Surge Protection Device (SPD) Types: Type 1, 2, 3, 4, 5 Explained

The surge protective device (SPD) is a kind of product that protects valuable electrical and electronic equipment against transients, originating from lightning and also from switching sources. Surge protective devices are devices consisting mainly of voltage-controlled resistors (varistors, suppressor diodes) and/or spark gaps (discharge paths).

Every piece of electronic equipment found in an industrial environment is subjected to power surges generated on the utility grid that is transmitted to the equipment via incoming power lines. Users are installing surge protection devices to protect their equipment from surges.

SPDs can be used either at the main circuit breaker zone or branch circuit breaker zone depending on ratings. For this reason, there are various types of surge protection devices available.

Surge Protection Device (SPD) Types

The types of surge protection devices are:

1. Type 1 SPD

SPD Type 1 must be connected upstream of the system, at the point of delivery of power energy. SPD protects buildings and people from the risk of direct lightning (fire and death) and is characterized by:

Iimp 10/350 Impulse current: Iimp corresponds to the peak value of a 10/350 μs current impulse waveform. This waveform represents a direct lightning strike

In 8/20 Nominal discharge current: The peak current (and waveform shape) through the SPD under conditions prescribed by EN 62305 to represent the surge current as a consequence of a lightning strike to the electric supply line.

Type 1 SPD Protected equipment examples: Electrical switchgear, switchboard, distribution, MCCs, emergency power backup, transfer switch, UPS system

2. Type 2 SPD

SPD Type 2 devices are designed to remove all the overvoltage from supply circuits that are not likely to be directly hit by lightning. SPD Type 2 is connected downstream to SPD Type 1 or SPD Type 1+2, (minimum distance 1 m) and they protect machines and tools connected to the ground and reduce the risk of economic loss.

SPD Type 2 is characterized by:

Imax 8/20 Maximum discharge current: Peak value of the highest current of an 8/20 μs waveform that an SPD can discharge at least once without breaking.

Type 2 SPD Protected equipment examples: Emergency power backup, transfer switches, control boxes, switchgear, generators, computer servers, building management systems, surveillance equipment, security systems, HVAC, building management systems, fire alarm panels, copiers, telephone systems, fax machines

3. Type 3 SPD

SPD type 3 devices are used to protect the end user from overvoltage. They may be installed in supply networks where SDP types 1 and/or 2 already exist. They can be installed in fixed or mobile sockets and have the following characteristic parameters.

Uoc: test voltage. This is the peak value of the no-load voltage of the combined test-generator; this has a waveform of 1.2/50 μs (figure 7) and can supply at the same time current with a waveform 8/20 μs

Type 3 SPD Protected equipment examples: X-Ray, CAT-scan, Life support equipment, Medical instrumentation, Computer servers, Elevators, Parking lot lighting, Printers, Communication systems, Motors, Pumps, Drives

4. Type 4 SPD

Type 4 SPDs are considered component SPDs. Component SPDs typically consist of one or more Type 5 components assembled. Type 4 SPDs are not intended to be used by themselves and must be integrated into other systems.

Type 1 component assembly is a Type 4 SPD that once installed inside another piece of equipment would be tested as a Type 1 SPD (would not require external overcurrent protection).

Type 2 component assembly is Type 4 SPD that once installed inside another piece of equipment would be tested as a Type 2 SPD. (would require external overcurrent protection)

5. Type 5 SPD

Type 5 SPDs are discrete component surge suppressors (such as MOVs) that may be mounted on a printed circuit board, connected by leads, or provided within an enclosure with mounting means and wiring terminations.

A single SPD also cannot fully protect against transients that enter a building along other conductors, such as grounding conductors or conductors powering loads on the roof, exterior walls, and grounds. For example, if lightning strikes the ground near a building’s grounding point, the voltage could enter the building and damage load equipment without ever reaching the service entrance SPD. For this reason, SPDs should be installed at the service entrance, distribution panels, and critical load equipment using a cascaded approach.

Types of Surge Protection Device Technologies

1. Varistor

This can be considered as a variable resistance that at nominal voltage has a very high ohmic value. But the resistance rapidly falls to near zero as the voltage surges. In this way, the varistor applies a near-short circuit that clamps the surge voltage. The varistor is however subject to progressive degradation due to the small leakage current that occurs at the nominal voltage, and with the number of interventions. With every overvoltage that occurs the leakage current rises and accelerates the end of life for the device – which is ultimately indicated by the change from green to red in the signal window.

2. Spark gap

This comprises two electrodes separated by air or gas. When a surge voltage occurs electrical arc bridges the gap and a surge current flow to limit the surge voltage to a low and constant level. The arc extinguishes only when the surge current falls below about 10 amperes. The gas guarantees a constant level of breakdown voltage since the arc is struck in a protected environment; not exposed to pressure or humidity variations or impurities as would happen if it had occurred in the air. There is, however, a delay before the device arcs and the surge current is diverted, and this is dependent on the magnitude of the original voltage surge and its rate of rising. Therefore, the voltage protection level can vary, although it is guaranteed to be less than up.

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