Automatic Transfer Switch Types: A Comprehensive Guide

An automatic transfer switch, or ATS, is a critical component in any backup power system. It ensures that power is seamlessly transferred from the primary source to a secondary source such as a generator in the event of a power outage. However, not all ATSs are created equal and there are several different types available on the market. This comprehensive guide will explore the automatic transfer switch types and their specific applications helping you to make an informed decision when selecting the right ATS for your needs.

Automatic Transfer Switch Types

Transfer switches are subdivided into three: compact ats, circuit breaker ats and contactor ats.

1. Compact ATS (Class PC)

Compact ATS is capable of making and withstanding. It is not intended for breaking short-circuit currents. It is also known as a change-over switch. The modern ATS reduces the installation time of the device in the panel while reaching a high level of reliability due to the limited number of components to be assembled and mounted. Compact ATS is typically the most expensive type.

2. Circuit Breaker ATS (Class CB)

Circuit breaker ATS is capable of making, withstanding and breaking short circuit currents. It is provided with overcurrent releases.

A circuit breaker ATS is usually assembled by an installer using several MCCBs or ACBs that must be accessorized. These accessories should be used in the circuit. Motor operator, shunt opening, shunt closing releases, auxiliary contacts, communication devices, mechanical and electrical interlock and external ATS controller can be used as accessories. Circuit breaker ATS is more expensive than contactor ATS.

3. Contactor ATS (Class CC)

Contactor ATS is capable of making and withstanding but is not intended for breaking short-circuit currents. Contactor ATS is usually based on 3 or 4-pole low-voltage contactors. To prevent the simultaneous closing of several contactors connected to unsynchronized power sources, the electrical and mechanical interlock shall always be used. Contactors have an in-built electromagnetic coil and it requires only one signal to operate. Therefore, the automatic transfer switching solution can easily be built with a simple control diagram based on electronic relay logic or with an external controller. Contactor ATS is generally the most economical solution.

Transition Types

There are two main transition types available: Open and closed transitions.

Open transition

The transfer from one source to a second source goes through a 0 position to ensure that the main and alternate sources do not overlap. An off time can be counted down to allow the load residual voltage to decrease below a non-critical value before transferring. Transferring the load too quickly to another source can lead to power transfers between the load and the supply which often causes damage. This can potentially damage sensitive equipment and cause protection devices to trip. The 0 position is a stable safe-off position that enables work to be carried out downstream of the transfer switch safely once padlocked. The off-time delay setting should be configured according to the equipment installed. The international standard IEC 60947-6-1, dedicated to transferring switching equipment, states that any time delay or off-time provided in the total operating transfer time from the normal to the alternative or the alternative to the normal supply, shall not be less than 50 ms. For applications that require a faster transfer time, it is recommended to include adequate measurement and protection in the installation. Typically sync check relays. If this time is not respected then the installation must have adapted synchronization and protection functions.

Closed transition (Synchronous transfer)

Depending on the local network regulations the normal and the alternative source may temporarily run in parallel for a period of <100ms. This is typically used for scheduled transfers, for example returning to the normal source from the alternative source. To allow a synchronized transfer the two sources must be in sync to allow the transfer:

Their phase angles must be in-phase (less than a 5° difference).

Their frequency and amplitudes are virtually identical (less than 0.2Hz and 5%V).

When synchronized within these limits a scheduled or return to normal supply transfer may take place without a blackout time allowing continuity of service.

When the normal source is lost or the power supplies cannot be synchronized (out of limits explained above) the transfer is carried out in an open transition.

Closed transition (Asynchronous Transfer)

This type of transfer mode is typically applied to applications with large asynchronous motor loads. A fast open transition transfer is used to allow a direct transfer without having to stop the motor. This transfer time is usually less than 50 ms and is achievable safely when using a transfer switch coupled with a sync check relay. Although the transfer is carried out in an open transition without overlapping contacts, the same conditions (in terms of voltage, phase angle and phase angle) as with a closed transition apply.

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