Coordinating cascaded surge protection devices: high-low versus low-high

Cascading surge protection devices located at the service entrance of a building and near the sensitive equipment are intended to ensure that each device shares the surge stress in an optimum manner to achieve reliable protection of equipment against surges impinging from the utility supply. However, depending on the relative clamping voltages of the two devices, their separation distances, and the waveform of the impinging surges, the coordination may or may not be effective. Computations with experimental verification of the energy deposited in the devices for a matrix of combinations of these three parameters is provided. Results show coordination to be effective for some combinations and ineffective for some others, which is a finding that should reconcile contradictory conclusions reported by different authors making different assumptions. From these results, improved coordination can be developed by application standards writers and system designers