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Home / Technical Articles / Automatic transfer switch (ATS) between two low-voltage utility supplies

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.

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

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


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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Yoseph Gebru
    Jan 28, 2021

    Dear all I want to know about ADS so can you help me please

  2. Juan Antonio Díaz Jiménez
    Mar 10, 2020


  3. Ajay
    Feb 06, 2020


    What difference it makes when an ATS using standard motor operated circuit breakers(ACB or MCCB) used instead of UL spec ATS like ASCO. Zenith etc..?

  4. Muhammad Abdullah Rafiq
    Dec 07, 2019

    Can we provide the interlocking between three circuit breaker in an auto transfer switch. Breaker Q1 form Main Power supply, Breaker Q2 alternate power supply and breaker Q3 Generator Power supply. Interlocking between Q1& Q2 for failure main supply, this Q1 open and Q2 close, and failure of main and alternate supply, under voltage relay on the down stream bus will within less than 5 sec give start signal to Generator and within 30Sec Q3 close and Q1 & Q2 become open. At the resumption of main or alternate supply, the relay sense the voltage on breaker Q1 & Q2 and with certain time delay open breaker Q3 and close wither Q1 or Q2. and after resumption of normal power, a signal should send generator for stop.

  5. Mohamed Ismael
    Mar 05, 2019

    dear sirs Can you please share detailed explanation of ATS CONTROLLER for 2 GENSETS + MAINS
    The automatic start is designed for automatic starting of one Genset when mains failure.
    – if the running Genset shut down to any fault that occurs the other set will automatically start and supply the load.

    Thanks & regards

  6. giusepp picciau
    Nov 20, 2018

    hello I would like some clarification in reference to the ATS system, in a configuration with two transformers that feed two bars divided by a tie breaker when a main breaker fails the tie breaker closes automatically so that one of the two transformers feed the other bar .
    I would like to know if you create a short voltage loss or is the ats system so quick to avoid it?

  7. Md. Hafizur Rahman
    Oct 15, 2018

    Dear sir
    We have two building, building -1 load 50kw, building -2 load 30kw which is power supply by individual transformer sources.
    I want to emergency power back up (when building-1 or building-2 or both load sheding) from a 200 KVA Generator. How i can do it. Please i want to your help.

    Note : 1.Two building distance 300ft
    2. 200kva generator with 350 Amp ATS set on building 1.
    3. Building 2 no generator.

    Md. Hafizur Rahman

  8. Mohammed
    Sep 06, 2018

    DEAR SIR ,



  9. Ben Chapman
    Jul 24, 2018

    Hi Sir,

    Is it possible to connect a back up generator to essential services bus (1) and utility supply to non-essential services bus (2), then use a tie breaker to connect the two buses so whenever the utility fails the tie breaker will disconnect the bus (2) and let essential services run on backup generator.

    Thank you.

  10. Sajid Roghani
    Mar 04, 2018

    i want to connect two 11kv voltage sources from different utilities injecting into the same 11kv feeder in run. which type of ATS i can use? is this possible.regards

  11. Darshan
    Nov 24, 2017

    Thank you for sharing ATS function in detail.

    Can you please share detailed explanation of ACB control circuit(internal control wiring of ACB(schneider or siemens ACB) for operation) for 2 incomer 1 bus coupler scheme along with drawing ?
    I am little perplexed to do this wiring by myself and need your help. I request you to please briefly explain the sequential operations of different internal elements of Air circuit breaker like spring charge, motor, closing coil, trip coil, anti pump, under voltage relay, SPP and contacts when closing & tripping while operating manually from TNC switch & in Auto mode.

    Thanks & regards
    Darshan V

  12. Akinola Ogunwuyi
    Nov 13, 2017

    Thanks for your blog, as a power engineer I am interested in power transfer between more than one source and the design principle of when they take up load.kindly advise.thanks

    • Edvard
      Nov 13, 2017

      You’re welcome Akinola. You didn’t mention the voltage level, but if it’s low voltage, you can take a search for terms ‘transfer switch’ or ‘ats’.

  13. Vinh tran
    Nov 08, 2017

    Nice to be your fan.

  14. nooredin
    Feb 13, 2017

    dear sirs , is there any magazine that i can receive monthly which covers all new news in electric engineering and technology

    thanks alot

    Nov 09, 2016

    Can we accept and consider a certain voltage range [say between +/- 10% from utility supply] to enable ATS to changeover to a standby generator. That means the ATS will consider any voltage beyond +/- 10% of utility supply as a blackout condition and automatically changeover to Generator [even when the Utility supply is available, beyond +/- 10% range]

    • Muhammad
      Nov 10, 2016

      you can do it by adjusting the over and under voltage relay. there are many version of that relay you can consider. But in your case, I think the most suitable one will be the percentage relay

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