Automatic transfer switch between two low-voltage utility supplies
Automatic transfer switch between two low-voltage utility supplies (photo by Giffen Switchgear, UK)

ATS to Emergency / Standby Source

Multiple utility services may be used as an emergency or standby source of power. Required is an additional utility service from a separate source and the required switching equipment.

Figure 1 shows automatic transfer between two low-voltage utility supplies. Utility source 1 is the normal power line and utility source 2 is a separate utility supply providing emergency power. Both circuit breakers are normally closed. The load must be able to tolerate the few cycles of interruption while the automatic transfer device operates.

Automatic switching equipment may consist of three circuit breakers with suitable control and interlocks, as shown in Figure 2.

Circuit breakers are generally used for primary switching in which the voltage exceeds 600 V. They are more expensive but safer to operate, and the use of fuses for overcurrent protection is avoided.

Figure 1 - Two-utility source system using one automatic transfer switch
Figure 1 – Two-utility source system using one automatic transfer switch
Relaying is provided to transfer the load automatically to either source if the other one fails, provided that circuit is energized. The supplying utility will normally designate which source is for normal use and which is for emergency.

If either supply is not able to carry the entire load, provisions must be made to drop noncritical loads before the transfer takes place. If the load can be taken from both services, the two R circuit breakers are closed and the tie circuit breaker is open.

This mode of operation is generally preferred by the supplying utility and the customer.

The three circuit breakers are interlocked to permit any two to be closed but prevent all three from being closed. The advantages of this arrangement are that the momentary transfer outage will occur only on the load supplied from the circuit that is lost, the loads can be balanced between the two buses, and the supplying utility doesn’t have to keep track of reserve capacity for the emergency feeder.

However, the supplying utility may not allow the load to be taken from both sources, especially because a more expensive totalizing meter may be required. A manual override of the interlock system should be provided so that a closed transition transfer can be made if the supplying utility wants to take either line out of service for maintenance or repair and a momentary tie is permitted.

If the supplying utility will not permit power to be taken from both sources, the control system must be arranged so that the circuit breaker on the normal source is closed, the tie circuit breaker is closed, and the emergency-source circuit breaker is open.

If the utility will not permit dual or totalized metering, the two sources must be connected together to provide a common metering point and then connected to the distribution switchboard. In this case, the tie circuit breaker can be eliminated and the two circuit breakers act as a transfer device (sometimes called a transfer pair).

Under these conditions, the cost of an extra circuit breaker can rarely be justified.

Figure 2 - Two-utility source system in which any two circuit breakers can be closed
Figure 2 – Two-utility source system in which any two circuit breakers can be closed

The arrangement shown in Figure 2 only provides protection against failure of the normal utility service.

Continuity of power to critical loads can also be disrupted by:

  1. An open circuit within the building (load side of the incoming service)
  2. An overload or fault tripping out a circuit
  3. An electrical or mechanical failure of the electric power distribution system within the building

It may be desirable to locate transfer devices close to the load and have the operation of the transfer devices independent of overcurrent protection. Multiple transfer devices of lower current rating, each supplying a part of the load, may be used rather than one transfer device for the entire load.

The arrangement shown in Figure 2 can represent the secondary of a double-ended substation configuration or a primary service. It is sometimes referred to as a “main-tie-main” configuration.

Availability of multiple utility service systems can be improved by adding a standby engine-generator set capable of supplying the more critical load. Such an arrangement, using multiple automatic transfer switches, is shown in Figure 3 .

Figure 3 - Diagram illustrating multiple automatic double-throw transfer switches providing varying degrees of emergency and standby power
Figure 3 – Diagram illustrating multiple automatic double-throw transfer switches providing varying degrees of emergency and standby power


About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


  1. Okonkwo Daniel
    Aug 28, 2015

    Having began my journey into the engineering world, I have found this site so much insiteful and knowledge-packed which has always been my companion. Thanks and keep the job going!!

  2. Arun
    Apr 13, 2015


    i deal with the same type of ATS and currently having an issue with it. Can you kindly pass me the control circuit diagram for ATS.

  3. ee_pkk
    Feb 28, 2015

    Hi Sir,
    I have one client which the LV switchboard include two number of 500A MCCB units as the incoming ATS for essential and city main power. During the building power test, the auto mode of ATS system was not fed to essential power when the city power is failed and the generator is up. However, the ATS system is changed to essential power normally by operated manual mode.

    Would you have any comment for above issue?

    Thanks for your help.

    Best regards,

  4. Ali
    Nov 24, 2014

    Thank you sir for your effort your really help me.

  5. Swamy Chouhan
    Nov 11, 2014

    Dear sir i want know more information regarding this equipment(Switch). i am using a 10KVA inverter on this inverter(Bi directional inverter) i am using 3 HP motor and general load. when Grid supply(Main source) off at that time i want to share inverter load only for 3 HP motor if motor is in ON condition only otherwise i want to work at normal condition . for requirement can i use This switch… please give the answer.

  6. Stephen musyoka
    Jul 01, 2014

    Am an electrical engineering student and working on this electrical filed, Your posts on your Facebook page has been of great importance and help in my practice in my electrical works. Keep it up for good work.

  7. trustmore gwezere
    Jun 03, 2014

    I would like to do an electrical engineering degree online is it possible and with which colleges

  8. Bus transfer in a large plant where there are many induction motors are presented, would be a very difficult job, if the tuning has not be done carefully. Please take care about it. You just need a very intelligent real time simulator to take care of these things.

  9. Hashim
    Jan 28, 2014

    Dear Sir


    Best Regard
    S.M Hashim

    • Please download POUYA from and then choose ORDER YOUR OWN network and then put your Single Line Diagram and the associated data there, they will provide you with the design and relay coordination of your plant, while you can also test the system in real time simulator. This is the best choice you can select.

  10. soe moe aung
    Jan 25, 2014

    I want to read new technology

  11. Prof. Shahram Montaser Kouhsari
    Jan 15, 2014

    Use POUYA software to tune the time of switching.

    Please visit an actual real time simulation of bus transfer in a water pumping station.

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