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Zone4info - Automatic Transfer Switch

Automatic Transfer Switch

Posted by Hameedullah Ekhlas August 17

 

Elementary transfer switches are typically classified as “break before make” switching actions. The

connection to the normal source of supply is broken before the connection is made to the emergency source.

This action is intended to prevent the emergency generator from supplying power to the normal utility

source. Likewise, as the switch transfers back to normal, the emergency source connection is broken before

the main power is reconnected. This process is referred to as open transition. This open transition is all right

for many loads, as the automatic transfer takes less than 10 cycles or one-sixth of a second. Closed transition

is needed with some loads. In this situation, when the load is to be transferred back to the normal source, asynchronizing action must occur to synchronize the two sources so they can both be connected at the same

time. The time that both sources are connected is very short, as short as 100 milliseconds, but the transfer is

made without interrupting the power to the load as it transfers back to normal. The first transfer, from normal

to emergency, is still open transition. Another variation on the transfer of load is referred to as soft loading,

meaning the load is added gradually, or softly, to the auxiliary generator to enable loading of the on-site

generation to meet the needs. Different modes of operation allow the soft loading to gradually transfer the

load to the on-site generator to help reduce demand from the utility. The on-site generator can be used for 

cogeneration to sell power back to the utility and the load on the generator can be adjusted or programmed 

to stay constant to supply on-site demand as well as sell surplus power to the utility. This type of transfer is

designed to have two sources connected simultaneously and the electrical load shifts from one to the other in

a smooth, predictable fashion. Depending on where the on-site generator is located, inside the building or 

separate from the building, the transfer switch can be located to efficiently serve the main normal power and 

the generator power feeds.

 

e4info - Automatic Transfer Switch

8/18/2019 Automatic Transfer Switch.pdf

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This means that the switch may be located outdoors in an enclosure suited for the need or it may be installed 

indoors in a suitable enclosure. Figure shows an automatic transfer switch. An elementary diagram of a

typical automatic transfer switch is shown in Figure (The figure does not include engine starting and other 

controls.) When the normal supply on the left side is energized, current flows from L1 through the

time-delay coil (TD) and back to L2. After a predetermined setting of time delay in closing contact, the relay

R coil becomes energized. Contact R then closes, and energizes the N coil. Power contacts N then close,

supplying the load from the normal or preferred source. When the R coil is energized, it also opens the

normally closed R contact interlock in the E coil emergency circuit. This safe action ensures that each power 

supply operates independently of the other. When the normal power fails, all coils on the left, or normal

supply side, become deenergized. Relay contact R drops to its normally closed position in the E (emergency

coil) circuit. Coil E is then energized, which closes the E power contacts feeding the load from an emergencyelectrical supply. The time delay action helps to ensure that the normal service does not supply the load 

intermittently with the emergency supply. In other words, the switch waits a preset time until the normal

supply is firmly established, before the load is reconnected to normal.

 

Reference:

 

Electrical Power Generation by Jeff 

e4info - Automatic Transfer Switch