Emergency Buttons
Helen Li
Electirc Technology
2016-06-22 11:56:13
Emergency Buttons
Emergency buttons are an important safety component of many electrical circuits, especially those that control hazardous equipment such as gas pumps, moving machinery, saws, mills, and cutting tools, conveyor belts, and many other types of equipment. They are designed to allow an operator or bystander to stop the equipment in a hurry should anything go wrong.
System Design
Emergency buttons are wired in series with the control circuit on a piece of equipment. Pressing the emergency button breaks the circuit and removes power from the holding relay that keeps the circuit energized. The concept is the same for an energized single phase circuit, three phase circuit and for higher voltages. Larger pieces of equipment would operate off a larger relay.
Using the Emergency Buttons
To start a piece of equipment, the operator presses the Start button. This provides power momentarily to a relay coil. As the relay coil is energized, it closes its normally-open contact so that power is provided through the normally-open relay contact to both the relay coil and the load. Load is the electrical term for any piece of equipment that uses electricity. As long as the relay is energized, the circuit is complete and power is provided to the load. Pressing the stop or emergency button breaks the circuit, allowing the relay to return to its normally closed position.
Difference between a stop button and an Emergency Buttons
There are several differences between a stop button and an emergency button. The stop button is generally placed right next to the start button. It is the same size as the start button and if you let go of it the circuit is reset so that it can be restarted if desired. The emergency button (often there are more than one for large pieces of equipment) is bigger and highly visible. It is generally located where it can be easily seen and pressed by the equipment operator or a bystander. Once pressed, the emergency button locks into position and does not reset until manually reset by the operator. The circuit is broken and the equipment cannot be restarted until the emergency button is reset.
Restoring normal operation
Once the Emergency Buttons is reset, the start button can be used to bypass the normally open relay contact and energize the circuit, which energizes the relay coil and completes the circuit through the relay contact. This provides power to the circuit until power is interrupted by pressing the stop button or Emergency Button.
