant capacitance) in circuit then I could see the capacitor being charged by the 500V and then stopping the current flow past it so limiting the testing to before the capacitor and possibly lowering the reading, whereas the continuity test may just be measuring the charging capacitor current and so giving a lower reading.
I believe that insulation resistance tests apply a pulse of voltage multiple times to gain a reading, however a continuity test should just apply a constant low voltage and this may cause electronics to respond differently.
Even a difference in accuracy should not be able to generate such a variance.
Just a few off the cuff thoughts.
 
Wot did you get on the reverse polarity continuity test? ;)
...
This is heading in right direction -!
..Strangely consider an DC/AC volts test for a small AC/DC offset as it will mess with DC Ohms continuity -MORE-
Than IR where any residual voltage (Common with Stored energy capacitive effects).. Residual is greatly swamped out by high test voltage !
---Now removing nerd hat--- (If that was ever of interest)
 
I am glad you had the wit to observe and question this fascinating anomaly - much lower insulation resistance at low voltage than high voltage - so that we could ponder on it - thank you!

My theory centres on the nature of an insulator and the few free or loosely bound electrons within its molecular structure (as compared to a conductor or semi-conductor). First at low voltage using the Megger on its continuity/resistance setting the applied emf from the internal battery (-I assume to be say 9V at most but whatever it is a low voltage direct current) will establish a very weak radial electric field between say L and E in L's insulation. Before the electric field is established, the electrons in the insulation have thermal kinetic energy and are moving randomly in all directions so there is no net flow of current nor any regions of higher or lower concentrations of electrons. Under the influence of the weak electric field the electrons experience a force Fe = Eq which influences them to move in the radial direction of the electric field. But the electric field is weak, there are not many free electrons and their thermal kinetic energy can make them some unruly and overcome the influence of the electric field. But enough electrons drift steadily radially to create a current flow between L and E. Not a large current per mm length of conductor but over a long length the current flow adds up. A reading of 65kOhms is observed.

When you switch the Megger to IR mode a very much larger dc voltage (250V or 500V) is applied across the L conductor's insulation. The radial electric field is many magnitudes stronger and the force Fe = Eq dominates the thermal kinetic energy motion so much that nearly all the free electrons are pulled towards the conductor of the L-E pair which is positive - let us say it is the E which is positive. All the free electrons are on the surface of the L's insulation, there are very few within the insulator and not many flowing into the insulation from the negative lead connected to L. So now within the insulation bounded by the inner and outer surfaces of the insulation there are (relatively to the low voltage case earlier) extremely few free electrons left to form a radial electron current, and what little current there is per mm of conductor does not add up to much even over a long length of cable. The result is that the measured insulation resistance = applied emf/radial current - is the high MegOhms value you mentioned in your post. The insulation has become polarised which did not happen for the low voltage continuity measurement.

Different reading when using continuity and IR? {filename} | ElectriciansForums.net
 
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Interesting theory M :)
My concern with the 65k is it's implied current flow. I struggle with seeing more e flowing with less voltage, unless we have a rectification effect perhaps (how I do not know ...). Were the meter leads reversed between the readings?
 
Polarization:

Polarization - http://www.physicsclassroom.com/class/estatics/Lesson-1/Polarization

A thought experiment:

Let's imagine there is a river and folk can travel from only one side to the other by canvas sail boat ferries. How they return is not relevant. The prevailing wind is North bank to South Bank. There are hundreds of ferries jostling and loitering for fares in the river. A ferry can pick up someone on the North bank and then sail to the other bank, disembark the passenger and then return to the floating pool of ferries. For 'usual' wind strengths ferries can continue with this routine and folk who arrive will be able to board a ferry and cross all day long. Thus there is a flow of people North Bank to South Bank.

Now imagine the wind is much much stronger. All the ferry boats will be blown to the South Bank, none (or a much much smaller number) will be in the pool and so folk arriving at the North Bank have to queue some time before a ferry manages to come alongside the North Bank, they can step into it and be sailed to the South Bank. Even then that ferry may now be unable to leave the South Bank. The net result is a much much reduced rate at which passengers cross from one side to the other, all because of a large increase in wind strength.

For people read 'electric charge'

for ferry read 'charge carrier'

for wind read 'electric field'

for river read 'insulator'

for banks read 'L and E conductors'

for jostling and loitering read ' random thermal motion of charge carriers'.

I accept there is no analogue for the skipper of the boat and his sailing skill. Like all analogies they have often have flaws but maybe this helps coax the grey cells.

To be clear - it is a theory I put forward earlier - I am not saying it is right.
 
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I agree the results are surprising but we are missing an essential piece of information: Did the reading return to 65kΩ when tested with the DMM *after* the 500V IR test, i.e. were the two tested values repeatable under their respective test conditions?

There are a couple of possibilities. A continuity test typically applies a few volts maximum, during which the power dissipated as heat into 65kΩ is negligible, say 1/65k = 15 microwatts. The power dissipated when subjected to 500V is 25,000 times greater, at 3.8W. In fact most IR testers won't output this much power; the voltage drops off when the resistance is this low, but there is enough to drive off a thin film of moisture at the exposed ends of cables and terminations. This can cause a moderate IR reading to rise under test and not immediately fall again afterwards.

Another mechanism something akin to what Marconi describes is called dielectric absorption, which also tends to cause varying resistance readings. When insulation is subjected to high DC voltage, some molecules in the material tend to align so that their electrostatic dipole lies parallel to the electric field created by the voltage. When the voltage is removed, these molecules don't all instantly return to their original positions, but as they jiggle back into random orientations, their stored energy creates a voltage across the insulation that can sometimes be read with a DMM, and which can cause incorrect readings on a resistance test. The effect isn't usually noticeable in low voltage electrical work, but in both high-voltage transmission and precision analogue electronics dielectric absorption is a daily fact of life that has to be worked around or compensated for.

Finally, a British Standard Woodlouse standing across two terminals measures 65.3kΩ. They can run quite fast when chased by 500V.

HHD: Can you add any info, such as relative polarity of the tests, type and age of fittings and cable etc. 65kΩ is low and its cause ought to be identifiable, regardless of the anomaly you describe. Have you checked the calibration of the MFT continuity ranges at all in case that is a gross measurement error?
 
I believe I will be going back to carry out the work. I'll be more thorough with my testing then and will reply further to the thread.

Many thanks to the replies offering some interesting information.
 

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