Surge protection devices (SPD), their classification and connection diagrams

Literally in every house there is at least a minimal set of equipment, such as a refrigerator, a TV and a washing machine, and they all cost quite decent money.

But few people think that the connected device is constantly at risk of being disabled. a short high voltage impulse (the cause may be a thunderstorm or a short circuit on lines).

It is very good that such a phenomenon does not occur so often, but the very idea that a TV set for 40,000 rubles can simply turn into a piece of plastic makes you wonder how you can protect it? In this article, I will tell you which products are specifically designed to protect equipment from such a misfortune.

What is the SPD intended for and what functions does it have?

This product serves to protect the electrical consumers connected to the wiring from the effects of surges, which can appear as a result of a direct or indirect lightning discharge into the power transmission line supports or as a result of transient processes during a short circuit in system.

SPDs can be referred to as: OPS (surge arrester) - mains overvoltage limiter, SPE - surge voltage limiter. But no matter how they are called, the triggering algorithm and their functions are identical.

What is the algorithm of SPD functioning

This device uses non-linear elements and, often, it is varistors that are used.

Varistor is a semiconductor resistor in which the resistance has a non-linear dependence, which is characterized by the acting voltage.

As can be seen from the above dependence, when the specified voltage value is reached, the varistor resistance drops to zero.

In order to better understand how it works, let's look at a simple diagram.

Here is a simple diagram of a single-phase network, where a lamp plays the role of a load. An SPD is also connected to the phase in this network.

During normal operation, U = 230 V, at this voltage the resistance of the varistor reaches hundreds MOhm, which means that no current passes through it.

As soon as a high voltage pulse occurs in the network, the resistance of the varistor drops sharply to zero and the dangerous current does not go into the network, but flows through the relay to the ground.

That is, in fact, an artificial short circuit is created, the current of which makes the circuit breaker trip.

According to this principle, the equipment is preserved from the effects of a high voltage pulse.

How are SPDs classified

According to GOST R 51992-2011, SPDs are subdivided into the following subclasses:

• First class (Class B).

These products are used to protect against direct lightning discharge into the power system. It is mounted in the ASU (input switchgear) or main switchboard (main switchboard). It is also mandatory for installation in buildings where there is an extremely high probability of being hit by a lightning discharge. The nominal discharge current is 30-60 kA, with the waveform being 10/350 μs.

• Second class (Class C).

This product is used to protect against residual atmospheric overvoltages that have passed through a Class B SPD. These devices are already being installed in RC houses. Normalized by a pulse current with a waveform of 8/20 μs. And the discharge current is 20-40 kA.

• Third grade (grade D).

The product is applicable for the protection of electrical equipment against impulse voltage that has passed through a second class SPD. It is usually installed either inside equipment or in junction boxes and wiring outlets.

For example, a surge protector is a third class SPD. Pulse current with a waveform of 8/20 μs. And the rated discharge current is 5-10 kA.

SPD connection diagrams

Important. Installing an SPD makes sense only if you have a high-quality grounding system, and an automatic switch must be installed in front of the SPD.

SPD connection diagram in a single-phase network of two-wire or three-wire design.

Connection in a three-phase network.

Important. If you are implementing a full-fledged multistage system of SPDs, then keep in mind that the distance along the cable between the modules must be at least 10 meters.

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