NEC in the Facility, April 6, 2010

April 6, 2010
Don't confuse surge arresters (Art. 280) with surge-protective devices (Art.e 285). The main distinction the NEC makes is the voltage level

Don't confuse surge arresters (Art. 280) with surge-protective devices (Art.e 285). The main distinction the NEC makes is the voltage level: If it's more than 1kV, it's a surge arrester. Otherwise, it's a surge-protective device (SPD).

This isn't an arbitrary distinction. A single device can't handle all voltage levels, due to those pesky laws of physics. Article 280 devices reduce voltages to levels that Art. 285 devices can handle.

Use Article 280 devices where the higher voltage levels are encountered (e.g., on the service drop). These devices reduce large transient voltages, typically by diverting the “peak” of the transient to ground. An example is the spark gap device. The desired power does not pass through this device, but the undesired power does. In a piping system, the equivalent would be a pressure relief valve.

Use Art. 285 devices where the lower voltage levels are encountered (e.g., on a distribution panel). These devices reduce smaller transient voltages by blocking or diverting the “peak” of the transient. Generally, the desired power does pass through these devices — they are in line with the flow of power. In a piping system, the equivalent would be a pressure control valve.

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