Surge Protective Device (SPD) is a device used to protect electronic equipment from power surges or transient voltages. They are connected in parallel with the load power circuit that needs protection and can also be used in power supply networks at all levels. This article will delve into the working principle of surge protective devices and their important role in electrical systems.

What are surges?

Surges are transient over voltages that can reach tens of kilovolts with durations in the order of microseconds. Despite their short duration, the high energy content can cause serious problems to equipment connected to the line like premature aging of electronic components, equipment failure or disruptions to service and financial loss.

Origin of surges

Lightning is known to be the most significant source of surges—bolts have been recorded to have a million to a billion volts and between 10,000 to 200,000 amps. However, lightning only makes up a portion of all transient events in a facility. Because transients can originate from both external sources (like lightning) and internal sources, facilities ought to have both a lightning protection system and surge protection installed.

Lightning: The most destructive source of surge. Based on the IEC 61643-12 standard, energy from lightning can reach up to 200 kA. However for reference, estimates indicate 65% are less than 20kA and 85% are less then 35kA.

Induction: Sources include cloud to cloud lightning or nearby lightning impacts where the current flow induces an over voltage on supply lines or other metallic conductors.

There is no way of really knowing when, where, the size, or the duration/waveform of a surge. Therefore within the Standards some assumptions have been made and 2 main waveform have been chosen to simulate different surge events:

- Conduction

  Conduction or 10/350μs simulates energy from lightning direct impact.

- Induction
  Induction or 8/20μs simulates energy from indirect lightning impact.

Internal sources:

- They come from utility grid switching, disconnection of motors or other inductive loads. Energy from these sources is also analyzed with the 8/20 wave form.

- Transient over voltages do not occur solely in power distribution lines, and are also common in any line formed by metal conductors, such as telephony, communications, measurement and data.

The Role of Surge Protective Devices

Surge protective devices control transient voltages by diverting or limiting surge currents, protecting sensitive electronic equipment connected to them, such as computers, televisions, washing machines, and safety circuits (such as fire detection systems and emergency lighting). These devices contain sensitive electronic circuits that are prone to damage from transient overvoltage; thus, surge protective devices play a crucial role in protecting electrical installation systems.

Without proper SPD, transient events can damage electronic equipment and lead to costly downtime. Therefore, the importance of surge protective devices in protecting electrical equipment cannot be overstated.

How does a SPD work?

There is at least one non-linear component of the SPD, which under different conditions, transitions between a high and low impedance state. At normal operating voltages, the SPDs are in a high impedance state and do not affect the system. When a transient voltage occurs on the circuit, the SPD moves into a state of conduction (or low impedance) and diverts the transient energy and current back to its source or ground. This limits or clamps the voltage amplitude to a safer level. After the transient is diverted, the SPD automatically resets back to its high impedance state.

The working principle of the surge protector is as follows:

- Normal Operation

  In the absence of a surge, the surge protection device has no effect on the system it is installed in. It acts as an open circuit, maintaining

  isolation between live conductors and ground.

- During a Surge
  When a voltage surge occurs, the surge protection device will reduce its impedance in nanoseconds and divert the surge current. At this

  point, the SPD behaves like a closed circuit, short-circuiting the overvoltage and limiting it to values acceptable to the electrically connected

  downstream equipment.

- Post-Surge
  Once the pulse surge ceases, the surge protection device will restore its original impedance and return to the open circuit state, continuing to

  monitor the voltage conditions in the electrical system.

3P or 4P? When is the N-PE pole required?

Surge Protection Devices (SPDs) are installed in parallel upstream from electrical equipment in a position such that, during any excessive voltage event, the SPD will act as a low-impedance path to earth. This channels the high voltage energy away from the downstream equipment before its voltage withstand rating is exceeded thus avoiding damage.

A common inquiry regarding SPDs is the distinction between the application of 3 pole and 4 pole devices. In the case of TN-C-S wiring systems, the neutral conductor is directly connected to earth (MEN link). Should an SPD be installed within 10 meters of this MEN link, only a 3 pole device is required. The additional N-PE pole provided by 4 pole devices is made redundant in this situation as there is already a path to earth through the neutral via the MEN link.

However, if an SPD is installed further than 10 meters from a MEN link, a 4 pole SPD is required. As the impedance to earth increases with cable length, a surge energy now has the potential to enter the network after the MEN link and damage the downstream equipment.

Classification of protectors

Protection devices are classified into types according to discharge capacity.

Type 1:
■ Tested with a 10/350 μs waveform (Class I test), which simulates the current produced by a direct lightning strike.
■ Ability to discharge very high currents to earth, providing a high Up - voltage protection level.

■ Must be accompanied by downstream Type 2 protectors. Designed for use in incoming power supply panels where the risk of lightning strike

   is high, for example in buildings with an external protection system.

Type 2:
■ Tested with a 8/20 μs waveform (Class II test), which simulates the current produced in the event of a switching or lightning strike on the

   distribution line or its vicinity.
■ Ability to discharge high currents to earth, providing a medium Up - voltage protection level. Designed for use in distribution panels located

   downstream of Type 1 protectors or in incoming power supply panels in areas with low exposure to lightning strikes.

Type 3:
■ Tested with a combined 1.2/50 μs - 8/20 μs waveform (Class III test), which simulates the current and voltage that can reach the equipment

   to be protected.
■ Ability to discharge medium currents to earth, providing a low Up - voltage protection level. Always installed downstream of a Type 2

   protection designed to protect sensitive equipment or equipment located more than 20m downstream of the Type 2 device.

SPD features based on the IEC 61643 standard

Protector parameters:

- Up Level of protection: Maximum residual voltage between the terminals of the protection device during the application of a peak current.

- In Nominal current: Peak current in 8/20 μs waveform the protection device can withstand 20 times without reaching end of life

- Imax Maximum discharge current: Peak current with 8/20 μs waveform which the protection device can withstand.

- Uc Maximum continuous operating voltage: Maximum effective voltage that can be applied permanently to the terminals of the protection

  device.

- Iimp Impulse current: Peak current with 10/350 μs waveform which the protection device can withstand without reaching end of life.

Where to start the protection design?

As the origin of the installation, the main switchboard is the place to start the design of SPDs on the network.

How to start the protection design?

As previously stated, the SPD protection design does not depend on the fault ratings given by the transformer it only depends on the level of exposure in front of surge. So, what SPD do we have to install in the main switchboard?

See the diagram above from IEC 63205-1 standard which displays the dispersion of the highest lightning considered: 200kA @ 10/350μs.

In the worst case scenario, 50% of this energy is conducted away to earth leaving 100kA potential across the networks 3 phase and neutral.

Here a 25kA @ 10/350μs (Iimp) Type 1 SPD is highly recommended for cases when a lightning strikes on or close to the building’s earth connection – in particular when a building has a lightning rod.

In the “Normal Scenario” it is assumed any direct lightning strike to the network will be at such a distance from the installation that another 50% of the energy is dispersed to earth via other conductors before entering your point of connection. In this scenario a device with a 12.5kA @ 10/350μs (Iimp) Type 1 is recommended. Furthermore, based on the IEC 61643-12 standard, 12.5 kA is the minimum kA rating when a Type 1 is needed.

If the level of exposure of the installation is lower than above described scenarios Type 2 SPD (Imax) may be considered along with risk and cost of equipment and downtime.

Do I need to install a third stage of surge protection devices?

A third stage of surge protection installed at the final load may be considered depending on what loads it, how critical, expensive, cost of downtime and sensitive it is. If the cost of the equipment and/or downtime is high then installing a third stage Type 3 (1.5/50μs) device will further reduce the risk of any last surge energy getting to your equipment.

Examples of applications that should include a 3rd stage of surge protection are:

■ Hospitals
■ Data Centers
■ Airports
■ Banking and Insurance
■ Transportation