Last month we discussed the operation and ratings of the automatic transfer switch (ATS). In this column we’ll review three significant specification options, and continue our discussion next month with application of the ATS to motor loads.
The three-pole transfer switch, having provisions to switch the three-phase conductors but not the neutral, is always suitable for use with three-wire loads that do not require a neutral conductor. It may also be used with four-wire loads if it includes a neutral bus with a solid connection of both source and load neutrals. However, this requires that the neutral of the standby source not be directly bonded to ground, which can create safety concerns and should only be specified after careful consideration by the designer. The preferred application for four-wire loads is use of either a four-pole switch or a three-pole switch with overlapping neutral contacts.