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Introduction Any instrument power distribution panel should have some transient protection. See Figure 3-1 or 3-2. Protector usefulness A protector is useful under the following conditions: • Should an ac feeder develop a short circuit, its circuit breaker may not open until peak currents have reached 10,000 amperes or more. The sudden interruption of such a large current when a circuit breaker opens injects a severe transient into the rest of the electrical system. • HPM servicing may require that the Power Supply Modules be turned off and on. Even at normal load currents, significant transients may be generated in the distribution panel. • Lightning may strike the facility power feeder and send significant transients into the instrument system. Power Supply Module performance The HPM Power Supply Module is rated to perform to all its specifications while handling a variety of transients, such as a 3 kV impulse for 8 x 20 microseconds. This provides a safety factor to allow for feed through when the surge protector operates. MOV protector A Metallic Oxide Varistor (MOV) is the preferred power line protector. Compared to a protector based on a spark gap, the MOV protector does not short circuit the power along with the transient. Use a 150 kA unit. Overcapacity here does not carry a penalty. A suitable protector can be purchased from Lightning Protection Corporation in Santa Barbara, CA at telephone number 805-967-5089. For a 120/240 Vac system, use Model 20208.