Surge protection 101

Surges or sudden bursts of voltage and current can be disastrous to electronic and electrical equipment and to the cables feeding them. Surge protectors limit the voltage getting through to equipment and often the feed lines, either by blocking it or diverting to earth voltages above a predetermined level.

The protecting device has a clamping voltage which determines the point at which excessive energy will be diverted or blocked. A low clamping voltage may give better protection but it’s at the expense of a shorter life and possible damage to what it is protecting.

Metal oxide varistors (MOV), one of the most common components of protectors, are often arranged to redirect voltages to earth rather than absorb them.

The device is a mass of semiconductor material of sintered, granular zinc oxide. They can comfortably conduct large currents even when fed a voltage greater than their design figure. They are sometimes connected in parallel but they must be in matched sets otherwise one unit may overwork to create a phenomenon called current hogging, which substantially reduces its operating life.

All MOVs have a finite life with their triggering voltage falling lower and lower over time and use. Thermal runaway can be a problem if a failing MOV is used as a power filter because it starts behaving like a short circuit and heats up. To overcome this MOVs are normally thermally fused.

Electrical spikes take a few microseconds to reach their peak voltage and similarly the protectors have a time lapse before they operate. In practice, most surge protectors kick in before the dangerous part of the spike reaches the equipment and most MOVs respond in microseconds.

Transient voltage suppression diodes, sometimes known as avalanche diodes, are the fastest protectors of all, reacting to sudden power spikes in picoseconds. However, they absorb a much lower energy level than MOVs, but if voltages are kept well within their rating they have a very long life. If the rating is exceeded, the diode may short circuit yet protection remains intact.

A glass discharge tube (GDT) comprises a glass bulb in which two electrodes are separated by a special gas mixture. When the gas is iodised by a high voltage, current flows between the electrodes. As with MOVs, the GDTs can deal with a few large transients or rather smaller transients. Lightning surges may result in a short. The downside of GDT devices is that they are quite slow to trigger, which means a high voltage can pass before they kick in. Pulses of 500 V at 100 ns are often allowed through, which may result in additional protection being required.

Once triggered, the device will carry on conducting until the voltage drops and the gas is quenched. It will continue to conduct at a voltage lower than that needed to ionise the gas. Some GDTs are sensitive to light, which lowers their trigger voltage. Telecommunications and power lines are their principal areas of use where their high current handling is a desirable feature.

Thyristors are also used as surge protectors often in crowbar circuits where protection against overload is needed. Its operation is similar to a gas discharge tube but it operates much faster. A low clamping voltage permits large currents to flow, generating minimum heat.

Selenium is another mass semiconductor, although its clamping qualities are not as good as MOVs. With a longer life than an MOV, this semiconductor is mainly used in high-voltage DC circuits such as the exciter field of an alternator.

The spark gap is a carbon rod held close to an electrode. A predetermined voltage decides the distance of the electrode from the rod. It is one of the oldest protection devices, with a history going back to the nineteenth century. Today, it is used in telephone circuits but it has obvious limitations in that it can never be used in an explosive atmosphere.

Protecting power feeds are series mode suppressors that differ from the solid-state and gas discharge types in being heavy-duty, low-pass filters that allow 50 Hz to the equipment and block higher frequencies. Inductors, capacitors and resistors suppress the spikes and as the inductors are in series with the circuit they slow the energy down, which is then spread out and slowly released from a bank of capacitors