Through its technological advances, the modern world has brought us a lot of new materials, substances and other goods to make our lives easier, better and a lot more comfortable.
Electricity is one of the most common and most relied upon of the modern inventions; it is however still one of the most dangerous if it is misused, or its applications of use are underestimated.
This is the reason why any item that carries electrical current is designed to do so safely, and with some type of insulation. The insulation limits the flow of current between the different conductors and between the conductors to ground. It is therefore very important that the insulation has the opposite transfer properties of the conductor. Conductors are usually metallic, and the most common are copper or aluminium, both of which are known to be very good conductors of electric current due to the materials’ high current carrying capacity and constant thermal properties.
The insulation is usually made from a non-metallic material. The majority of electrical insulation is made of PVC, plastic, or rubber. It should resist current and keep it within the path alongside the conductor. Many of the best insulators are man-made.
In theory, insulation should not - and does not - carry electrical current. However, in reality there is not an ideal insulation material. Every kind of insulation has some resistance. The resistance of the insulation material needs to be very high but it is not infinite. Obviously the higher the resistance, the better the insulation capacity it has.
Manufacturers of wires, cables, and motors have continually improved their insulation. For areas where electricity is being used in industry (and in the home) manufacturers have developed far better products and at a much lower cost. However, the insulation, while in use, is subject to many effects which can cause it to fail. Mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapours, moisture from the air and general wear and tear are all causes.
A combination of electrical stress and the degradation of insulation are constantly happening, as one of the major causes of insulation failure is general wear and tear. As pinholes or cracks develop, moisture and foreign materials penetrate the insulation, providing a lower resistance path for leakage current. Once started, different degradation processes aid each other; this will then permit excessive current through the insulation.
The leaking current cannot be detected by smell or by sight; this is what makes the issue dangerous as only the consequences can be noticed - often too late.
If there is a planned maintenance program in place, this will pick up the gradual reduction in insulation resistance; this will also allow for a service program of replacement or renewal prior to the part having a full field failure. If there is no maintenance program then the item with poor and degraded insulation may become dangerous to the touch when voltage is applied. The item may not only be a danger to the user, but it may also be dangerous to any person in the area its being used.
A sudden drop in insulation resistance is very rare; the most common cause of this is when a device has been subject to complete submersion.
This is the main reason why an insulation-testing program should be carried out. A regular program of insulation resistance testing is recommended to cut down and reduce the risk of electrical shocks, to maintain the safety of personnel, and to reduce repair times due to failure. It helps to detect the onset of insulation breakdown which enables for a scheduled maintenance program to be implemented, which may include one or a number of the following steps: In depth cleaning, renewal, drying, and rewinding. It is also helpful for evaluating the quality of the repairs before the equipment is put back into operation.
The normal steps for a class one PAT test are visual checks, earth bond, insulation resistance, (occasionally flash) and leakage test. Any of the tests which cause a fail should stop the test, and prevent the item being used in general service.
For a class 2 test, the sequence is visual, insulation resistance, (flash), leakage.
The insulation test is always carried out as either the first or the second powered test, and is carried out in order of electrical safety. All of the mainstream PAT testing equipment carries out an insulation resistance test, commonly known as DCIR. It is carried out with DC test voltage; this voltage ranges from 250, 500, up to 1000V.
The voltage is applied through the live-neutral connections and is measured against the earth. The higher the reading the better, although for a class 1 appliance the reading must be above 1M Ohm, and for a Class 2 appliance it must be above 2M Ohm.