FAQ

Contactors Frequently Asked Questions and Information...

Change-over contactors are used to switch one load between a main supply and a standby supply. It consist of two separate contactors which are both electrically and mechanically interlocked to prevent both being connected at the same time.

Automatic change-over contactors will automatically select the main supply when available and only need the supplies and load to be installed.
Manual change-over contactors are full interlocked but require the installer to select which supply is to be connected using additional control circuitry.

Applications for Change-over contactors.

Backup Supply.
When two supplies are available for a load an automatic change-over contactor will select the main supply and switch to the standby supply when the main supply fails. The main supply will be re-connected when it becomes available again.

Emergency Generator.
When an emergency generator is used an automatic change-over will connect to the generator on failure of the main supply, and reconnect to the main supply when it becomes available again.

Options available for changeover contactors.

Delay timers.
Delay timers are available from 1 second up to 1 hour as standard and can be used in the following ways.
Fitted to the standby contactor the time delay will prevent the standby contactor from connecting the load to the standby supply for a set time. This may be important to allow a generator to run up to speed before being connected to the load.
Fitted to the main contactor the time delay will prevent re-connection to the main supply until the main supply has been stable for a set time period.

Pilot lamps.
Red and green pilot lamps can be fitted to show contactor state and supply availability as required. Pilot lamps are also available in other colors.

Phase Failure detection.
PFR (Phase Failure Relays) can be fitted to main and standby contactors to detect the loss of one phase in three phase applications. This will cause the supply affected to be disconnected.

Safety Contactors

Safety contactors are also known as Load Shedding Contactors or Drop-out Contactors.

A Safety contactor is pre-wired with a start/close key switch and stop/open mushroom head button mounted on the cover. Additional external start switches and stop buttons may be wired in as required.

The contactor will connect the load to the supply when the key switch is operated. The load will be disconnected if the supply fails (drops out) or a stop button is pressed. The contactor will not close again until the key switch is operated.

Any contactor with a normally open auxiliary can be wired as a safety contactor making 1250A safety contactors available as standard. Residual current circuit breakers (RCCB) up to 100A are also available as well as pilot lamps.

Applications for Safety Contactors.

Machine shop or computer room supply.

Supply to a machine shop or computer room may be controlled using a safety contactor. Supply is available when a key stitch is operated. The key can be removed once the supply is on. If an emergency occurs or at the end of the day one of the push button switches spread throughout the room can be pressed cutting the supply to the room.

Load Shedding.

If a factory has a 500A backup generator but draws 800A when all its equipment is in use, a safety contactor may be used to automatically turn off non-essential equipment keeping demand, in the event of a power failure, below 500A. When the normal supply is restored the safety contactor may be operated to bring non-essential equipment back on line.