Minimum Insulation Resistance

it depends on your customers requirements in the SOW. for example, typical is a

megger test at 1 or 2 MΩ

Yes.

Yes. Depending on the signal and current levels of your instrumentation it can be surprisingly high. I worked on a low voltage system once that 10^9 ohms of insulation leakage current was too small.

WOW! ive never seen a megger that could read 10^9 Ω. the megger would put out, for ex., 1 KV and measure an current below 10^(-6) amps ??? that must be one expensive megger.

I wasn't using a megger. It was the custom multiple channel instrumentation circuitry we were using. It was probably much more expensive than any megger. I still have the one giga-ohm load resistor we used to verify testing.

Then you haven't looked at the state of the art in megohmeters lately. There are relatively inexpensive meters that measure up to GΩ and even TΩ at 1000V, 5kV, and up to 10kV. Many of them run off a handful of standard batteries - our 5kV meggers use 12 AA cells. Check these out for example:

http://www.aemc.com/products/index.asp

Megger® must have similar offerings as well, and other manufacturers. They've come a long way from the days of the old hand-crank 500V megger.

That was about twelve years ago that I used a 1G test load for quick testing. I was also limited to 100V due to ionization break down for all of my tests. Besides the OP is asking about low voltage insulation measurements not measurements at 10kV.

_{Oops, you were replying to Kapy not me.}

Insulation resistance depends on material, on temperature, on ambient medium, on length and.. so on. For thermoplastic insulation 600 V power and control cable see UL 83.For thermoset- insulation see UL 44.In IEC World cables see IEC 60502-1 standard [it has to be calculated using volume resistivity of the insulation and conductor diameter and insulation thickness].

Vide the Indian Electricity Rules, 1956, Rule No. 48 (i): The minimum insulation resistance for systems/equipments below 650V, when tested with a test voltage of 500V DC, should be one mega ohm.

That will probably work fine in India for most applications. The OP has not told us where the circuitry lies or the application. So it is still anybodies guess.

It depends upon the equipment.

Thumb rule in maintenance industry is designed voltage +1 Mega ohm.

Minimum design voltage of LV system or equipment is 1kV. Hence desirable value is 2 Mega Ohm (measured with 500V DC megger)

As said before it depends upon equipment. For examples:

1. If it is long length of cable, you may find insulation value between phases or phase to earth a few Kilo Ohms. In such cases after testing IR, to make sure that insulation is intact - immediately after disconnecting megger we check the charge voltage and rate of decay. If it shows almost full megger voltage and discharges very slowly (may take up to few minutes, depending upon length) - it is considered that insulation is good and cable can be charged.

2. Certain equipment, for example rotor of magnetic field of a medium or large size turbo generator or even salient pole machine is tested very carefully for IR value.

I have been always using a megger with 50V, 100V, 250V and 500V multiple range and have been recommending to my colleagues.

To avoid damage - or causing a carbon track, always in case of rotor of a used generator, first I check with multi meter between terminal and earth - if value is found in few mega ohms, next I apply 50V DC, If value is same as of multi meter increase to 100V and never above 250V DC.

If IR value of multi meter is doubtful - dry out and check again after drying.

If IR drops at any step to half or lower of previous step stop applying further voltage - clean, dry out, varnish and then test again. In most machines even test at 100V DC most of time is OK as working voltage of field seldom exceeds this value.

Similarly, we take IR before and after High Pot test. We all know that IR after high pot shall not drop below before the test - specially with DC high pot. (we ground +ve of test set or the megger).

Reaon for grounding Positive of Megger or DC Test is that Conductor being at negative potential with respect to earth - any moisture in insulation during testing will travel towards the conductor (osmosis effect), resulting in lowering its IR value. AND GIVING US SIGNAL THAT INSULATION MUST BE TREATED BEFORE TAKING IN SERVICE.

Hence if there is little reduction in IR after Hi pot test - probably there are conductive impurities (like moisture). Winding requires good cleaning and dry out and treatment with impervious resin to protect moisture absorption.

If there is considerable reduction of IR after Hi Pot test - insulation is week and probably damaged during hi pot - requires careful investigation.

The basic reason to carry out Polarising Index measurement of Insulation before subjecting it to Hi Voltage, is to avoid any damage during high voltage test due to moisture or otherwise week insulation.

The method I was taught as an M&Q apprentice was the total leakage current should not exceed 1/10,000^th of the FLC.

1MΏ on an old large machine is often impossible, I would have been changing motors every five minutes had we stuck to a simple "one size fits all" value. The plants were mainly cold and damp so a motors insulation value would fall during shutdowns. A few hours running the IR value would back up again.

Re Megers reading high values. My 5000V model will read in tetra Ώ, on 1000V you're stuck with just gigΏ

PS it hurts when it bites you!