Understanding the Working Principle of Surge Protection Device

Surge protection devices (SPDs) are an essential component of any electrical system, as they protect sensitive equipment and appliances from dangerous power surges. These devices work by diverting excess voltage away from the electrical system, preventing damage to the equipment and ensuring a safe and stable power supply. The working principle of surge protection devices is based on the use of specially designed components, such as varistors and metal oxide varistors (MOVs), that are able to detect and respond to voltage spikes in the electrical system.

How Does a Surge Protection Device Work?

The operating principle of a surge protection device (SPD) is as follows:

• During normal operation (e.g., in the absence of surges), the surge protection device has no influence on the system where it’s installed. It acts like an open circuit and maintains the isolation between the active conductors and the earth.
• When a voltage surge occurs, the surge protection device reduces its impedance within a few nanoseconds and diverts the impulse current. The surge protection device behaves like a closed circuit, the overvoltage is short-circuited and limited to an acceptable value for the electrical equipment connected downstream.
• Once the impulse surge has stopped, the surge protection device will return to its original impedance and return to the open circuit condition.

Example of operation

Without a surge protection device, the surge reaches the electrical equipment. If the surge exceeds the electrical equipment’s impulse withstand voltage, the isolation is reduced and the impulse current flows freely through the device, damaging it.

With the use of a surge protection device between the active conductors and earth (TT network), the overvoltage is limited and the discharge current is safely diverted, establishing an equipotential connection between phase and earth.

Technologies Used in Surge Protection Devices

A surge protection device contains at least one non-linear component, its electrical resistance varying in the function of the voltage which is applied to it.

Surge protection devices based on spark gaps

They are called switching surge protection devices. The spark gap is a component composed of two electrodes in close proximity that isolate one part of the circuit from the other up to a certain voltage level. These electrodes can be in the air or encapsulated with a gas. During normal operation of the system (at rated voltage), the spark gap does not conduct current between the two electrodes. In the presence of a voltage surge, the impedance of the spark gap rapidly decreases to 0.1-1 Ω with the formation of an electric arc between the electrodes, typically in 100 ns. The electric arc is extinguished when the surge finishes, restoring the isolation.

Varistor surge protection devices

Varistors are components that have their impedance controlled by the voltage, with a characteristic continuous but not linear “U in function of I”. surge protection devices based on varistors, also known as voltage limiting, are characterized by a high impedance when there is no surge present (normally above 1 MΩ). When a surge occurs, the varistor’s impedance falls rapidly below 1 Ω within a few nanoseconds, allowing the current to flow. The varistor regains its isolation properties after discharging the surge. A peculiarity of varistors is that a negligible current is always flowing through them, known as residual current, IPE (100 to 200 µA).

Comparison between spark gaps and varistors

The main characteristic of spark gaps is their capacity to manage large quantities of energy from direct lightning strikes, while varistors have a very low level of protection (therefore high performance) and are fast-acting. We will now examine the difference between the two technologies.

Isolation properties

A varistor, although it presents a very high impedance at rest, always has a minimum continuous current, Ic, flowing through it (e.g., 0.5 NA). This current tends to increase as the varistor wears until it reaches high levels. For this reason, Varistor SPDs must always be protected against short circuits and cannot be used for N-PE connection upstream of the RCDs. + include internal protection that guarantees a safe end of life.

A spark gap is a true open circuit when at rest, ensuring that there is no current flow at all either in normal operating conditions or when it reaches the end of its life; for this reason, an SPD may be installed upstream of an RCD (therefore protecting it from the flow of impulse or discharge current) only if the connection between the active conductors and earth provides for a spark element.

Resistance when conducting

Even in the discharge phase, the resistance of the varistor remains appreciably greater than zero, limiting the possibility to reduce the surge overvoltage to 3-4 times the rated mains voltage.

When the arc is ignited, the resistance of the spark gap becomes negligible.

Response time

The response time of the varistor is very rapid, with a few nanoseconds. Spark gap technology is generally slow but accelerated by electronic devices.

Ignition / limiting voltage

Ignition / limiting voltage of the varistor is low, thanks to the fast response time. Ignition / limiting voltage of the spark gap is generally high, thanks to the excellent insulating properties of the air, but reduced with the aid of the electronic device.

Extinction of the short-circuit

Varistors are not characterized by a follow-through short circuit current, as their impedance returns to very high values as soon as the surge ceases. SPDs with spark gap technology must necessarily be designed in a way that enables the interruption of the following current (such as an arc extinguishing chamber)

End-of-life

A varistor progressively loses its isolating performance; at the end of its life, it can therefore become a low-impedance short-circuit. A spark gap is no longer able to ignite the arc at the end of its life, due to the wear of its electrodes or because the electronic ignition circuit has faded. It, therefore, becomes a permanently open circuit.

Need for backup protection

Back-up protection is necessary in order to ensure short-circuit end-of-life safety. In case of a short circuit end of life of the varistor, the thermal disconnector is generally not able to open the circuit.

Back-up protection of the spark gap is to be provided in all cases to ensure safety in the case of a fault with the SPD and to interrupt the electrical arc if the short-circuit current in the installation point is greater than the SPD’s performance for interrupting the short-circuit follow-through current (Isc>If).