Automatic Transfer Switch – What is it?

1. Automatic Transfer Switch – Definition

A Transfer Switch is an electrical switch which will transfer power load between two different sources. These transfer switches come in two different variants, manual and automatic. The manual version can be activated using a lever or switch that is activated manually. An automatic transfer switch will activate when the unit senses power has been lost or gained from the primary unit. In most cases an Automatic Transfer Switch will be installed near a backup generator to provide power in the event of a power outage.

Automatic Transfer Diagram

automatic transfer switch
src: https://www.outsidesupply.com

2. Operation of an Automatic Transfer Switch

Not only can an Automatic Transfer Switch transfer power or load to a backup generator, an ATS unit can also communicate with a backup generator and force it to start up. This is firmly based on the voltage that is monitored on the primary supply. The ATS will isolate the backup generator form the main utility when the generator is on and running providing temporary power. The ability of control an Automatic Transfer Switch has varies. The unit can be manual or a mix of automatic and manual. The Switch Transition Mode of a transfer switch may be Open Transition or Closed Transition.

A transfer switch can be setup to to provide power to an entire electrical sub panel or to critical circuits only. Some Transfer Switches allow load shedding or can prioritize optional circuits. Complex Emergency Switch Gear can be used in larger backup generator installations which will permit soft loading. This will allow the load to be smoothly transferred from the utility to synchronized generators and vice versa. These setups are particularly useful in utilities that experience a high amount of demand at peak times during the day.

2.1 Open Transitions

Open Transition Transfer Switch is also known as a break before make transfer switch. This type of switch breaks contact with with one source of power before it makes contact with another. It also prevents back feeding from an emergency generator back into the utility.

2.2 Closed Transitions

A Closed Transition switch is also known simile to the Open Transition above as a make before break transfer switch. Most Emergency systems use an open transition configuration. This is because there is an inherent momentary interruption of power to the load when its being transferred from one utility to another.

Some loads may be effected by the slightest loss of power . There are also some operational conditions where it be beneficial or important to be able to transfer loads with no interruptions of power when the transfer is needed. For this type of setup a closed transition transfer switch can be provided within the setup of the system.

It is normally required that the closed transition or overlap time be less that 100 milliseconds. If this source is not present or acceptable the switch must operate in the break before make mode stated above. to ensure no back feeding happens.

3.0 Soft Loading

Soft loading transfer switch makes use of CTTS. This is more commonly used to synchronize and operate onsite generation in parallel with utility power, and to transfer loads between both sources while attempting to minimize voltage or frequency transients.

4.0 Static Transfer Switch

Static Transfer Switches use power semiconductors to transfer a load between two sources. As there are no moving parts the transfer can be complete fairly quickly. Static transfer switches can be used where it will be reliable and independent sources of power are readily available, and is necessary to protect the load from a few power frequency cycles interruption time or from any surges in the main power source.

5.0 Craig and Derricott ATS

Total Control and Distribution can now supply three different types of Automatic Transfer Switches from the Manufacturer Craig and Derricott. These ATS Systems are Standard Automatic Switches, Automatic Transfer Switch with Single Line Bypass and an Automatic Transfer Switch with Dual Line Bypass.

5.1 Standard Automatic Transfer Switches

The ‘Standard’ range utilizes electro mechanical contactors and provides all of the essential requirements for automatically providing a replacement power source. Facilities are provided to control the ‘start-up’ of standby generators manually and to set the under voltage values and the required time delay. Main LED indicator lights show the status of the supplies.

5.1.1 Technical Features

  • Electro-mechanical interlocked 4 pole AC1 rated contactor
    • Applied standards: BS EN / IEC 60947-4 & BS EN / IEC 60947-6-1
    • Class of equipment PC
    • Rated Voltage 230V or 400V ac
    • Utilization category AC31B (for other categories, please contact Craig &
    Derricott)
    • Rated frequency 50Hz
    • Volt-free changeover, x3 contacts for BMS systems – Rated 250V 7A
    • Supplies available indication

To view the full specifications view the datasheet here

5.2 Automatic Transfer Switches with Single Line Bypass

The Single Line Bypass Automatic Transfer Switch (ATS) units are available in current ratings from 32A to 400A in both Single and Three Phase options. The units are supplied in a mild steel IP65 painted enclosure (RAL7035).

5.2.1 Technical Features

• 2 Position key switch for Auto and Test
• Adjustable under-voltage and Time delays
• 2P and 4P configurations
• Changeover time adjustable to 50m/s
• Removable Top and Bottom Gland plates
• Locks supplied with one key per enclosure
• Easy connectors for incoming and outgoing supplies
• IP2X internal panel
• 3 x Volt free changeover contacts (COM, N/O & N/C)
• Supply availability/on load LED indication panel for visual representation
of supply status
• Mechanically and electronically interlocked 4 Pole AC1 rated contactors
• Auxiliary power supply for Generator battery charging or jacket water
heater
• Applied standards: EN 60947-4-1, EN 60947-6-1 & EN 60947-3

To view the full specifications view the datasheet here.

5.3 Automatic Transfer Switches with Dual Line Bypass

The Dual Line Bypass Automatic Transfer Switch (ATS) units are available in current ratings from 32A to 400A in both Single and Three phase options. The units are supplied in mild steel IP65 painted enclosures (RAL 7035).

5.3.1 Technical Features

2 Position key switch for Auto and Test
• Adjustable under-voltage and Time delays
• 4P configurations
• Changeover time adjustable to 50m/s
• Removable Top and Bottom Gland plates
• Locks supplied with one key per enclosure
• Easy connectors for incoming and outgoing supplies
• IP2X internal panel
• 3 x Volt free changeover contacts (COM, N/O & N/C)
• Supply availability/on load LED indication panel for
visual representation of supply status
• Mechanically and electronically interlocked 4 Pole AC1
rated contactors
• Auxiliary power supply for Generator battery charging or
jacket water heater

To view the full specifications view the datasheet here.

6.0 Instructional Video – How an Automatic Transfer Switch works with a Generator

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