Selection of Surge Protective Device (SPD)- (Part 1)



  • A device which diverts or limits surge current is called Surge protective devices (SPD).
  • SPD protect electrical equipment against over voltages caused by lightning or Switching. It is wired in parallel to the equipment which is needed to be protected.
  • Once the surge voltage exceeds SPD’s rating it starts to conduct energy directly to the electrical grounding system. An SPD has a very low resistance during this time and give low resistance path the energy to ground. Once the surge is over it gives high resistance path to current.
  • SPD is previously known as Transient Voltage Surge Suppressors (TVS) or Secondary Surge Arresters.
  • Underwriter laboratories ,UL 1449 Listed SPDs are now designated as either Type 1, Type 2 or Type 3 and intended for use on AC power systems rated Less than 1000vrms


  • SPD is used to limit transient over voltages of atmospheric or Switching Surge and gives path to the excessive current to earth hence limit the overvoltage to a value that is not hazardous for the electrical installation.

Causes of Surges:

  • (1) External Surge:
  • lightning strikes :Direct Stroke , Indirect Stroke
  • (2) Internal Surge:
  • Switching Surge:
  • Switching on/off of inductive loads.
  • Tripped circuit breakers and fuses.
  • Short circuits.
  • Malfunctions caused by the power company.
  • Insulation Failures:
  • Arcing Ground:
  • Ignition and interruption to electric arc.

Difference between Surge arrestor (Lighting Arrestor) and Surge Suppressor:

  • Surge arresters and Surge Suppressor both are used to protect equipment from surges. But, there is confusion between the application of surge arrestors / Lighting arrestor and surge suppressors.
  • The main differences between a lightning arrester and a surge arrester are its fault clearing time and it’s position
  • Both are doing the same job, but still both are not same.

Lighting Arrestor / Surge Arrestor:

  •  Surge Arresters are widely also known Lightning arresters.
  • Surge arresters are devices installed on Over head lines, substations etc to avoid a Lighting surge and other Surges of an additional current/ voltage/charge due to various faults occurring.
  • In the past year when nonlinear / solid-state devices (computers, PLC and drives) were not used. The Electrical Load is mostly Linear Load. Utility companies and end users were concerned with how to protect electrical distribution systems from lightning surges to ensure that voltage surges did not exceed the basic insulation level (BIL) of the conductor wires, transformers and other equipment.
  • Hence Surge arrestors / Lighting arrestors were developed for use in low, medium and high voltage applications at various points in the transmission and distribution system.
  • Surge Arrestor provide low resistance path between the phase conductor and ground. LA did not concern with the loads if it cleared within a few cycles.
  • Arrestors are still used in the electrical industry primarily along the transmission lines and upstream of a facility’s service entrance.
  • Arrestors are available in various classes depending upon their withstand capability (e.g., station vs. distribution class). At the service entrance location on low voltage systems (600V and below), Lightning arrestors were designed to protect the electrical distribution system and not the sensitive solid-state equipment.
  • Economically, surge arresters are better than surge Different surge arresters are available based on their withstanding capability. The main problem with them is that they are designed for protecting large electrical distribution systems from lightning surges, and not for sensitive solid state equipment.
  • Applications: The surge arrester is best to protect insulation of transformers, panel boards, and wirings. However, it doesn’t work well for solid state components.

 Surge Suppressor / Surge Protector (called TVSS):

  •  In today’s we mostly use solid-state (nonlinear) loads like electronic equipment, drives, PLCs, computers, electronic ballasts, telecommunication equipment. Non Linear is about 70% of utility loads. The solid-state components will be damaged by the surges.
  • Using Surge suppressors at the service entrance and key branch panels, the surge will be effectively reduced to under 100V.
  • If a TVSS and lightning arrestor are both used at a service entrance switchboard, the TVSS will “turn on” earlier and shunt most of the surge current. Many water-treatment plants, telecommunication facilities, hospitals, schools and heavy industrial plants utilize TVSSs instead of surge arrestors to provide protection against the effects of lightning, utility switching, switching electric motors.
  • Applications: They are used in water treatment plants, hospitals, schools, and telecommunication facilities.

 Size of Surge Protection Device (SPD) does not depend on Panel Size:

  • The kA rating of an SPD (surge rating) is one of the most misleading terms. We normally use 50KA SPD to protect 50KA panel.
  • The kA rating of the surge arresters has nothing to do with the fault current rating of electrical distribution board. We can fit a 40kA surge arrester in a domestic board with a fault current rating of less than 5kA
  • When a surge enters a panel, it does not know the size of the panel. So It is totally miscalculation for use 50KA SPD for 50KA Panel
  • There is a normal Practice that larger panels need larger SPD, but surges are indifferent to panel size.
  • The largest surge that can enter a building’s wiring is 10kA, as explained in the IEEE C62.41 standard. So why would we need a SPD rated for 100KA or 200kA.