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Single Line Diagrams of Emergency and Stand-by Power Systems With Automatic Transfer Switch (ATS)

Updated: Apr 4

Emergency and Stand-by Power Systems

Emergency and stand-by power systems with Automatic Transfer Switch (ATS) typically feature a main utility source, a backup generator, an ATS, and essential loads, all represented on a single line diagram for clarity and understanding.

Emergency and stand-by power systems are generally designed into the overall 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 figure1. 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 arrangement extends the same inherent weaknesses of the radial system to the emergency system since a single failure of one piece of equipment can result in loss of service to the emergency/standby loads. Note that the single generator shown may be several engine-generator sets operating in parallel, if necessary.

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.

Figure-1: Simple emergency/standby system arrangement


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:

Figure-2: Example of a more complex emergency/standby system arrangement

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:

Figure-3: Medium voltage emergency/standby system implementation

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.


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 electrical system is comprised of “alternate sources of power and all connected distribution systems and ancillary equipment, designed to ensure continuity of electrical power to designated areas and functions of a health care facility during the disruption of disruption of normal power sources,”.

An emergency system is “a system of circuits and equipment intended to supply alternate power to a limited number of prescribed functions vital to the protection of life and safety”. The emergency system is a part of the essential electrical system.

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

Figure 4a – Minimum requirement per NEC and NFPA 99 for an essential electrical system for hospitals 150 kVA or less

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 supplies, which itself part of the essential electrical system, supplies the life safety branch, which is “a subsystem of the emergency system consisting of feeders and branch circuits… intended to provide adequate power needs to ensure safety to patients and personnel”.

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).

Figure 4b – Minimum Requirement per NEC and NFPA 99 for Essential Electrical system for Hospitals over 150 kV

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 / stand-by power supply.o

Figure-5: Automatic transfer switch single line diagram representation

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.

Because automatic transfer switches are designed to continuously carry the loads they serve, even under normal conditions, care must be used in sizing these so that the potential for failure is minimized.

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.


Anatomy of an Automatic Transfer Switch (VIDEO)


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

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