Megger Insulation Resistance Test

Good Insulation Resistance?

As you know good insulation has high resistance and poor insulation relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture).

With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative.

The Megger insulation tester is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms. For good insulation, the resistance usually reads in the megohm range.

The Megger insulation tester is essentially a high-range resistance meter (ohmmeter) with a built-in direct-current generator. This meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied.

This method is non-destructive; that is, it does not cause deterioration of the insulation.

The generator can be hand-cranked or line-operated to develop a high DC voltage which causes a small current through and over surfaces of the insulation being tested (Fig. 2). This current (usually at an applied voltage of 500 volts or more) is measured by the ohmmeter, which has an indicating scale.

Fig. 3 shows a typical scale, which reads increasing resistance values from left up to infinity, or a resistance too high to be measured.

What is “Good” Insulation?

Every electric wire in your plant – whether it’s in a motor, generator, cable, switch, transformer, etc. – is carefully covered with some form of electrical insulation. The wire itself is usually copper or aluminum, which is known to be a good conductor of the electric current that powers your equipment. The insulation must be just the opposite from a conductor: it should resist current and keep the current in its path along the conductor.

To understand insulation testing you really don’t need to go into the mathematics of electricity, but one simple equation – ohm’s law – can be very helpful in appreciating many aspects. even if you’ve been exposed to this law before, it may be a good idea to review it in the light of insulation testing.

Common sense tells us that the more voltage we have, the more current there’ll be. Also, the lower the resistance of the wire, the more current for the same voltage. Actually, this is ohm’s law, which is expressed this way in equation form:

e = I x R

where,
e = voltage in volts
I = current in amperes
R = resistance in ohms

Note, however, that no insulation is perfect (that is, has infinite resistance) so some electricity does flow along the insulation or through it to ground. Such a current may only be a millionth of an ampere (one microampere) but it is the basis of insulation testing equipment. note also that a higher voltage tends to cause more current through the insulation.

This small amount of current would not, of course, harm good insulation but would be a problem if the insulation has deteriorated. now, to sum up our answer to the question “what is ‘good’ insulation?”

We have seen that, essentially, “good” means a relatively high resistance to current. Used to describe an insulation material, “good” would also mean “the ability to keep a high resistance.” So, a suitable way of measuring resistance can tell you how “good” the insulation is. Also, if you take measurements at regular periods, you can check trends toward its deterioration (more on this later).

What Makes Insulation Go Bad?

When your plant electrical system and equipment are new, the electrical insulation should be in top notch shape. Furthermore, manufacturers of wire, cable, motors, and so on have continually improved their insulations for services in industry. nevertheless, even today, insulation is subject to many effects which can cause it to fail – mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapors, moisture from processes, or just the humidity on a muggy day.

In various degrees, these enemies of insulation are at work as time goes on – combined with the electrical stresses that exist. As pin holes or cracks develop, moisture and foreign matter penetrate the surfaces of the insulation, providing a low resistance path for leakage current.

Once started, the different enemies tend to aid each other, permitting excessive current through the insulation. Sometimes the drop in insulation resistance is sudden, as when equipment is flooded. Usually, however, it drops gradually, giving plenty of warning, if checked periodically. Such checks permit planned reconditioning before service failure.

If there are no checks, a motor with poor insulation, for example, may not only be dangerous to touch when voltage is applied, but also be subject to burn out. What was good insulation has become a partial conductor.

Resource: Megger handbook