What is a surge protective device and what does it do?

A surge protective device limits brief overvoltage transients in an electrical installation. Usually abbreviated to SPD, it helps reduce the risk of equipment damage from sudden voltage rises caused by lightning or switching events. This guide covers low-voltage SPDs used in building installations and machine electrical systems.

A building does not need to suffer a direct lightning strike for a surge to occur. Nearby lightning, switching on the distribution network, and the operation of inductive loads through relays and contactors can also produce transients. Relevant equipment includes home routers and televisions, as well as industrial PLCs, electronic power supplies and variable frequency drives.

How does an SPD work?

Many panel-mounted SPDs form a protective path connected in parallel with the supply circuit. Under normal conditions, this path has a high impedance. During a surge, internal components such as a varistor or spark gap become conductive and divert surge current. This limits the voltage rise at the protected point.

Depending on its design, an SPD may provide protection between line and neutral, line and earth, or neutral and earth. Understanding the roles of live, neutral and protective earth is therefore essential. Limiting a surge does not mean holding the equipment terminals at normal supply voltage throughout the event. For the related principles used on electronic boards, see varistors and TVS diodes.

What is the difference between Type 1, Type 2 and Type 3 SPDs?

The following descriptions use the IEC/EN classification. The type number identifies a device's test duty and role in the protection arrangement; it is not a ranking from lower to higher quality. These descriptions should not be substituted for the separate UL classification used in other markets.

Combined devices such as Type 1+2 SPDs can meet more than one test classification. By contrast, a power strip's marketing description alone does not establish its protective capability; its technical documentation must identify that capability. For a fuller comparison, see SPDs, voltage protection relays and surge-protected sockets.

What do Uc, Up, In and Imax mean?

Uc — maximum continuous operating voltage: This is the voltage that can be continuously applied across the relevant protective terminals. For example, Uc = 275 V AC does not mean that the SPD disconnects the building's supply when the voltage reaches 275 volts. Uc is not an adjustable overvoltage trip setting.

Up — voltage protection level: This describes the voltage level appearing across the SPD under specified test conditions. It must be considered alongside the protected equipment's impulse withstand capability. A lower Up can provide a lower remaining surge voltage in a suitable application under comparable conditions, but poor installation can reduce that benefit.

In and Imax — discharge current ratings: In is the nominal discharge current and Imax is the maximum discharge current; Type 2 ratings use an 8/20 µs test waveform. A marking such as 40 kA does not mean the SPD can continuously supply a 40,000-amp load. The ampere rating here concerns a very brief surge current.

Iimp — lightning impulse current: This is an important Type 1 rating associated with the 10/350 µs waveform. Ratings expressed in kA are not directly interchangeable when their test waveforms differ. Check whether a quoted value applies per pole, per protection mode or to the device as a whole.

Why can poor installation undermine a suitable SPD?

Connection length affects protection performance. Surge current changes rapidly, creating an additional voltage across the inductance of the connecting conductors. This can add to the voltage limited by the SPD. Connections therefore need the short, direct arrangement specified by the manufacturer. Increasing conductor thickness alone does not eliminate the effect of excessive lead length; conductor cross-section and current-carrying capacity must also be assessed.

Earthing and equipotential bonding are important, particularly for protection paths involving earth. Visible earth contacts on a socket do not prove that the connection is sound; verifying socket earthing is a separate task. An improvised neutral-to-earth bridge must not be used to compensate for a missing protective conductor.

The SPD itself can fail, so its coordination with short-circuit protection must be checked. Any required backup fuse or circuit breaker is selected using the manufacturer's coordination information; some devices incorporate the relevant protection. Selection also considers the TT, TN or IT earthing arrangement, the prospective short-circuit current and the protection modes required.

Which protective functions does an SPD not replace?

Installing an SPD does not remove the need for an appropriate MCB, RCD or RCBO. Overcurrent, residual current and transient overvoltage are different phenomena. Consequently, a breaker remaining closed does not prove that no surge capable of damaging an electronic board has occurred.

Persistent undervoltage, overvoltage or a broken neutral requires separate assessment. An SPD does not regulate the supply like a voltage regulator, or provide backup energy like a UPS. An EMI filter or zero-phase reactor serves an interference-suppression function; its presence alone does not establish surge protection.

External lightning protection and SPDs also have different roles. The external system manages the effects of lightning on the structure, while SPDs form part of the protection for electrical and electronic systems. Having an external lightning protection system does not eliminate the need to assess surge protection inside the building.

Can the same SPD protect solar power and communication circuits?

The DC circuit between solar panels and a solar inverter must be assessed separately from the inverter's AC grid connection. The DC side requires a device suitable for photovoltaic service and the system voltage. An SPD selected for an AC circuit must not be assumed suitable for the DC side. Verify the intended application and connection arrangement in the technical documentation.

Surges can also arrive through communication cables. Connections such as Ethernet and Modbus communication over RS-485 belong in the protection assessment. A data-line protector must suit the signal voltage, data rate, connection arrangement and any PoE supply. A mains SPD in the distribution board cannot, by itself, protect every other cable entering a device.

How can you tell whether an SPD remains operational?

Use the status indication and the manufacturer's maintenance instructions. Many products have a colour-changing window, another local indicator or a remote signalling contact; interpret these according to the specific model. The installation may remain energised after a protective cartridge reaches the end of its service life. Therefore, equipment continuing to operate does not establish that the SPD is healthy.

Replacement is not necessarily required after every surge, but installation date alone cannot establish remaining service life either. Electrical stress, status indications and the manufacturer's inspection requirements must be considered together. When the device indicates that protection has been lost, the appropriate cartridge or complete SPD needs replacement.