Also sometimes known as a surge protector, surge suppressor, or surge diverter, a surge protective device or SPD is designed to protect electrical devices or equipment from voltage spikes. If there is any unwanted voltage that goes past a certain threshold, the surge protector will block or short to ground the unwanted voltage. Having the Surge Protective Devices is important because surges can cause damage to electrical equipment, could increase the risk to human life where, for instance, medical equipment is adversely affected, and for businesses, could potentially cause downtime to production and other work-related activities.

A surge is a short-term high voltage impulse, and are sometimes also referred to as spikes, transients, or glitches. Because of the inrush of high voltage, circuits and sensitive electronic equipment can be damaged. Anomalous or abnormal surge events can be caused by external factors such as lightning and utility power surges, but more often than not, these begin internally. These may not be as powerful as those caused by lightning, but they can happen more frequently, and over time, with continued exposure, electrical equipment or device can become severely damaged. SPDs can help prevent this situation from happening, thus minimizing the need to replace valuable and expensive electrical equipment.

Another reason for the importance of SPDs is that they are often required by insurance companies, and when installed, may reduce premiums. The opposite is also true, however, and the lack of SPDs may actually drive up the cost of your insurance premiums.

SPDs can be used at all levels of a power supply network, but they are most often used parallel to the power supply circuit of the load it is intended to protect. They can be used for power supply networks, telephone networks, telecommunication and signaling networks, and other power transmission and distribution systems.

SPDs are designed to have high impedance levels, and once tripped, this high impedance is reduced so that the surge current passes through the SPD instead. The SPD then diverts the current waves away from the protected load and to the earth, or distributes it to other live conductors when in differential mode, thus limiting the voltage to non-hazardous levels.

There are three types of SPDs under the NEC:

  • Type 1: these are permanently connected SPDs which are designed to protect against external surges caused by lightning or utility capacitor bank switching. They are often installed between the secondary of a service transformer and the line side of the service equipment or the service disconnect overcurrent device.
  • Type 2: these are also permanently connected, installed on the load side of service equipment. The purpose of Type 2 SPDs is to protect against residual lighting energy and internally generated surges. These are used as the main protection for low voltage electrical installations and are usually installed in the main distribution switchboard.
  • Type 3: otherwise known as point-of-utilization SPDs, they are installed a minimum of 10 meters from the electrical service panel to the point-of-utilization, or at the terminals of sensitive equipment. These have a low discharge capacity and are used for sensitive loads, as well as a supplement to Type 2 SPDs

The choice of installing a Type 1 or a Type 2 SPD depends on whether or not the building is equipped with a lightning protection system. If it is, Type 1 SPD is installed, if without, a Type 2 SPD is installed. In both cases, Type 2 and Type 3 SPDs are also installed as additional protection. A determination of which types to install, in which locations, and how many SPDs will be installed are determined only after conducting a risk assessment, which is regulated. Type 2 SPDs are often considered mandatory in any case, to be installed in the incoming-end switchboard. SPDs should then be installed at a distance of every 30 meters towards the equipment that is to be protected.