Surge Protection Devices — Types / Role / Importance

With the expansion of electric power in today's world, electrical power systems are exposed to many critical conditions like overvoltages and voltage surges due to lightning strikes or abrupt switching conditions. The main concern is the overvoltage condition which can cause major damage to the system's equipment. Therefore, it is necessary to install a device that guarantees protection from the condition where over or under-voltage occurs.

 A surge protector is a protective device that is connected to protect a system against overvoltage. It is the component of the electrical protection system and is used to protect equipment in power transmission and distribution systems. These devices, in general, protect the electrical equipment against voltage transients.

Surge Protective Device:

Surge Protective Devices are the devices that are used to protect the system from voltage spikes. It is an umbrella term that is used to represent any protective device used for overvoltage protection. SPD is designed to limit transient overvoltage and divert current waves to earth, to limit the amplitude of this overvoltage to a value that is not dangerous for the electrical installations and switchgears.

The term surge protection device (SPD) is used to describe electrical devices typically installed in power distribution panels, process control systems, communications systems, and other heavy-duty industrial systems, for protection against electrical surges and spikes, including those caused by lightning. 

​The surge protective devices are categorized as:

• Surge Arresters
• Surge Protectors

Principle:

 According to the National Electrical Code (NEC), a surge arrester is defined as: "A protective device for limiting surge voltages by discharging or bypassing surge current, and it also prevents the flow of current while remaining capable of repeating these functions".

Surge Arresters VS Surge Protectors:

​Surge protectors and surge arresters are used for the same work, i.e., protecting equipment from surges. However, many people are confused about their applications. This problem occurs especially in industrial facilities, water treatment plants, and some other important areas.

Surge Arresters:

 Surge arresters are usually installed in the substations to protect the equipment by eliminating the effects of lightning and switching over-voltages.

Surge Protectors: 

The main job of a surge protector system is to protect electronic devices from "surges". A surge protector attempts to limit the voltage supplied to an electric device by either blocking or shorting current to reduce the voltage to a safe threshold.

How does a Surge Protector work?

A surge protector allows the electrical current to flow from the outlet to a number of electrical and electronic devices plugged into the power strip. If the voltage from the outlet surges or spikes rises above the accepted level, the surge protector diverts the extra electricity into the grounding wire. 

In most of the surge protectors, Metal Oxide Varistor (MOV) are used to divert the extra voltage.

Types of Surge Protection Device:

According to the standards, surge protective devices are classified into three different types:

• High voltage SPD
  
• Medium voltage SPD
  
• Low voltage SPD

Low voltage SPDs do not limit the voltage like high voltage and medium voltage SPDs. Low voltage surge arresters are further divided into three classes:

Type 1 SPD: This type of SPD is used in industrial buildings to protect insulation levels against external surges caused by lightning. They can be installed between the secondary side of the utility transformer and the line side of the main service equipment overcurrent protective device, as well as the load side of the main service equipment. It protects the system from direct lightning strokes.

Type 2 SPD: The type two of low voltage SPDs is normally installed on the load side of the main service equipment overcurrent protective device. These surge devices may also be installed at the service entrance point but should be installed on the load side of the main service overcurrent protective device. These types of SPDs prevent the spreading of overvoltage to the installations and protect the system from damage.

Type 3 SPD: These types of SPDs are usually installed after the main breaker and are used as a complement for type 2.

Electrical Surges: how do they occur?

The most reoccurring cause of surge is lightning. During a lightning storm, it may strike somewhere near a power source and affect the voltage running through it. When a lightning surge hits an electrical system, it damages the devices connected to the system, resulting in loss of efficiency.

Electrical devices work at a specific voltage range. When these devices obtain a voltage higher than the specified voltage necessary for their operation, they get damaged. However, the electrical systems that are protected by a surge arrester do not get damaged because the arrester guarantees the safety of the electrical system bypassing excessive voltage to the ground.

The surge arrester does not absorb all the high voltage that passes through it, but it diverts it to the ground to minimize the effect of the voltage. It works with Metal Oxide Varistor (MOV). MOV is mainly a semiconductor that is extremely sensitive to voltage. MOV functions as an insulator under normal voltage. At high voltage, it works as a conductor as well as a switch that remains open under normal AC voltage and closes when high voltage is passed. 

How does a Surge Arrester Work?

A surge arrester is connected in parallel with the equipment that needs to be protected. These surge arresters limit the over-voltages occurring in the equipment. The energy associated with the surge is transferred to the ground by the arrester, protecting the equipment eventually.

The highly non-linear characteristic of an arrester allows it to limit the voltage across its terminal nearly a constant value over a wide range of arrester currents. The voltage across the equipment to be protected is almost the same as the voltage across the arrester.

A surge arrester usually contains a ground terminal as well as a high-voltage terminal. When an electric surge occurs, the surge arrestor sends the high voltage current directly to the insulation or to the ground to prevent the system damage.

To eliminate insulation failure, the arrester should be applied properly so that the equipment insulation isn't exposed to over-voltages. It is important to select the arrester parameters correctly in order to avoid problems in the system.

Importance of Surge Arresters:

A surge arrester protects the equipment from surges or transient voltages in electrical power systems, occurring from lightning or a switching surge. Not only does it operate the extra voltage into the earth wire, but it also allows the normal voltage to continue its path.