Electrical Transfer Switch Use, Operation, Diagram


Electrical Transfer Switch has a very important role in a reliable, continuous power supply system which works with two electrical power sources, one is primary, or utility power source and another is back up power source. In this article, we are going to know what is an electrical transfer switch or electrical changeover switch, its operation, connection, use, and applications.
We are going to discuss about electrical transfer switch which used to transfer electrical power only, don't confuse with transfer switch which used in data centers, electronic devices, communication system, etc although they both are works almost same principle.


What is Electrical Transfer Switch


The transfer switch is an electrical switch that switches an electrical load from one source to another source.

If we consider a building as a load which is taking power supply from electricity board as a primary source and it also has a generator as a backup source then here an electrical transfer switch required which will switch the building electrical load from primary source to the generator when the primary source goes out.


Why Transfer Switch Required


Transfer switch required for two purposes,

                                        1. Connecting  2. Disconnecting

This means it is required to connect a load to a source and to disconnect or isolate from another source.

If we take the same example as above explained, when the primary source fails, the transfer switch connects the building load with the generator and disconnect from the primary source. If it is not disconnected from the primary source, the building generator will feedback power towards the primary source and it will create a problem. So transfer switch keeps isolate the generator from the primary source and helps to provide a temporary power supply to the building from the generator.


Where Electrical Transfer Switch is used


The electrical transfer switch is used in Commercial buildings, Hospitals, Shopping malls, companies having both primary and backup power sources.

Electrical transfer switches also used in electrical power transmission and distribution systems. Of course, they are different from the domestic or industrial transfer switch.

Soft loading and Static Transfer switches are used in UPS, sensitive electronic devices, programmable devices, testing devices, etc.


How Electrical Transfer Switch Works


There are different types of electrical transfer switches available, their working also different. 
The most commonly used electrical transfer switch is the Open Transition Transfer Switch which works on the principle of Break-Before-Make. It means when the primary power source fails, it first breaks the contact between the load and primary source, then it makes contact between the load and backup source. In this case, the load experiences a power loss or interruption for a very short time(a few milliseconds). Automatic Transfer Switch(ATS) works on the above same principle.

Another type is Closed Transition Transfer Switch which works in the principle of Make-Before-Break. It is not used for back up power providing purpose. It is used when both power sources available but we need to switch the load from one source to another source without any power interruption.
It first makes the contact between load and second source then breaks the contact between the load and first source. So you may understand for a certain time load is connected with both power source.
The operation of the closed transition transfer switch is a little bit complex because here we need to match voltage, phase angle, frequency of both power sources.

Another type of transfer switch is the soft loading transfer switch which is used to run a load with two power supply synchronously. Here the parallel operating principle is used. Here loads are divided between two sources.

Static transfer switches are the works on the same principle as others but it does not have any mechanical parts, it uses power semiconductor devices like SCR, IGBT, etc. It provides high speed, reliability, and low power loss, noise, interruption.


Function of Transfer Switch


The basic function of a transfer switch is to connect a load with the backup source when the main primary source falls and also keep isolate the load from the primary source.

The function of an Automatic Transfer Switch(ATS) is to sense either primary source falls then connect the load with the backup source automatically. Again connect the load with the primary source when it is available.

Industrial Electrical Transfer Switch not only connect or disconnect the load, even they send the signal to the backup source(Generator) to turned on when primary source falls and to turned off when primary source available.


Types of Electrical Transfer Switch


There are different types of transfer switches available, although their function almost the same. Some different types of electrical transfer switch are,

   1. Open Transition Transfer Switch
   2. Closed Transition Transfer Switch
   3. Static Transfer Switch
   4. Soft Loading Transfer Switch
   5. Manual operated transfer switch
   6. Automatic Transfer Switch(ATS)


Electrical Transfer Switch Diagram


Here you can see the connection diagram of a three-phase automatic transfer switch,

transfer switch diagram


Transfer Switch Applications


Electrical transfer switches are used to shift or divert or switch the electrical power from one source to load to another source. If you have a standby generator in your home for backup purposes then you must use a transfer switch(automatic or manual) to connect your loads to the generator when the main power supply fails.

Static Automatic Transfer Switches are better than old manual transfer switched because it provided faster operation, low power fluctuations. For home use purposes a minimum of two-pole 63A transfer switch should be used which will work with both phase and the neutral wire.


How To Choose a Transfer Switch


Choosing of electrical transfer switch or changeover switch is very easy. First, calculate your total load, no. of phases, and voltage. As per your requirements search product rating and select to purchase. For home use, generally, 2-pole, 230V, 63A Transfer switch is used. For large building or industries purpose 3-phase, up to 1000A, 440V transfer switch or changeover switch is used.


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Electrical Transfer Switch Use, Operation, Diagram Electrical Transfer Switch Use, Operation, Diagram Reviewed by manoranjan das on 12/18/2020 Rating: 5
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