Megger insulation resistance test

Megger insulation resistance test

Good insulation resistance?

as you know good insulation has high resistance and poor insulation has relatively low resistance.

Actual resistance values ​​may be higher or lower, depending on factors such as temperature or moisture content of the insulation (resistance decreases in temperature or humidity).

Megger insulation resistance test

Remember that good insulation has high resistance; poor insulation, relatively low resistance. Actual resistance values ​​may be higher or lower, depending on factors such as temperature or moisture content of the insulation (resistance decreases in temperature or humidity).

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

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, resistance is usually read in the megohms range .

The Megger Insulation Tester is basically a high resistance rangefinder (ohmmeter) with a built-in DC generator. This meter is specially constructed with current and voltage coils, which allows true ohms to be read directly, regardless of the actual voltage applied.

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

The generator can be operated by hand or on line to develop a high DC voltage which generates a low current through and on the surfaces of the insulation under test ( Fig. 2 ). This current (usually at an applied voltage of 500 volts or more) is measured by the ohmmeter, which has an indicating scale.

What is “good” insulation?

Every electrical wire in your plant — whether in a motor, generator, cable, switch, transformer, etc. — is carefully covered with a kind of electrical insulation. The wire itself is usually copper or aluminum, which is known to be a good conductor of the electrical current that powers your equipment. Insulation must be the exact opposite of a conductor: it must withstand current and keep it in its path along the conductor.

Understanding Insulation Test You really don’t need to go into the math of electricity, but a simple equation — ohm’s law — can be very helpful in understanding many aspects. Even if you have been exposed to this law before, it may be a good idea to review it in light of insulation testing.

The role of insulation around a conductor is very similar to that of a water pipe, and Ohms’ law of electricity can be more easily understood by comparison with water flow. In Figure 1 we show this comparison. The pressure exerted on the water by a pump causes a flow down the pipe ( Fig. 1a ). If the hose were to leak, you would be wasting water and losing some pressure. With electricity, the voltage is equivalent to the pressure of the pump, causing electricity to flow along the copper wire ( Fig. 1b ).

As in a water pipe, there is some resistance to flow, but it is much shorter along the wire than through the insulation.

Common sense tells us that the more voltage we have, the more current there will be. Also, the lower the resistance of the wire, the higher the current for the same voltage. In fact, it is Ohm’s law, which is expressed in the form of an equation:

e = I x R

where,

e = voltage in volts

i = current in amperes

R = resistance in ohms

Note, however, that no insulation is perfect (i.e. it has infinite resistance) so electricity will flow along the insulation or pass through it to ground. Such a current may be as little as a millionth of an ampere (a microampere), but it forms the basis of insulation test equipment. Also note that a higher voltage tends to cause more current through the insulation.

This small amount of current would, of course, not detract from good insulation, but would be a problem if the insulation deteriorated. Now to summarize 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 insulating material, the term “good” also means “the ability to maintain high strength”. So, a proper way of measuring resistance can tell you how good “insulation” is. Also, if you take measurements at regular times, you can check for trends in its deterioration (more on that later).

What makes insulation bad?

When your plant’s electrical system and equipment are new, the electrical insulation should be top-notch. In addition, manufacturers of wires, cables, motors, etc. have constantly improved their insulations for services in industry. Even today, however, insulation is subject to many effects that can cause it to fail — mechanical damage, vibrations, excessive heat or cold, dirt, oil, corrosive fumes, moisture from processes or just moisture from a sunny day.

To varying degrees, these enemies of insulation come to work over time — combined with existing electrical stresses. As pinholes or cracks develop, moisture and foreign objects penetrate the surface of the insulation, providing a path of low resistance for leakage current.

When cast, different enemies tend to help each other, allowing excessive current through the isolation . Sometimes the drop in insulation resistance is sudden, such as when the equipment is flooded. Usually, however, it gradually decreases, giving a lot of warning, if checked periodically. These checks allow planned reconditioning before a service failure.

If there are no controls, a motor with poor insulation, for example, can not only be hazardous to the touch when voltage is applied, but also be subject to overload. What used to be good insulation has become a partial conductor.