Topics: Insulating resistance calculate in open circuit
on General Discussion
Insulating resistance calculate in open circuit
For open circuit current is zero. so how cum we get resistance value, when i=0. please explain?
08-19-2013 09:51 PM
An insulation resistance value would be expected and desirable on conductors in an electrical distribution system. When measuring the insulation resistance of conductors phase-to-phase and phase-to-ground you would want to see values in megohms-gigohms depending on system design voltage.
08-19-2013 09:52 PM
it is suggested you may refer any basic book on measuring instruments to recollect what you must have learn but seems to be forgotten. Now. About. Functioning of insulation tester. You may raise further doubt to your seniors in commissioning team.
08-19-2013 09:55 PM
If the voltage applied is adequate, you are measuring the capacitive leakage resistance of cable insulation...
08-19-2013 09:56 PM
To my opinion Mr. Robert comment is the ideal full answer to this query. But i would like to add that the Insulation resistance is actually measuring the leakage current which known also as " Meggering" test. Such test is performed to ensure that the insulation of conductors , accessories and equipment is electrically sound.
However, insulation resistance is the resistance between conductors that made up of countless millions of resistances in parallel along the conductor.Therefore is expected to be low for long cables and if conducted as whole for installation.
Moreover, the minimum insulation resistance as per IEE / BS7671 as follows:
a)- SELV / PELV > 0.25 M Ohms while the required Test voltage is 250 V DC.
b)- LV (,500V) > 0.5 M Ohms while the required Test voltage is 500 V DC
c) - Over 500V > 1 M Ohms while the required Test voltage is 1000 V DC
08-19-2013 09:57 PM
I need to say that I agree in completely with sir Robert and sir Osman except I would like to add that insulation resistance inversely depends of temperature which means that insulation resistance increases with decreases of temperature, so when we measure insulation resistance by using megaohmmeter (megger) we need to measure it in cool state (under room temperature).
08-19-2013 09:59 PM
Yes dear Bojar,
I do agree with you with reservation.
However, not only the temperature has effect on the measured value as far as the insulation resistance measurement is concerned such as how long the cable under test " very long cables give low resistance that may be less than prescribed limit or in case you conducted the test for whole electrical installation where it may be composed of a couple parallel circuits-branch circuits), care is required by the Engineer in such cases.
Moreover, I may record that "Temperature" is the most external factor "measurand" that has an affect on measurement or any production process without any exception or reservation, so it is not required to be ignored and all necessary correction to be applied where applicable. It is a Vital & Serious metrological value / factor.
Although I agree with the general idea of your statement as far as the insulation resistance measurement is concerned, I believe that the insulation resistance is directly propotion to the tmperature which means that insulation resistance increases with increases of temperature since we have agreed that the insulation resistance is actually the measurement of leak currents which decreases within a reasonable high temerature ( R = V/i) . As you know the conductors resistance is increased with increase in temerature and at the same time the conductors conistitute and integral part of the insulation resistance which resisting the leakage current and on the otherhand the insulation material become more reliable and expand where any cracks will closed and thicness increase.
Note: High temperatue = Tempertaure under (not reach) the critical point on which the material commence deterioration.
08-19-2013 10:00 PM
I need to say that I disagree with your opinion in aspect of changing of insulation resistance in depends of temperature what I will confirm with next arguments:
In my previous company, where I worked one year as electrical design engineer and as test engineer, I came to some conclusions in aspect of relation between insulation resistance and temperature. When owner brings his own electrical machine into our company, after dismantling, all parts of electrical machine we send on washing and, after that, we send them into drier where, in according to appropriate standards, all parts of electrical machine dried under 150°C during appropriate time. After their drying, all parts of electrical machine need to cool up thus their temperature need to reach appropriate air temperature, so as it could be possible to measure insulation resistance and to determine state of insulation of electrical machine (is there some connections between windings and shield of electrical machine or is there some connections between windings which are separated at connection board). Otherwise, if we measured insulation resistance before cooling of all parts of electrical machine, then we can't measure true insulation resistance and we can't determine is everything all right in aspect of insulation of electrical machine because of influence of temperature. I'm not sure what looks like mathematics' expressions which describe relation between insulation resistance and temperature, but I'm pretty sure that relation between insulation resistance and temperature is inversely proportional.
But, when we are talking about inside resistance of windings and its relation with temperature, then I need to say that I agree in completely with your opinion, so my opinion is that inside resistance increase with increase of temperature.
08-19-2013 10:02 PM
Dear Eng. Bojar
Thank you for your response.
I do agree with you that any measurement (not limited only to "IR") taking place in conditions deviate from the design parameters and / or operated conditions,, adequate correction to be applied where applicable.
Yes, the Temperature and humidity can affect the readings of the insulation resistance, the measurements should be preferably be made above the dew point at standard temp. about 20 °C.
I understand that you did not exercise the measurement of the insulation resistance for the two cases by your self and no records to substantiate your conclusion.
Nevertheless, may I remind you that Electrical equipment (transformers, cables, motors, etc.) are recommended to be in hot state (of course within the limit - avoiding heat stress) in order to avoid condensation or to ensure dryness and a good insulation resistance accordingly. As far Electrical M/c or Motors are concerned, as in your case, Polarization Index Test (PI) and/or dielectric discharge Test is the most type of insulation Test recommended, PI ratio PI for 10minutes to PI for 1minute is the measurement of the insulation quality e.g. If the ratio PI.10/PI.1 was less than 1 the insulation condition would consider dangerous while it will be excellent if ratio is greater than 4. In light of the value of the obtainable PI ratio, a decision of whether the dryness process is required or not, can be taken.
IEEE recommends the following insulation resistance formula For such M/Cs:
Rm = Kt(kV+1) where
Kt = Insulation Resistance Temperature Coefficient at winding temperature , the value of it has a linearly relation with Temperature, approximately Kt = 1.5t-10 , t is temp. in kelvin. Therefore if Temp =0, Kt has negative value of -10 and it is clear that Kt increase with increase in temperature.
kV = Rated M/c line voltage.
Furthermore, responding to Mr. william question which adressing the basing question that R= Infinitive if I =0.
08-19-2013 10:08 PM
Hi, sir Osman,
I am very grateful to you because of your explanation of manner of measuring insulation resistance, but I can tell you that I have measured only insulation resistance of all parts of electrical machines by using megaohmmeter (megger) during one minute and I have tested all parts of electrical machines by using appropriate AC high voltage power sources during one minute, so I can't discuss with you about polarization index because I don't have appropriate data about polarization index. Measurement of insulation resistance is performed in no-voltage conditions. In any case, thank you a lot for suggestion which is very useful in improving of my knowledge.