Transfer of Power

Transfer switches comprise the critical path for essential loads in an emergency power supply system (EPSS). The most common applications include a primary source from the local utility and an alternate source from an on-site generator. The typical application for an EPSS system is associated with the life safety system of hospitals, large assembly-based facilities and high rises.

11/01/2005


Transfer switches comprise the critical path for essential loads in an emergency power supply system (EPSS). The most common applications include a primary source from the local utility and an alternate source from an on-site generator. The typical application for an EPSS system is associated with the life safety system of hospitals, large assembly-based facilities and high rises. Additional applications include improved reliability for continued operations, data preservation, load shedding and basic convenience. There are several types of switches available. Therefore, it is important to understand the needs and objectives of the facility owner.

Types and characteristics of Transfer switches

Basic Features

Manually Operated

Automatic Operation

Bypass-Isolation (automatic operation)

Static Switch

Applications

When some downtime is acceptable. Emergency power for convenience only.

Immediate switching is required.

Downtime not allowed during maintenance.

Downtime of sensitive equipment or process is not acceptable. Two active electric sources.

Advantages

Cost-effective.

Quick response to power failure. Human action is not required. Time delays available to protect motor loads and prioritize importance of loads.

Quick response. Load not disturbed during periodic maintenance. May be operated manually if automatic switch fails.

Very quick switching speeds. Human action is not required.

Limitations

Human failure. Slow response time. Selective load shedding.

Sensitive electronics may be interrupted during switch operation.

Large space allocation required.

Requires two active sources for complete effectiveness. Not cost effective for most applications.


Applying switched neutrals: three-pole vs. four-pole

Apply three-pole switch (neutral is not switched) when:

All existing transfer switches in the facility are three-pole.

Emergency power source is not wired as a separately derived system.

Single utility substation.

Apply four-pole switch (switched neutral) when:

Existing four-pole switches in the facility.

Emergency power source is wired as a separately derived system.

Ground fault sensing equipment is installed ahead of the transfer switch.

Multiple normal sources.





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