How Do Emergency Stop Buttons Work

Figure 1, Energized circuit with E-Stop

Emergency Stop Buttons

Emergency stop (E-Stop) 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 stop buttons are wired in series with the control circuit on a piece of equipment. Pressing the E-Stop 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 E-Stop

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 E-stop button breaks the circuit, allowing the relay to return to its normally closed position.

Difference between a stop button and an E-Stop

There are several differences between a stop button and an E-Stop 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 E-Stop 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 E-stop 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 E-stop is reset.

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Restoring normal operation

Once the E-stop 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 E-Stop.