Per Mode and Per Phase Ratings of SPD

                                                     Per Mode and Per Phase Ratings of SPD

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. How do 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:

Each is shown in Figure 1.

Many three-phase applications use SPDs that offer complete line-to-neutral, line-to-ground, and neutral-to ground pathways, for a total of seven modes of protection. This arrangement is shown in Figure 2. SPDs that also provide line-to-line pathways offer 10 modes of protection. This brief will use seven-mode SPDs in subsequent examples.

Per Mode Rating Defined

An SPD’s per mode rating is based on the total amount of energy it can shunt from one circuit conductor to another. If an MOV capable of shunting a 50 kA of current is used in each pathway, then the per mode rating of the SPD in Figure 3 is 50kA.

If multiple MOVs are used between the same conductors, then the per mode rating will be the sum of the capacity of the MOV’s used in these pathways. Figure 4 shows a seven-mode SPD with three 50 kA MOVs installed between each pair of conductors. The per mode rating of this MOV is 150kA.

Per Phase Rating Defined

A different way to rate an SPD is to state the total capacity of the protective components serving each of the three phase conductors. Using the same seven-mode SPD with 50 kA MOVs, Phases A, B, and C are each served by two MOVs (one to neutral, one to ground). The per phase rating for the same SPD is 100 kA, as shown in Figure 5 below. Applying the same rating scheme to the SPD in Figure 4 above produces a per phase rating of 300 kA because each phase conductor is served by six 50 kA MOVs.