Underwriters Laboratories (UL) requires certain markings be on all listed and recognized SPDs.

SPD Type – Describes the intended location of the SPD in regard to the main breaker
Type 1 – Between the secondary of the service transformer and the line side of the main breaker
Type 2 – Permanently connected SPD on the load side of the main breaker
Type 3 – Point of utilization SPD, like a receptacle, must be within 10 meters (30 feet) of the panel
Nominal System Voltage – Should match the utility system voltage being protected
MCOV – The Maximum Continuous Operating Voltage, this is the most voltage allowed before the device starts to clamp. Typically 15-25% more than the nominal system voltage
Nominal Discharge Current – The peak value of the current through the SPD having an 8/20 waveform where the device is still functional after 15 surges. The manufacturer chooses which level to test: 5 kA, 10 kA, 20 kA
VPR – The Voltage Protection Rating, which is the average limiting voltage of an SPD when subjected to a 6 kV, 3 kA 8/20 combination waveform generator. VPR is a clamping voltage measurement rounded up to one of the standardized table of values. All VPR tests are conducted with 6-inch leads outside of the SPD enclosure
A short circuit current rating (SCCR) is the amount of fault current to which an SPD can be subjected and still safely disconnect from the power source
Enclosure Type – NEMA rating for environmental conditions and locations where the SPD can be installed

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