Simplified – SPD Per Mode and Per Phase Ratings
To mitigate the effects of transient overvoltages, Surge Protective Devices (SPDs) are used throughout electrical distribution systems, such as at service entrances, transfer switches, and downstream panelboards. Manufacturers state surge capacity ratings as either Per Mode or Per Phase. This paper describes how these terms apply to SPDs used in three-phase, four-wire Wye systems.
SPD Modes Defined
In the context of SPDs, the term mode refers to the types of pathways available for shunting overvoltages. These pathways are most commonly formed by bridging two conductors through a Metal Oxide Varistor (MOV). These components are non-conductive at nominal circuit voltages, but become conductive when higher voltages are present. When that occurs, the varistor shunts excess voltage from the conductor of higher potential to the conductor of lower potential.
In a three-phase four-wire systems, four pathways are possible:• Line-to-Neutral
Each is shown in Figure 1.