Surge trap Surge Protection Devices or SPDs are protection devices to absorb high-energy power spikes that could damage sensitive electronic equipment such as process controllers, instrumentation, and computers. They divert high-energy power away from an appliance by providing a low-impedance path to the common point earth ground. Frequently, panel boards use several metal oxide varistors or MOVs, connected in parallel, to act as surge suppressors or traps.
AC surge suppressors most commonly use an MOV comprising solid-state zinc oxide having multiple junctions. When conducting, MOVs offer a low impedance path, and come in packages for specific voltage and current handling capacities. Surge suppression devices found in DC applications mostly are single junction diodes and/or gas discharge tubes that ionize at preset voltages.
Installation of most AC surge traps are typically at the entrance to a utility service for protecting the entire facility, in distribution switchboards and panel boards for the protection of sensitive loads downstream, and/or in wall outlets for protecting an individual and specific piece of equipment such as a solid-state controller or a computer.
NEMA standards define the surge current capacity of a surge trap as the maximum level of current it can withstand for single transient event. The level indicates the protection capacity of the surge suppressor.
The suppressed voltage rating (SVR) or the clamping voltage of a surge trap is the voltage it permits passing on to the attached load during a transient event. The ability of the surge trap to attenuate a transient is its performance measurement and the clamping voltage provides this. The Underwriters Laboratories or UL confirms the clamping voltage during tests it conducts while evaluating surge traps.
The short circuit rating and the surge current capacity of a surge trap are the criteria a user should consider while selecting a device for its performance and safety. The user should make sure the surge device they have selected is not fuse limited, as many manufacturers use fuse limiting in front of the device for passing UL testing conditions.
Installing a surge trap SPD is always in parallel with the load. The surge trap SPD remains idle and does not conduct when the operating voltage is within the normal levels. The SPD turns on whenever the system experiences an overvoltage and conducts the extra current to the ground, while allowing the load to experience the correct voltage. The performance of a pressure relief valve in a steam system offers an operational similarity.
It is easy to retrofit an existing panel with a surge trap SPD, provided the panel has adequate space. Typical control panels in industries have a mains disconnect feeding a power distribution block, which then connects to individual loads. Users can mount the surge trap SPDs on the standard 35-mm DIN-rail that the panels typically use.
Manufacturers recommend mounting surge trap SPDs as close as possible to the power distribution block with #8-#14 AWG wires, not exceeding a length of 20 inches. Users must make sure of not twisting any wires together, and of not forming any loops, as these can result in higher voltages that the SPD let through.
