Single line diagrams of emergency and standby power systems with automatic transfer switch (ATS)

Emergency and standby power systems

Emergency and standby power systems are generally designed into the over-all electrical system for one of the following two reasons:

1. Legal Requirements – As required by the NEC, NFPA 101, NFPA 99 and other local, state, and federal codes and requirements. These are concerned with the safety of human life, protection of the environment, etc.
2. Economic Considerations – Continuous process applications often require a continuous source of electrical power to avoid significant economic loss. In some cases even a momentary loss of power can be disastrous.

Various ways of arranging emergency and standby power systems exist. The most common arrangements are given here.

1. Basic arrangement – radial system
2. More complex systems
3. Hospital arrangements

1. Basic arrangement – radial system

The most basic arrangement for an emergency or standby power system is shown in figure 1. This can be recognized as an extension of the single-source radial system, with the transformer omitted.

The transfer switch transfers the emergency / standby loads to the alternate source upon failure of the normal source.

This simple system may be expanded to the other system types like expanded radial systems with:

• 1 utility source and a single primary feeder
• 1 utility source and multiple primary feeders or
• 2 utility sources and multiple primary feeders.

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2. More complex systems

The basic arrangement from figure 1 may be extended to the other system arrangements. For example, the secondary-selective system could be equipped with an emergency system as shown in figure 2:

In figure 2, the emergency / standby load at the bottom of the figure will always be supplied by one of the normal sources if possible, and by the generator(s) if not. This will avoid the generator starting time for this load if one utility source were to fail. The two emergency / standby loads in the middle of the figure will be supplied by their respective switchboard busses or by the emergency source.

Emergency / standby systems are not limited to the low voltage level. For example, the primary selective / primary loop / secondary selective system can be expanded to include an emergency system, as shown in figure 3:

In figure 3 above there is a great deal of flexibility in the system operation. However, instead of automatic transfer switches metal-clad switchgear is used increasing the complexity of the system.

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3. Hospital arrangements

NFPA 99 and the NEC have very unique requirements for the design of a hospital emergency system. The emergency system is classified into the essential electrical system and the emergency system itself.

The minimum arrangement, for hospitals 150 kVA or less, is shown in figure 4a. The minimum requirement over 150 kVA is shown in figure 4b.

The essential electrical system supplies the equipment system, defined as “a system of circuits and equipment arranged for delayed, automatic, or manual connection to the alternate power source and that serves primarily 3-phase power equipment”.

The  emergency system also supplies the critical branch, which is “a subsystem of the emergency system consisting of feeders and branch circuits supplying energy to task illumination, special power circuits, and selected receptacles serving areas and functions related to patient care”.

For hospitals of 150 kVA and less the equipment system, life safety branch, and critical branch may be on the same transfer switch. Note that the transfer switch(es) for the equipment system above 150 kVA is required to be delayed (figure 4b).

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Automatic transfer switch – ATS

An automatic transfer switch is defined as “self-acting equipment for transferring one or more load conductor connections from one power source to another”. The automatic transfer switch is the most common means of transferring critical loads to the emergency / standby power supply.

An automatic transfer switch consists of a switching means and a control system capable of sensing the normal supply voltage and switching over to the alternate source should the normal source fail. Automatic transfer switches are available in ratings from 30-50 A, and up to 600V.

Automatic test switches with adjustable pickup and dropout setpoints and integral testing capability are generally preferred. An automatic transfer switch is generally an open-transition device that will not allow paralleling of the two sources. Manual versions of transfer switches are also available.

A single line representation of an automatic transfer switch is shown in figure 5 above.

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Anatomy of an Automatic Transfer Switch (VIDEO)

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Reference // Emergency and Standby Power Systems by Bill Brown, P.E., Square D Engineering Services