What is insulation and why does it degrade?

What is Insulation and why does it degrade?

Electrical insulation degrades over a period of time because of various stresses, which are imposed upon it during its normal working life. The insulation has been designed to withstand these stresses for a period of years, which would be regarded as the working life of that insulation. This often runs into decades.

Abnormal stresses can bring about an increase in this natural aging process that can severely shorten the working life of the insulation. For this reason it is good practice to perform regular testing to identify whether increased aging is taking place and, if
possible, to identify whether the effects may be reversible or not.

The purpose of diagnostic insulation testing is:
•    To identify increased aging.
•    To identify the cause of this aging.
•    To identify, if possible, the most appropriate actions to correct the situation.

In its simplest form, diagnostic testing takes the form of a “Spot Test.” Most electrical maintenance professionals have made spot tests where a voltage is applied to the insulation and a resistance is measured. The diagnosis in this case is limited to “the insulation is good” or “the insulation is bad.” But having made this diagnosis what do we do about it? It’s a bit like going to the doctor with a bad cough and the doctor simply telling you, “You’ve got a bad cough.” You wouldn’t be happy to come away with only that information. You expect the doctor to examine you, carry out a few tests, and tell you why you have a bad cough and what to do about it to cure the cough.

In insulation testing, a spot test on its own is the equivalent of the doctor telling you that you are well or you are sick. It’s minimal information. This is the sort of test that is typically applied to low-voltage circuits where the cost of a failure is low and equipment can be replaced easily and inexpensively. Since the equipment being tested is low voltage equipment, these tests are typically performed using a 500 or 1000 V test voltage and will be familiar to all electrical maintenance personnel.

However, if the doctor records the results of his examination and compares them with those from previous visits, then a trend might be apparent which could lead to medication being prescribed. Similarly, if insulation resistance readings are recorded
and compared with previously obtained readings, it may be possible to see a trend and to prescribe remedial actions if such are called for.

WHAT IS INSULATION?

Every electric wire in a facility, whether it’s in a motor, generator, cable, switch, transformer, or whatever, is covered with some form of electrical insulation. While the wire itself is a good conductor (usually made of copper or aluminum) of the
electric current that powers electrical equipment, the insulation must resist current and keep the current in its path along the conductor. Understanding Ohm’s Law, which is expressed in the following equation, is the key to understanding insulation testing:
E = I x R
where,
E = voltage in volts
I = current in amperes
R = resistance in ohms

For a given resistance, the higher the voltage, the greater the current. Alternatively, the lower the resistance of the wire, the more current that flows for the same voltage.
No insulation is perfect (has infinite resistance), so some current does flow along the insulation or through it to ground. Such a current may be insignificantly small for most practical purposes but it is the basis of insulation testing equipment.

So what is “good” insulation? “Good” means a relatively high resistance to current flow. When used to describe an insulation material, “good” also means “the ability to maintain a high resistance.” Measuring resistance can tell you how “good” the insulation is.

What Causes Insulation to Degrade?

There are five basic causes for insulation degradation. They interact with each other and cause a gradual spiral of decline in insulation quality.

Electrical Stress

Insulation is designed for a particular application. Overvoltages and undervoltages cause abnormal stresses within the insulation, which can lead to cracking or delamination of the insulation.

Mechanical Stress

Mechanical damage such as hitting a cable while digging a trench is fairly obvious but mechanical stresses also may occur from running a machine out of balance or frequent stops and starts.
The resulting vibration from machine operation may cause defects within the insulation.

Chemical Attack

While you would expect insulation to be affected by corrosive vapours, dirt and oil can also operate to reduce the effectiveness of insulation.

Thermal Stress

Running a piece of machinery in excessively hot or cold conditions will cause over expansion or contraction of the insulation which might result in cracks and failures. However, thermal stresses are also incurred every time a machine is started or stopped. Unless the machinery is designed for intermittent use, every stop and start will adversely affect the aging process of the insulation.

Environmental Contamination

Environmental contamination covers a multitude of agents ranging from moisture from processes, to humidity on a muggy day, and even to attack by rodents that gnaw their way into the insulation.

Insulation begins to degrade as soon as it is put in service. The insulation in any given application will have been designed to provide good service over many years under normal operating conditions. However, abnormal conditions may have a damaging
effect which, if left unchecked, will speed up the rate of degradation and will ultimately cause a failure in the insulation. Insulation is deemed to have failed if it fails to adequately prevent electrical current from flowing in undesirable paths. This includes
current flow across the outer or inner surfaces of the insulation (surface leakage current), through the body of the insulation (conduction current) or for a variety of other reasons.

For example, pinholes or cracks can develop in the insulation or moisture and foreign matter can penetrate the surface(s). These contaminants readily ionize under the effect of an applied voltage providing a low resistance path for surface leakage current which
increases compared with dry uncontaminated surfaces. Cleaning and drying the insulation, however, will easily rectify the situation. Other enemies of insulation may produce deterioration that is not so easily cured. However, once insulation degradation has started, the various initiators tend to assist each other to increase the rate
of decline.

Image credit: Dr Stan Zurek