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This circuit demonstrates the function of a emergency stop (E-stop) relay, like the ones in the following links:
http://img.directindustry.com/images_di/photo-g/787-7443115.jpg
https://cdn.shopify.com/s/files/1/1034/1611/products/2279978-40.jpg?v=1501620901
The internal circuit of the E-stop relay is different per kind, but could be like the one in the following link:
https://i.stack.imgur.com/PGxa6.png
In the E-stop relay from above mentioned diagram, there are 3 coils; K1, K2 and K3.
The N.O. contacts of coils K2 and K3 are considered the "safety contacts". An example: with these contacts power supplies to electric motors or solenoid valves can be switched off when a E-stop button is pressed.
These N.O. contacts are connected in series for safety. When one contact fails to open, the other one will (very, very, very little chance that the second N.O. contact fails too) and the power to motors/solenoid valves will still shutoff.
An E-stop button should always consist of 2 N.C. contacts connected in parallel.
N.C. because, in case the cable will break, the circuit is opened and the E-stop relay will see this as "E-stop button pressed" and shuts down everything connected to the N.O. contacts of coils K2 and K3.
Why 2 contacts? Because the E-stop relay sends out an positive and negative voltage. These voltages pass through the N.C. contacts of the E-stop and will return to the designated terminals on the E-stop relay. When these voltages are shorted or mixed up, the E-stop relay will see this as "E-stop button pressed" or as a fault. Again, everything connected to the N.O. contacts of coils K2 and K3 will shut down. This is called "failsafe". It is very hard to "cheat" on the E-stop relay, because it is meant for safety!
Also, the E-stop button should latch in the pressed/activated position. When pulling back the E-stop button actuator, it is done conscious and prepares the circuit for reset. Resetting the circuit may never be an automatic sequence! It should always be done manually. Again, this is for safety.
When the E-stop relay is powered up and the E-stop button is not pressed (circuit is closed), the reset button can be pressed. This activates coil K1, and on his turn, activates coils K2 and K3.
N.C. contacts of coils K2 and K3 will prevent coil K1 from being activated again untill the E-stop button is pressed and released, or the E-stop relay is powered off and on again. This is also for safety.
For example: when only 1 voltage is going through the E-stop button to the E-stop relay (wire break or bad connection of the other voltage) the reset button can be pressed, but coil K1 will activate only K2 or K3. That coil is activated and its N.C. contact prevents coil K1 from being activated again. Now with only K2 or K3 activated, the circuit to motors and solenoid valves is not closed (only one N.O. contact of K2 or K3 is closed). Also the E-stop relay cannot be reset again untill both voltages are pressent on the E-stop button, the E-stop button is pressed and released so both voltages will go through the E-stop button and return to their designated terminals. When the reset button is pressed, K1 will now activate K2 and K3.
When switching off motors, the E-stop relays should be always in the main current circuit and control circuit. Power to the motor should be shutoff to get the highest safety but also its control circuit should be shut down. This to prevent automatic startup of the motor when the E-stop circuit is reset. Especially for VFD motors or Y/Δ motors!
In the simulated circuit the next components will be present:
* the horizontal relay coils on the left side of te screen; these represent coil K1 of the E-stop relay diagram;
* the horizontal relay coils in the middle; these represent coil K2 of the E-stop relay diagram. These coils will need a positive voltage from the E-stop button;
* the horizontal relays coils more to the right; these represent coil K3 of the E-stop relay diagram. These coils will need a negative/0V voltage of the E-stop button. This is simulated by the DC power supply in the middle of the screen, connected to one side of these coils;
* the vertical relay coils on the right of the screen represent the normal control circuit of the motor, with LED indicators;
* the 2 latching switches on the left represent the N.C. contacts of the E-stop button;
* the momentary switch on the left represents the reset button;
* the 2 change over switches can be operated to simulate wrong wiring of the E-stop button.
Take your time to understand the circuit and its behaviour to wrong signals from the E-stop button. You will see that cheating on a E-stop relay is very difficult. Slow down the simulation speed to about 1 to 10 μs/s if you want to have a more step by step view of the resetting of the coils. Keep in mind that you need to hold the reset button for a few seconds when slowing down the simulation speed.
If you have questions about the simulated circuit, or about E-stop circuits, feel free to ask!
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