The Edco H modules are ultra high-speed fused solid-state secondary protectors for KEY/PABX systems. The units are available in three different clamping voltages. The Edco H-Series is constructed with solid state silicon foldback technology, polyswitch resettable fuses — PTCs — and failure fuses. These protection modules are intended to be used in conjunction with proper primary protectors (UL 497A). The protective elements switch from the normal high-impedance state to a low-impedance state upon a detection of a surge, then switch back to high impedance after the surge has passed. The Edco H-Series single pair modules fit onto a standard Edco MI-50 split block and replace bridging clips. In addition, these modules require the Edco MGB Ground Rail for each Edco MI-50 block used.