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Topic Title: how does the insulation resistance test work?
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Created On: 01 February 2008 08:58 PM
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 01 February 2008 08:58 PM
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lsuk

Posts: 59
Joined: 01 May 2007

Trying to understand how the insulation resistance test works

this is how I think it works, i maybe right or maybe wrong?????????

bascially, your short circuiting neutrel and phase.

Sending 500v down the cable, also sending a constant small current down.

So if the voltage leaks, current staying the same. an ohms reading will alter?

is this right?

regards luke
 01 February 2008 10:24 PM
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rocknroll

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bascially, your short circuiting neutrel and phase.


Not quite, you are measuring the capacitive leakage between conductors, as you should know the insulation acts as a dielectric.

The insulation tester applies a known voltage (DC) to the circuit under test, the meter then measures the leakage current through a known shunt resistor and calculates the result
R = V/I.

The actual measurement is Meg Ohm Micro Farad, but Meg Ohm for short.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 03 February 2008 at 01:19 AM by rocknroll
 01 February 2008 11:20 PM
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lsuk

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I'm beginning to understand more, but when testing I.E a ring circuit between live conductors, at the tails you connect one lead to live and the other to neutrel. How does it send voltage and current down the cable if theres no closed circuit.
 01 February 2008 11:21 PM
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intrinsic4225B

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Originally posted by: rocknroll

Not quite, you are measuring the insulation capacitive leakage between conductors, as you should know the insulation acts as a dielectric.


How does one measure the capacitive leakage current with a DC test voltage?
 01 February 2008 11:22 PM
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lsuk

Posts: 59
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sorry, not a ring circuit, I mean a radial.

Trying to picture a test on a 3 metre twin and earth cable.

you test at one end, the other end is open. how does it form a complete circuit, it order to retrieve the return current?

regards luke
 01 February 2008 11:44 PM
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intrinsic4225B

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The current flows via the insulation - although this may sound like a rather odd statement, insulators are not perfect - they will always pass a small leakage current.

If you take a healthy cable of say 100MΩ insulation resistance between conductors and apply a test voltage of 500V DC the current flowing between the conductors can be calculated from Ohms Law:

V = I/R

I = V/R

I = 500V/100 000 000 Ω

I = 0.000 005 A

As you can see, this is a tiny amount of current - 5µA - five millionths of an ampere! In the example above, 100MΩ has been used as an example value of insulation resistance - for many modern polymeric cables in good condition this is quite a pessimistic value and real values could be much higher, leading to even smaller leakage currents through the insulation - perhaps even nanoamperes - billionths of an ampere!

Edited: 01 February 2008 at 11:51 PM by intrinsic4225B
 01 February 2008 11:57 PM
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iansettle

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Regulation 713-04-03 (16th edition) Measurement shall be carried out with direct current.The testing apparatus shall be capable of suppying the test volage indicated in Table 71A when loaded with 1mA. This is how the minimum values are given.
 02 February 2008 12:13 AM
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mikejumper

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Originally posted by: lsuk

How does it send voltage and current down the cable if theres no closed circuit.


It might be easier to think of it as applying
a voltage across two separate conductors rather
than putting a current down a circuit.

If you take a light bulb out of a pendant and
turned the switch on you'll have 230v between
live and neutral but no current.

When you apply 500v across live and neutral you
would not expect any significant amount of current
to flow unless there is a breakdown of insulation
between the conductors making a closed circuit.

Best I can offer at this time of night.
 02 February 2008 12:21 AM
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lsuk

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oh right, does that mean its testing whether the voltage current is leaking into the other conductor, transfering from live to neutrel at some point ?
 02 February 2008 12:23 AM
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lsuk

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so the current is jumping from one conductor to the other via the insulation? leaking current into one another
 02 February 2008 12:23 AM
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rocknroll

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Insulation resistance:

The ratio of the DC voltage applied to the terminals of a capacitor and the resultant leakage current flowing through the dielectric and over its surface after the initial charging current has ceased expressed in megohms or as time constant megohm x microfarads.

Leakage current:

Measure of the stray direct current flowing through a capacitor after DC voltage is impressed on it.

NB: when two or more conductors are together in a cable they act as a capacitor, due to this capacitive effect a leakage current flows though the conductor insulation, just a warning if the conductors are seperated by some distance 25mm+ or testing one cable in one T&E and another in another T&E or even singles your wasting your time doing an insulation resistance test, they need to be close coupled

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 02 February 2008 at 12:59 AM by rocknroll
 02 February 2008 12:57 AM
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intrinsic4225B

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Both the examples given above refer to the resistive leakage current via the imperfect insulation material of the dielectric - it is not capacitative leakage such as occurs in power supply filters on AC supplies etc (although these may also have a component of resistive leakage).

I don't follow this (MΩ x µF) expression in relation to the units of insulation resistance at all!

Two conductors separated by an insulator do indeed create a capacitor. However, if the insulation resistance is measured using a DC voltage then once the capacitance of the cable has charged to the same voltage as the test voltage the flow of current due to capacitative effects ends and any remaining current flow is due only to the resistive leakage through the insulation.

This is why insulation measurements on long cables are often seen to rise as the application of the test voltage is continued beyond the intial application as the capacitative element of current flow decays. When a steady value of insulation resistance is obtained, it is due to the resistive element of current flow only.

If insulation resistance were to be tested with an AC test voltage, then the value would be lower as the cable would appear to pass a capacitative current in addition the the resistive current. The measured value of insulation resistance obtained would also be dependent on the value of capacitor formed by the cable and the frequency of the test voltage.

Edited: 02 February 2008 at 01:55 AM by intrinsic4225B
 02 February 2008 02:44 AM
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dg66

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one thing to get right ,there is no such thing as an insulator ,everything is conductive what we term as insulators are just crap conductors,ie ceramic,pass enough voltage(electrical pressure) and it will conduct.

-------------------------
Regards

Dave(not Cockburn)
 02 February 2008 09:43 AM
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oldgit

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Originally posted by: lsuk

oh right, does that mean its testing whether the voltage current is leaking into the other conductor, transfering from live to neutrel at some point ?



Yes, its testing exactly what it says; that there is sufficient insulation between the conductors. That is why you do not link them when testing.

Capacitive effects aside, what matters in practice is that you do not, for instance, have a short circuit between phase and neutral.

Mike
 02 February 2008 02:57 PM
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potential

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Originally posted by: intrinsic4225B
I don't follow this (MΩ x µF) expression in relation to the units of insulation resistance at all!


Neither do I. Never heard of it.

Surely the whole point of insulation testing with dc is to wait until any capacitive effect has disappeared after 60 seconds or so?
Resistance testing is not to measure capacitance, you are supposed to wait until capacitance no longer effects the reading.
If you need to measure capacitance you use a capacitance tester that uses ac.
 02 February 2008 03:46 PM
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tattyinengland

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farrago - I'm afraid I agree with DG66 - his explination is good. It may be that over this side of the pond we use different terminology so causing some confusion.
 02 February 2008 03:57 PM
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tattyinengland

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Farrago -I think what DG66 is trying to say is...If you had a thin sheet of any insulating material with two conductors of differing potential on either side of it, even an insulating material such as ceramic, if the difference in potential between conductors is large enough curent will pass through that insulating material from one conductor to the other. Not so? What the insulation resistance test does is test how good this insulating material between these two conductors of differing potential is. In low voltage systems over here there is little point in testing the insulations resistance beyond 200M Ohm.

Are you from a high Voltage background? This maybe where the confusion lies? No offence ment and nor do I presume to lecture. Cheers.

Edited: 02 February 2008 at 03:58 PM by tattyinengland
 02 February 2008 04:04 PM
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rocknroll

Posts: 9677
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Basic electrical theory really.

Two conductors seperated by a dielectric/insulation is a capacitor, DC voltage charges up the conductors the positive conductor deficient of electrons and the negative has an excess of electrons, in 5 time constants at 99.2% of the applied voltage no current will flow unless there is a leakage.

Insulation resistance is based on the formula t = RC

Some study of time constant RC circuits would be an advantage to you.

Have not got any more room, need some space for my post-nominals

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 02 February 2008 at 06:21 PM by rocknroll
 02 February 2008 04:37 PM
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intrinsic4225B

Posts: 1724
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Originally posted by: rocknroll

Two conductors seperated by a dielectric/insulation is a capacitor, DC voltage charges up one conductor with positive electrons and the other is charged up with negative electrons, in 5 time constants at 99.2% of the applied voltage no current will flow unless there is a leakage.


This is agreed - and any current flowing after the capacitance charges is resistive leakage current flowing via the imperfect insulator between the conductors.


Insulation resistance is based on the formula t = RC


I don't understand where you are going with this one - the above describes the charge of discharge time of a capacitor - the R in it being the resistance of the circuit through which the capacitor is charged of discharged, not the resistance of the dielectric.


Some study of time constant RC circuits would be an advantage to you.


A subject with which I am familiar - but I can't see how it can be used to determine the insulation resistance. I would be most interested to see a worked example (using arbitrary values of capacitance etc) of how the insulation resistance can be calculated using the formula for the time constant of an RC circuit.
 02 February 2008 04:51 PM
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intrinsic4225B

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Originally posted by: farrago

dg66 what the ***** are you talking about? It will jump to ground around insulative materials not conduct or pass through it. That's why it's called TRACKING!


No insulation material is perfect - a small leakage current will always pass. As shown in the worked example above, this current may be miniscule - in the order of millionths or billionths of an amprere.

Increasing leakage with increasing voltage will occur until a potential gradient which exceeds the dielectric strength of the insulation (kV/mm) is reached, at which the dielectic breakdown occurs and the insulation fails.

Tracking is normally the flow of current over the outer surfaces of an insulator - a ceramic bushing on outdoor switchgear for example. This may lead to the formation of carbonised tracks on the surface of the insulator from which the name tracking is derived.

I understand this is why insulators for high voltage systems have convoluted shapes to give a longer surface path to help prevent tracking and why it is also important to keep such insulators clean.
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