SPD stands for Surge Protection Device.

It is a device that protects the electrical installation, and connected devices, within a property, i.e the consumer unit, all wiring and sockets, lights etc, from an electrical ‘surge’ known as transient over voltages.

A surge protection device (SPD), also called a surge protector, is an electronic device that provides safety protection for various electronic equipment, instruments, and communication lines. When a spike current or voltage is suddenly generated in the electrical circuit or communication circuit due to external interference, the surge protection device can conduct and shunt in a very short time, so as to prevent the surge from damaging other equipment in the circuit.

A surge protection device (SPD) is an indispensable device in the lightning protection of electronic equipment. The function of a surge protection device is to limit the instantaneous overvoltage that penetrates into the power line and signal transmission line within the voltage range that the equipment or system can withstand, protecting the protected equipment or system from impact and damage.SPD is suitable for AC 50/60HZ power supply system.

Types of Surge Protection Device

There are different types of SPD according to IEC/EN 61643-11, classified as follows:

- Type 1 / Class I / Class B
- Type 1+2 / Class I+II / Class B+C
- Type 2 / Class II / Class C
- Type 2+3 / Class II+III / Class C+D
- Type 3 / Class III / Class D

Main parameters

1. Nominal voltage Un: The rated voltage of the protected system matches. This parameter indicates the type of protector that should be selected. It marks the effective value of AC or DC voltage.

2. Rated voltage Uc: The maximum effective value of voltage that can be applied to the designated end of the protector for a long time without causing changes in the protector characteristics and activation of the protection element.

3. Rated discharge current In: The maximum impulse current peak value that the protector can withstand when a standard lightning wave with a waveform of 8/20μs is applied to the protector for 10 times.

4. Maximum discharge current Imax: When a standard lightning wave with a waveform of 8/20μs is applied to the protector for one impact, the maximum impulse current peak value that the protector can withstand.

5. Voltage protection level Up: the maximum value of the protector in the following tests: 1KV/μs slope of flashover voltage; residual voltage of rated discharge current.

6. Response time tA: It mainly reflects the action sensitivity and breakdown time of the special protection element in the protector. The change in a certain period of time depends on the slope of du/dt or di/dt.

Type 1 SPD

Type 1 Surge Protective Device are intended for installation between the secondary of the service transformer and the line side of the service equipment overcurrent device, as well as the load side, including watt-hour meter socket enclosures, and are intended to be installed without an external overcurrent protective device.

Type 1 devices are dual-rated for Type 2 applications as well, providing the highest ratings available for installation at the service entrance.

Type 1 SPD can discharge lightning current with waveform 10/350 μs. It will be installed in the primary distribution board at the origin of the electrical installation.

A Type 1 SPD does not in itself offer the required protection level and must be used in conjunction with coordinated type 2 devices. An installation with a lightning protection system will require a Type 1 SPD.

Type 1 surge protection device is developed as the best solution available to protect service entrance at an industrial site, especially those with existing lightning protection systems or meshed cage applications.

Type 1 surge protection device is particularly useful in a high lightning density area where the risk of heavy surge current or even direct strike is high (eg: buildings equipped with lightning rods).

Type 1 SPD is characterized by a 10/350 µs lightning current waveform and installed at the load center’s main circuit breaker, e.g. main distribution board.

Type 1+2 SPD

Type 1+2 surge protection device can protect all electrical installations against lightning strikes by discharging the current created from a lightning surge and keeping it from spreading to the equipment.

Type 1+2 SPD is characterized by a 10/350 µs and 8/20 µs lightning current waveform.

Type 1+2 surge protection device installed at the origin of the AC installation equipped with LPS.

Type 1+2 surge protection devices use Metal Oxide Varistor (MOV) and/or Gas Discharge Tube (GDT) circuits to protect electrical devices from spikes in alternating current power.

Type 1+2 SPD is designed to be connected in multi-pole configuration to protect single-phase, 3-phase and 3-phase+Neutral AC networks, and for TN-C, TN-S, TN-C-S TT, and IT power supply systems.

Type 2 SPD

Type 2 Surge Protective Devices are intended for installation on the load side of the service equipment overcurrent device, including SPDs located at the branch panel.

Type 2 SPD can discharge 8/20 μs current wave, they can prevent the damage of transient overvoltage in the electrical installations and protects equipment connected to it.

It usually employs metal oxide varistor (MOV) technology. The device would normally be installed in sub-distribution boards and in the primary distribution board if there was no requirement for a type 1 device.

Type 2 surge protection device is characterized by an 8/20 µs lightning current waveform.

Type 2 SPD is designed to be installed at the service entrance of low voltage systems or close to sensitive equipment to protect against transient overvoltages.

Type 2 surge protection device is for nominal working voltage (50/60Hz) Un = 120V 230V 400V ac applications and for maximum continuous operating voltage (50/60Hz) Uc = 150V 275V 320V 385V 440V ac applications.

Type 2 SPD is generally installed in sub-distribution or machine control cabinets, for indoor use or fixed into a waterproof box for outdoor use.

Type 2+3 SPD

Type 2+3 surge protection devices are normally installed right before the protected equipment.

Type 2+3 SPD is characterized by current waves (8/20 μs) and a combination of voltage waves (1.2/50 μs).

The technical parameters are usually nominal discharge current (8/20 μs) In and open-circuit voltage (1.2/50 μs) Uoc

Type 3 SPD

These SPDs have a low discharge capacity. They must therefore only be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.

Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs) and load current IL.

Type 3 surge protection devices are normally installed right before the protected equipment.

Type 3 SPD is generally installed near the protected load, to protect sensitive equipment for 24V 48V 60V 120V 230V, in coordination with Type 2 surge arrester installation head.

Arresters tested and found AC Type 3 surge protection devices are intended to be installed near sensitive equipment, in coordination with Type 2 surge arrester installation head.

Primary components

1. Spark gap

It is generally composed of two metal rods exposed to the air separated by a certain gap. One of the metal rods is connected to the power phase line L1 or the neutral line (N) of the required protection equipment. The other metal rod is connected to the grounding line ( PE) phase connection. When the instantaneous overvoltage strikes, the gap is broken down, and a part of the overvoltage charge is introduced into the ground, avoiding the voltage increase on the protected equipment. The distance between the two metal rods in the spark gap can be adjusted as required, and the structure is relatively simple, but the disadvantage is that the arc extinguishing performance is poor. The improved spark gap is an angular gap. Its arc extinguishing function is better than the former. It relies on the electric power F of the circuit and the rising effect of the hot air flow to extinguish the arc.

2. Gas discharge tube (GDT)

A gas discharge tube (GDT) is composed of a pair of cold negative plates separated from each other and encapsulated in a glass tube or ceramic tube filled with a certain inert gas (Ar). In order to improve the triggering probability of the discharge tube, there is an auxiliary triggering agent in the discharge tube. This gas-filled discharge tube has a two-pole type and a three-pole type.

3. Metal oxide varistor (MOV)

It is a metal oxide semiconductor with non-linear resistance with ZnO as the main component. When the voltage applied to both ends reaches a certain value, the resistance is very sensitive to voltage. Its working principle is equivalent to the series-parallel connection of multiple semiconductor P-Ns.

4. Transient voltage suppression (TVS) diode

A TVS diode has the function of clamping and limiting voltage. It works in the reverse breakdown zone. Because of its low clamping voltage and fast action response, it is particularly suitable for use as the last few levels of protection components in multi-level protection circuits.

5. Choke coil

The choke coil is a common mode interference suppression device with ferrite as the core. It consists of two coils of the same size and the same number of turns symmetrically wound on the same ferrite toroidal core, which forms a four-terminal device. It has to suppress the large inductance of the common-mode signal, while the small leakage inductance for the differential mode signal has almost no effect. The use of choke coils in balanced lines can effectively suppress common-mode interference signals (such as lightning interference) without affecting the normal transmission of differential mode signals on the line.

What is the difference between type 1, type 2, and type 3 surge protection devices?

SPD Type 1 provides primary protection against high-level surges caused by direct lightning strikes. It is often put on the main distribution board to safeguard a building’s whole electrical system.

SPD Type 2 provides secondary-level protection, the majority of common surges generated by electrical switching or nearby lightning strikes could be effectively protected, which are installed at the sub-distribution panel or electrical panel.

SPD Type 3 is designed specifically aimed at protecting specific terminal equipment, they should be installed close to sensitive loads as a supplement to Type 2 SPDs.

Overall, the selection of the appropriate SPD type will depend on several factors including the location of installation, maximum discharge current, voltage protection level, and application.

It’s important to note that the actual applications of each type can change based on the particular requirements of a given facility or piece of equipment that has to be protected.