Surge Protection Schemes

These schemes are connected in parallel to power systems to provide a path of least resistance to potentially damaging transients to prevent sensitive electronic equipment from being exposed to overvoltage’s.

Working Principle of a Surge Protection Device (SPD)

Figure 1 (below) shows the V-I characteristic for two SPD devices popularly used in power systems, zinc oxide and silicon carbide.  Both types of SPD have a window of voltage operation within which very little current is drawn, and beyond which, saturation occurs.

The principle is to set the window to be the threshold to be the positive and negative peaks of the sinusoidal voltage for the normal operating voltage for a given installation.

Once the voltage exceeds this ‘knee point’, either in the positive or negative direction, saturation occurs, and the current drawn by the SPD increases dramatically. This limits any further increases in voltage.  In the case of the Zinc Oxide SPD, the increase in voltage beyond the conduction point is very limited.

Application of SPD

Figure 2 (below) shows an example of a surge protection scheme where different grades are distributed throughout an electrical installation.

The main incoming supply has a Class 3 device which basically protects against direct or indirect (induced) lightning strikes.  This type of surge protection is capable of absorbing high levels of energy, the trade-off for which is the comparatively high let through voltage – often as high as 1500V.

At the sub-distribution level, Class 2 devices are applied to provide protection against the energy let through by Class 3 surge protection or local switching transients.  This type of protection can still absorb significant energy, but the let through voltage tends to be lower – typically 900 – 1200V.

Final circuits tend to have the most sensitive load connected to them and as such, need to have the highest level of protection.  A Class 1 SPD will offer such protection providing a let through voltage of typically 600 – 750V, but the amount of energy that can be absorbed by a Class 1 device is significantly lower than the Class 2 or Class 3 SPD’s.

Quite often, installations will have a Class 3 surge protection device on the main incoming supply and people may be under the impression that this provides adequate protection for the whole load.  The reality is that, in most cases, a graded surge protection scheme such as that shown in Figure 2 is required to provide comprehensive protection for modern sensitive loads.