Surge diverters, or surge protection devices (SPDs), are intended to protect electrical systems from surges by safely diverting them to earth. Under normal conditions, these devices are dormant, acting as high-impedance components that do not affect the electrical system in any way. When a surge strikes, the device switches to a low-impedance mode, allowing the surge to pass harmlessly to earth, thereby protecting the electrical system from damage. Lightning arresters, on the other hand, are specialized devices that are designed to handle the very high energy levels associated with direct or indirect lightning strikes. They offer a low-impedance path for the lightning currents to earth, thereby protecting the electrical system from destructive overvoltages.

What Causes Surges in LV Systems?

In LV systems, surges can be caused by two different sources. The first source is lightning. Lightning can cause high-energy spikes in electrical systems even if the system is not directly struck by lightning. Lightning produces a rapidly changing electromagnetic field, and when this field encounters conductors, it causes voltages that can be much higher than the system’s nominal voltage. This is especially true in areas of Australia that are susceptible to thunderstorms or in rural areas where overhead power lines are more prevalent. The second source of surges is switching or network disturbances. These can be caused by events such as the switching on of large inductive loads, fault clearing by protection systems, or changes in the distribution network. Although switching surges generally have lower energy levels than lightning-induced surges, they are more common and can contribute to degradation over time.

How Are Surge Diverters and Lightning Arresters Different?

While the terms surge diverter and lightning arrester are sometimes considered interchangeable, they each have their own specific role. Surge diverters are intended for general surge protection, such as from internal switching or induced lightning effects. Lightning arresters are designed for the high-energy impulses of direct lightning strikes. In contemporary Australian low voltage systems, it is not uncommon to find devices that incorporate both functions into one unit, providing a comprehensive level of protection against both general surges and lightning.

How Do Surge Protection Devices Work?

The functioning of these devices is quite simple and efficient. Metal Oxide Varistors (MOVs) are the most widely used components in SPDs. MOVs have nonlinear resistance and are conductive only above a certain voltage level. When a surge is detected, the MOV turns conductive, and the surplus energy is directed to earth. Gas discharge tubes are commonly used along with MOVs, especially in devices that protect against high-energy lightning surges. These tubes ionize when subjected to high voltage, forming a low-impedance path for the surge current. After the surge, both MOVs and gas discharge tubes return to their high-impedance state, ready to face another surge.

What Types of Surge Protection Are Used in LV Installations?

In Australian installations, surge protection devices are generally divided according to the amount of energy they are meant to protect against and their position within the electrical system. Type I surge protection devices are generally used to protect against high-energy surges caused by direct or indirect lightning strikes. These devices are normally positioned at the point of main electrical entry as the first line of defense. Type II surge protection devices, also known as general surge protection diverters, are designed to protect against surges that originate from the power grid or switching events. These devices are normally positioned at the distribution boards or sub-boards to protect the downstream circuits. Type III surge protection devices are point-of-use protection devices that are normally positioned near sensitive equipment such as computers or telecommunication devices as a final line of defense against low-energy surges.

Where Should Surge Protection Devices Be Installed?

The positioning and connection of surge protection devices are very important for their successful operation. The positioning of a Type I surge protection device as close as possible to the service entry will keep the length of the exposed conductors short and ensure maximum diversion of the incoming surge current. The Type II surge protection devices are normally positioned at the sub-distribution boards to offer localized protection, while the Type III devices are positioned close to the sensitive equipment to ensure that even a small surge does not cause any disturbance. The earthing and equipotential bonding of these devices are very important for their safe operation, as they depend on a low-impedance path to earth for their operation. The lead lengths of these devices should be kept as short as possible to avoid inductive voltage drops during surge currents. The surge protection devices should also be protected by overcurrent devices like fuses or circuit breakers to avoid damage in case of faults.

How Do You Choose the Right Device for Australian LV Systems?

Choosing the appropriate surge protection device requires a number of important factors. The continuous operating voltage (Uc) is the maximum voltage level that the device can tolerate without acting. For a 230-volt single-phase network, a Uc rating of 275 volts is adequate, but for 400-volt three-phase networks, a Uc rating of at least 440 volts is required. The maximum discharge current (Imax) shows the highest surge current that the device can handle, while the nominal discharge current (In) shows the device’s capacity to handle repeated surges over time. The device’s configuration must correspond to the supply configuration, with single-phase networks requiring single or two-pole devices and three-phase networks requiring three or four-pole devices depending on whether neutral protection is required. Modern surge protection devices are equipped with status indicators that enable maintenance personnel to check the status of the device following a surge event.

Do You Really Need Surge Protection in Australia?

Although surge protection is not generally obligatory in Australia, it is highly recommended in areas with high lightning activity, where overhead power lines are prevalent, or where critical electronic equipment is employed. The Australian Wiring Rules (AS/NZS 3000:2018) offer advice on the installation of SPDs and recommend their use in circumstances where the risk of lightning strikes is high or where the potential risk of equipment failure is significant. A risk assessment might take into account the local lightning strike rate, the nature of the supply, and the level of risk associated with the protected loads. In rural areas, where overhead power lines are prone to lightning strikes, more powerful devices are warranted, whereas in urban areas with underground power supplies, devices with lower energy ratings may be sufficient.

How Should Surge Protection Be Maintained and Tested?

Maintenance and testing are also crucial aspects of surge protection. SPDs will deteriorate with time, especially if they have been subjected to a series of surges. It is common practice for many SPDs to have visible or remote indication of when they need to be replaced. It is also important to ensure that the SPDs are inspected during routine electrical maintenance.

What Are Common Misconceptions About Surge Protection?

It should be noted that there is no SPD that can protect against damage from a direct lightning strike. Whole-structure lightning protection systems may be required in such cases. In addition, plug-in surge protectors that are often used to protect home electronics against surges do not provide structural protection and cannot substitute for a well-coordinated SPD installation at the switchboard level. High-quality SPDs, despite their cost, are a cost-effective investment because the protection they provide can save expensive damage to sensitive equipment.

How Is Surge Protection Applied in Australian Installations?

Domestic applications in urban settings can be protected using Type II devices with ratings of 20 to 40 kiloamperes, whereas rural or storm-susceptible domestic applications may need the use of combined Type I+II devices rated between 65 to 100 kiloamperes. Commercial establishments usually involve the use of Type I+II devices at the service entrance, in addition to the use of Type III devices near sensitive electronics. Industrial applications involve the use of multi-stage protection schemes, which involve the coordination of Type I, Type II, and Type III devices.