Choosing between 3-pole and 4-pole transfer switches
There are many design choices to make when planning a backup power system. But perhaps one that isn’t understood as well as it should be is whether to specify a 3-pole or 4-pole automatic transfer switch (ATS). At the heart of this choice is one simple consideration: whether or not your emergency power system will be separately derived.
On systems using a 3-pole ATS, the neutral is continuous through the entire system. This is known as a solid neutral, and it’s bonded to ground at only one location: the facility’s utility service entrance. If there is a ground fault when the loads are on the emergency source, the fault current travels through ground to the bonding point at the service entrance, then back to the emergency source on the neutral.
On systems using a 4-pole ATS, each source’s neutral is bonded to ground at its source, so each source is considered to be “separately derived.” Regardless of which source the customer load is switched to, if a ground fault occurs, the fault current will travel through ground directly back to the source that is presently supplying the loads. This is known as a switched-neutral system, and the neutral switching can be open or overlapping (closed).
According to the 2011 National Electrical Code (NEC), a separately derived power system is: “a premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct connection from circuit conductors of one system to circuit conductors of another system, other than connection through the earth, metal enclosures, metallic raceways, or equipment grounding conductors.”
The NEC (and many other local codes and standards) provides the system designer with guidance to determine whether the emergency system must be a separately-derived source. It’s important to consult the appropriate code when planning your system to ensure there is clarity regarding the need to have the emergency system as a separately derived source.
Understanding ground faults
The grounding for the emergency system and the ground-fault protection scheme is what determines if a 3-pole or 4-pole transfer switch should be selected. Ground-fault protection is a complex topic. But one simple way to see how it affects a system is to model the three phases as one phase and assume that all the current produced by a source, a transformer, or a generator returns to its point of production along the neutral line.
First, consider the example in Figure 1. The current produced by the transformer leaves along the phase line, does its work at the load, and then returns to the transformer along the neutral. The switchgear shown in Figure 1 is the service entrance for the facility, and based on its current and voltage ratings, the NEC requires that this service entrance have ground-fault trip (indicated as GF). In this example, because all phase current flowing through the ground-fault sensor equals the current returning on the neutral, the algebraic sum of the current flow through the ground-fault sensor equals zero and there is no ground fault for the sensor to detect.
Next, consider the example in Figure 2 in which a ground fault is introduced at point A. The current will leave the phase, but at point A, it will leave and return to its source along the ground. It will find its way back to the neutral at the neutral-to-ground bond at the service entrance (shown as point B). Because the neutral-to-ground link is on the source side of the ground-fault sensor, the ground-fault sensor registers only the outgoing phase current and cannot detect any current returning on the neutral. Therefore, the algebraic sum of the current flow through the ground-fault sensor equals the outgoing phase current only.
And if the algebraic sum of the current flow through the ground-fault sensor is greater than the ground-fault trip setting, the ground-fault sensor will trip its associated breaker.
Using a 3-pole transfer switch
Next, consider adding a 3-pole ATS and a generator to the aforementioned simple circuit (see Figure 3). Because there is now a 3-pole ATS in this system, the neutral is continuous and the generator is not considered to be a separately-derived source. There is no neutral-to-ground link at the generator. The only ground connection at the generator will be the equipment ground for the generator.
In Figure 3, note how normal current flows while the system is operating on generator power. Because the generator is producing the power, the current flows out of the generator phase, does its work at the load, and returns to the generator along the neutral. Again, the algebraic sum of the current flowing through the generator’s ground-fault sensor equals zero, and there is no ground fault.
Now consider what happens if there is a ground fault at point A in Figure 4. The current will leave at point A; however, it needs to find a way back to the generator (along the neutral). Its only option is to flow along the ground to return to the system at the neutral-to-ground bond at the service entrance (shown at point B).
When back in the system, the ground-fault current will flow along the neutral, through the solid neutral in the ATS, and back to the generator. As with normal current flow, the algebraic sum of the current through the generator’s ground-fault sensor equals zero. This means that the ground fault is not being picked up by the ground-fault sensor on the generator. The ground-fault sensor in a generator is often integrated into the circuit breaker, so in this case, the breaker will not open during a ground fault. In fact, the ground fault may not be correctly sensed by the system until the ATS returns to utility power. However, it might be seen at the normal-source breaker, causing the breaker to trip, even though the fault is not fed from the normal source. As previously mentioned, because the neutral is continuous in a 3-pole setup, the generator is not a separately-derived source and, therefore, there is no neutral-to-ground link at the generator.
Using a 4-pole transfer switch
For the generator’s current-based ground-fault sensor to detect the ground fault (and trip the associated generator-mounted circuit breaker), a system using a 4-pole transfer switch is required. In this case, because the neutral is switched with the phases, the generator is a separately-derived source and must have its own neutral-to-ground link. With this link in place, the sensor will be able to detect the ground fault from the previous example.
Consider what happens during a ground fault when a system is operating on generator power and a 4-pole transfer switch is installed (see Figure 5). As with previous examples, consider what happens if there is a ground fault at point A. As before, the current must return to the generator along the neutral.
Contrary to the example shown in Figure 4, however, the neutral in the ATS is open between the utility service entrance and the generator. This means the ground-fault current cannot return using the utility service entrance neutral-to-ground link (as it did with the system with the 3-pole ATS). Instead, the current returns to the generator through its neutral-to-ground link (at point B in Figure 5). Because the generator’s neutral-to-ground link is between the source (the generator) and the ground-fault sensor, the algebraic sum of the current flow through the ground-fault sensor equals the phase current only. And because the algebraic sum of the current flow through the ground-fault sensor is greater than the ground-fault trip setting, the ground-fault sensor will trip its associated breaker.
While there are many factors that determine whether to use a 3-pole or 4-pole transfer switch, it should be emphasized that in systems with multiple ATSs, it is important to stick with one or the other neutral switching schemes. In other words, all the transfer switches serving 3-phase, 4-wire loads should be of the same type-either all 3-pole or all 4-pole. This is essential for maintaining the integrity of the ground-fault scheme.
When multiple generators and paralleling switchgear are used, the same rules for determining the use of 3-pole vs. 4-pole transfer switches should be applied. If the emergency power system is a separately-derived source, then a neutral-to-ground link may be in place at each generator, or there may be a single neutral-to-ground link in the paralleling switchgear. Above all, remember to consult with a proven supplier. There’s a famous expression among carpenters that you should “measure twice and cut once.” The same applies here. If you get your system design right at the planning stage, then you’ll encounter fewer problems in the operational stage.
Mike Pincus is the manager of business development for Kohler Power Systems-Americas. He was previously the manager of the switchgear engineering department and has worked for the company since 1995. Pincus holds a BS in Electrical Engineering from the University of Wisconsin-Madison and an MBA from the University of Wisconsin-Milwaukee. He is a member of IEEE and a registered professional engineer in the state of Wisconsin.