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Phase Rotation Sequences for Temporary Power

 
When connecting a portable or temporary generator to backup three-phase power, connections are often made through a quick connect panel and a manual transfer switch. In doing so it’s important to connect each genset conductor to the corresponding conductor of a building’s power distribution system to avoid unintended performance impacts and potential equipment damage. The following narrative provides an overview.

Phase Sequence

Three-phase power distribution systems typically rely on three phase conductors, a neutral conductor, and a grounding conductor. When connecting power sources and loads, both must use the same phase sequence, which refers to the order in which each phase reaches peak voltage during an alternating current cycle. Figure 1 applies to three-phase generators and motors, and shows an ABC phase sequence and the resulting sine waves.
 
Phase Sequence and Motor Loads

Importantly, switching the position of any two conductors reverses the rotation of motors on the circuit, as shown in Figure 2. While ABC is the most common phase sequence, utilities sometimes use a different order, such as CBA. Regardless of the phase sequence used, both must use that the power source, distribution equipment, and load equipment use the same phase sequence.
 
Unintentional changes in phase sequence can have multiple consequences. Equipment that is designed for only a specific rotation direction could be damaged. For example, lubrication systems in refrigeration or HVAC compressors may only be effective when the rotating parts of their oil pumps move in the designed direction. Running this equipment in the wrong direction could result in improper function, inadequate lubrication, and failure of the equipment.

The results can be even more dramatic if a load transfer occurs between two active power sources where one is wired out-of-sequence. This can result in instantaneous reverse current being applied to running motors. The resulting high currents could trip overcurrent protection devices and the resulting mechanical stresses could damage mechanical equipment. For these reasons, it is imperative to ensure that power sources and power distribution systems are connected in the proper phase sequence.

In the event of a loss of a phase (single-phasing), current would increase on the two remaining phases, which could result in damage to motors. Phase rotation relays can detect such conditions and send signals that can be used to annunciate them.

Properly Connecting Temporary and Portable Gensets

To ensure proper phase sequencing, manufacturers design connection panels with several features. Purchasers can order connection panels wired for the phase sequence used in a particular application, perhaps where a utility uses a sequence other than ABC.

Regarding phase sequence, Article 700.3(F)(3) of the National Electrical Code® states: The connection point for the portable or temporary alternate source shall be marked with the phase rotation and system bonding requirements. Consequently, connectors are typically color-coded or otherwise marked to show the proper arrangement of connections for the application. Connecting the devices according to a color-coded or marked scheme properly arranges the phase, neutral, and ground conductors.

Manufacturers may also offer phase sequence monitors, which provide visual and audible indication when phases are mismatched. This enables personnel to evaluate and correct the connections before transferring loads between power sources.

Notably, ground and neutral connections should always be made before phase conductors are connected. Likewise, connections should neither be connected nor disconnected when energized.
 
Summary

Backup power systems are often designed with capabilities for connecting a temporary or portable generator. If the equipment is connected using an improper phase sequence, motors could reverse direction, which may disrupt facility operation and damage load equipment.

Connection panels can be ordered with a phase sequence relay, and are typically prewired in either an ABC or CBA arrangement. These typically feature color-coded connectors or other markings to facilitate proper arrangement. When installing a connection panel, a phase sequence test should be completed to verify proper arrangement. A label should be placed inside the connection enclosure indicating the connection arrangement to ensure that personnel connect conductors using the correct phase sequence.

For further information, contact an ASCO Power Technologies representative.

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