What are Transfer Switches?

Transfer switches transfer electrical power back and forth between two or more power systems or buses such as a utility power line and a backupgenerator. They are used in applications that require a backup power source where loss of power could cause problems.

Some transfer switches allow switching from a primary to a secondary, or even a tertiary power source. Others are used to switch from a regular power source to a temporary generator.

How a Transfer Switch Works

A transfer switch can be a simple switch that is operated by a person or can be complex and include sensors and controllers. In addition to the switching of the power, some transfer switches can command a backup generator to turn on when the main power fails, and will likewise shut-off a generator and reconnect to the main utility source once it reestablishes power.

Types of transfer switches include open transition, closed transition, and static. Open transition switches are the most common, and operate by breaking contact with one power source before switching to another. Closed transition switches operate by switching to another power source before switching off the first, and are important for processes that cannot tolerate any momentary loss of power; this also requires the power sources to be synchronized. Static switches use power semiconductors to transfer load between sources. Because they require no mechanical parts, they are inherently faster but are also more complex in design.

Selecting transfer switches requires the buyer to consider the electrical specifications, the mounting style, the control type, and various features of the switch. This guide is designed to help buyers with this process.

Electrical Specifications

The primary considerations when selecting a switch are the electrical specifications which must be matched for the system they are working with. Specifications include current ratings, voltage ratings, poles, and phases.

  • Maximum current rating is the maximum nominal or nameplate current capacity of the switching device. Higher ratings are given to switches designed to be able to operate with higher currents.
Design Tip: If you are unsure of the required current rating, it is better to over specify than to under-design the switch
  • Maximum AC voltage rating refers to the maximum AC voltage capacity of the device. Higher ratings are given to switches designed to be able to operate with higher AC voltages. Most systems operate at standard voltages such as 120V or 230V
  • Number of poles designates the number of poles the device can switch.
  • Phase is the number of AC voltages a switch distributes. A single-phase system will distribute a single AC voltage. A three-phase system will distribute three AC voltages that are 120° out of phase with each other.

Mounting Style

A transfer switch must be designed to be correctly mounted in order for it to physically fit the system.  Mounting styles include ground, pole, rack, wall, and submersible.

  • Ground mounted switches are designed to be mounted or installed from the floor. They permit a variety of bushing orientations for single-sided access or for separate components.
  • Pole mounted switches are mounted onto a pole or tower
  • Rack mounted switches are designed to mount on an equipment rack
  • Wall mounted switches are intended to be mounted on a wall or other flat, vertically oriented surfaces.

Control Type

Determining a type of control is another part of the selection process. Depending on the type of process, a switch might be controlled manually, automatically, or with a SCADA system.

  • Manual switches are regulated by physical control requiring an operator. It is used for systems in which control and predictability cannot be easily programmed or automated or where fast switching is not required. Manual switches tend to be the least expensive type of transfer switch.
  • Automatic switches rely on some processes to be controlled automatically without requiring an operator. Automated switches are often used with automatic standby generators that use single-phase or three-phase power and standard voltages during fail over.
  • Supervisory control and data acquisition (SCADA) switches are used to control distributed systems from one master location. This allows for safe, convenient, and efficient operation of more involved systems that need to be coordinated.

Switch Features

Certain switch features may be important to consider when selecting the right transfer switch for a specific application or system.

  • Wiping contacts are designed so that the sliding of contacts over one another results in contact cleaning. Because of this they are self-cleaning and usually low resistance. However, wiping action does result in mechanical wear. This type of contact is good for switches that only see occasional use.
  • High inrush current switches are capable of handling a high inrush current, which is the maximum input current resulting from turning on an electrical device. This type may be required when a number of devices with high starting currents like HVAC equipment and motors are being powered.
  • Locking devices have a locking mechanism that requires a key to operate the switching functions. This prevents unauthorized or accidental tampering of the switch functions.
  • Dustproof construction includes features such as seals to increase resistance to dust.
  • Weather and water resistance devices include features such as hermetical sealing against moisture to prevent environment-related damages.

Various certifications including CE and CSA are also included as features of certain products.