SURGE PROTECTION DEVICES

Surge Protection devices are required in installations where sensitive (critical) electronic equipment is being utilized. Transients may occur either in repeatable fashion such as commutation voltage spikes, inductive load switching, etc., or at random such as lightning or from the utility.

There are two major categories of surge suppressors or also called transient suppressors: a) those that attenuate transients, thus preventing their propagation into the sensitive circuits; and b) those that divert transients away from sensitive loads and so limit the residual voltages.

Attenuating a transient, that is, keeping from propagating away from its source or keeping it from impinging on a sensitive load is accomplished with filters inserted in series within a circuit. The filter, generally of the low-pass type, attenuates the transient (high frequency) and allows the signal or power flow (low-frequency) to continue undisturbed.

The attenuation of a Surge Protection Device is stated typically as the amount of reduction it will offer over a frequency range. A typical PQ Global specification could state that the Filter in the SPD offers EMI/EFI Filtering up to –50db from 10KHz-100 MHz. The attenuation is given in a log (non-linear) scale of dB.

Diverting a transient can be accomplished with a voltage-clamping type device. A voltage-clamping device is a component having variable impedance depending on the current flowing through the device or on the voltage across its terminal. With a voltage-clamping device, the circuit is unaffected by the presence of the device before and after the transient for any steady-state voltage below the clamping level*.

Surge Protection Devices are classified by the magnitude of transients they can tolerate. ANSI/IEEE C62.41 provides both manufacturers and users with a guide to determine the tolerance that a Surge Protection Device can be subjected to. They are classified as Category C (for service entrance panels where it must tolerate lightning from outside), Category B (for inside distribution panels where transients are mostly generated by internal switching), and Category A (for outlets).

It is very important that Surge Protection Devices be installed properly in effort-to effectively operate according to specifications. Please see installation instructions on the Home Page.

*Harris Semiconductors. “Transient Voltage Suppression Devices”.