TechArchive

Original article date: January 2000

The SD250 battery disconnection switch combines a 250A DC manual disconnect switch and a coil-operated contactor in a single unit. Alan Quinn takes a closer look.

Modern battery-powered electric vehicles are inherently very reliable and safe, but even when sophisticated electronic controllers are used, you want to have a means of disconnecting the battery in the event of an emergency, such as a vehicle failing to stop or an electrical short circuit.

In many countries, it is mandatory to fit one or more devices to achieve an emergency disconnection of the battery. The options are as follows:

• Manual disconnecting switches, such as the Albright SD and ED device.
• A minimum of two contactors with their contacts in series. This usually means a main line contactor and an additional contactor for each major electrical circuit.
• A battery plug, providing this is within easy reach of the vehicle driver.

Other practical considerations are the need to make the vehicle electrically “Dead” with a keyswitch when not in use and the ability to isolate the battery during routine maintenance.

A highly cost-effective means of achieving all the safety, legal and practical requirements is the Albright SD combined manual and electrical disconnecting switches. Alternatively, a manual disconnecting switch such as an Albright ED, together with a separate line contactor is worth considering.

The SD and ED ranges of disconnecting switches have been designed to provide a rapid means of disconnecting batteries or other power supplies in the event of serious electrical faults. Though primarily intended for use with battery-powered vehicles, they are also suitable for use with static power systems. All types are capable of safely rupturing full load battery currents in the event of an emergency.

SD switches combine the dual function of a manual disconnect and coil-operated line contactor. The benefits of this design include compact size and reduced installation costs, combined with an electrical capacity sufficient for most small and medium-sized electrical vehicles.

With the operating knob depressed (OFF), no electrical functions can take place, However, if the knob is ON, the option of energising the coil and thus closing the main contacts becomes available. The coil energisation can be carried out either through the vehicle keyswitch or as a result of a signal from the vehicle electronic controller.

When an SD switch is used as an emergency battery disconnecting switch, manually depressing the operating knob will override the energised coil such that the main contact and the auxiliary contact will open, until such time as the knob is again moved to the ON position.

Operating coils are normally continuously rated, although intermittently rated versions are also available.

An integral auxiliary contact is fitted. This has one normally open contact which closes when the knob is put into the ON position. The status of the auxiliary contact is not affected by the energisation or de-energisation of the coil. An additional double circuit normally open/normally closed changeover auxiliary contact can be fitted.

ED switches are manually operated devices with a simple over-centre spring mechanism which provides a snap action for both opening and closing the main contacts.

Simply pulling the operating knob upwards closes the main contacts and depressing the operating knob opens them again.

A double circuit, normally open, normally closed microswitch auxiliary can be fitted which has a DC resistive rating of 15A at 24V.

The auxiliary contact can be set to operate either before (default) or after the main contacts open, according to the circuit requirements.

• Albright International

January 2000