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What Causes Transients

The transient condition can be caused direct connection or through inductive, resistive or capacitive coupling. For instance a lightning strike to electrical equipment is direct, however a strike to earth through pipe-work is a resistive coupling; as is a discharge between clouds giving rise to an inductive coupling. With any 'coupling', it is the effect of the movement of electrons that causes a build up in potential difference within the susceptible equipment. If the build up in potential difference is enough to exceed the rated insulation value of a device, then a malfunction will occur.

How Can We Prevent Damage

The final failure in insulation can be minimized by design of a system that will prevent a build up in charge between the insulated conductive paths. This can be achieved through a combination of measures; each of which is essential to ensure an effective transient protection system.

1. Identify the anticipated causes of potential failure,

2. Select a surge diverter that provides a balance of risk to cost benefit,

3. Build an effective protection shield for lightning impulse conditions,

4. Build an effective earth and surge protection system.

1) Identification of the anticipated transient is essential in selecting the appropriate protection device. The questions to ask are;

2) Select a surge diverter that provides a balance of risk benefit as to cost benefit.

3) Build an effective protection shield for lightning impulse conditions.

4. Build an effective earth and surge protection system.

What Must We Be Aware of As Potential Problems

For an installation where there are a combination of earthing systems and high incidence of lightning, there are many difficulties designing and installing effective protection systems. Installation of a shunt surge diverter may be inadequate for the potential problem. If there is a failure of one device and not another, we must ask the question, Why did this particular device fail? We will often find the problem is not related to the installation or absence of the surge diverter but could be related to the earth configuration of the total system.

Some typical surge 'Failures' that will not be protected by surge protection devices are;

To adequately protect an installation, all of the rules must be followed;

General Comment

Manufacturers build devices that will provide a degree of protection, generally in line with Australian Standards and IEEE category classifications. The 5 categories define risk and the protection recommended relative to the exposure of the power supply system. Although effective in most applications, the general nature of the categories does no cater for special applications. Additionally, the IEEE categories are 'recommendations' for protection under the prescribed conditions. There is a requirement to engineer specific applications that fall outside the prescribed conditions, as the IEEE gives no undertaking as to the adequacy of the recommendations outside of the defined scope.

By their very design, Metal Oxide Varistor (MOV) surge protection devices deteriorate each time they are subjected to a transient. The higher the risk area, the more they operate and the earlier they fail.

    The IEEE-587 Categories of protection