Which method is best for switching the neutral: 3-pole transfer switches with overlapping neutral contacts or 4-pole switches? The answer may surprise you.
You want to preserve the integrity of your ground fault protection system without compromise, so what do you do when it comes to switching the neutral? Among many benefits, you've heard 3-pole transfer switches with overlapping neutral contacts operate with little or no arcing. You've also heard 4-pole transfer switches offer positive isolation of the source neutrals. Which is best? Base your decision on what benefits are important to you. Regardless, you should know the operational details of each type of transfer switch.
To help you make a knowledgeable decision, let's take a closer look at the advantages and disadvantages of these two methods.
Three-pole transfer switches with overlapping neutral contacts. This type of transfer switch offers many advantages, including the following:
• It minimizes abnormal switching voltages;
• Its neutral pole is always connected to at least one source; and
• It operates with little or no arcing, which minimizes the risk of erosion or degradation of the neutral contact.
However, a 3-pole transfer switch with overlapping neutral contacts has several drawbacks. One disadvantage is the design and construction of the neutral pole and contacts. On some manufacturer's switches, it's not identical to that of the power poles and contacts.
Specifically, these designs incorporate a separate neutral switching mechanism connected by a linkage to the main power pole switching mechanism. Malfunctions of this accessory can occur because the neutral contact does not operate by the same mechanism as the power contacts. This could lead to the following conditions:
• A permanent connection of both source neutrals to the load neutral, compromising ground fault interrupting (GFI) devices, and
• A permanent open load neutral.
So, what could happen? Suppose a system suffers a cable fault downstream, and one of the phase poles shorts to neutral. The overlapping contact (if not fully rated the same as the power contacts) could melt down and create a permanent open-load neutral condition. However, you can avoid this problem if you make sure the overlapping contact is rated the same as the power contacts.
Keep in mind that a 3-pole transfer switch with overlapping neutral contacts is not considered a true 4-pole switch, according to the NEC and UL-1008.
Four-pole switches. This type of automatic transfer switch offers the following benefits:
• A fully rated neutral pole (with design, construction, and ampere capacity rating identical to the power poles);
• A single contact mechanism that switches all four poles simultaneously; and
• Positive isolation of source neutrals; even during transfer, preventing transfer of fault conditions from one source to another.
The 4-pole switch also has its drawbacks, including:
• A potential for abnormal surges due to momentary open-transition switching of all contacts;
• A potential for abnormal surges due to momentary open-transition switching of all contacts, as well as neutral contact erosion; and
• Dangerous voltage transients may occur.
There's been much discussion on these dangerous voltage transients occurring whenever the neutral conductor is switched from one source of power to another, without the benefit of an overlapping neutral arrangement. Nevertheless, let's look at this issue more closely.
To generate transient voltages, you must have a load that is capable of storing electrical energy. We know a resistive load has no such characteristic, so let's consider an inductive load. The energy stored in a transformer primary, secondary, and the leakage reactances are small contributors to the transient situation. The largest contributor to possible transient voltage generation is energy stored in large motor windings or similar loads.
However, switching tests of inductive loads show no appreciable voltage transients occur. In fact, the transient voltages normally found in distribution circuits in industrial or commercial installations are far more severe than the ones the switched neutral might produce. In fact, the magnitude of AC voltage transients found on most industrial, commercial, and residential power systems far exceeds anything detected during 4-pole switch tests.
What about neutral contact erosion? Since you must apply the same criticism to the other poles in the switch, the assumption that neutral contact arcing and erosion of the load switching contact could result in discontinuity and eventual disconnection of the neutral conductor may be unrealistic.
The successful operation of any disconnecting device, especially under reactive load conditions, is extremely dependent upon rapid "make" and "break" contact action with properly supported arc extinction. The transfer switch does this by operating the neutral contacts with the same high-speed transfer mechanism as the power contacts. Which method is best for you? Ultimately, both offer segregation of the grounded circuit of the two sources feeding the transfer switch. Now, the choice is yours.
McMorrow is supervisor, ATS Applications, Russelectric, Inc., Hingham, Mass.