Master Control Relay (Safety Relay)

[Richards]

It's common practice to install a master control relay (MCR) in a process control cabinet. Sometimes the MCR is called a Safety Relay. Its purpose is to disconnect power to anything that could cause a safety issue. The MCR can be added to an Acorn. I've drawn a simple schematic that shows one way a relay/contactor could be used as an MCR.

I normally use two 24VDC power supplies with my Acorns. One 24VDC power supply supplies power only to the Acorn's Power pins (H9). The second power supply supplies all other 24VDC power (H1/H4, devices connected to H2 and H3, devices controlled by the Acorn Relay board). Both power supplies are turned on by the master disconnect switch. That way, power is always applied to the Acorn if the controller is powered on. After the controller is powered on, pressing the Start button turns on the MCR if the N/C Stop button and the N/C E-Stop switch are not being pressed. The MCR is self-latching via a set of contacts inside the relay. When the MCR turns on, an indicator lamp turns on showing that the MCR is on. If the MCR is on, power is applied to the Output Power Rail which supplies power to devices connected to the Acorn's Relay Board. If the Stop button or the E-Stop button is pressed, power to the MCR's coil is interrupted and the relay's contacts open. I normally use a 3PST contactor with auxiliary N/O switch instead of a relay. The 3-poles on the contactor allow multiple voltage/power sources to be controlled.

You'll note that the E-Stop has two poles allowing it to control both the MCR and to supply an E-Stop signal directly to the Acorn.

Using an MCR is optional. I use it because at one time OSHA required a mechanical circuit to control power to all outputs in a control box. They didn't allow a PLC (Acorn or other PLC) to control an MCR because it wasn't possible to know what the PLC would do when the E-Stop was pressed. On the other hand, he mechanical circuit clearly shows how power is disconnected when the E-Stop is pressed.

mcr.png (5.07 KiB) Viewed 444 times

[cnckeith]

thanks, great post!

as an alternative, install a disconnect (knife switch) on the cabinet or wall with fuses/circuit breakers. this is another way to safely disconnect all power from the cnc electrical cabinet. as seen on our complete system schematics such as this one:
https://www.centroidcnc.com/dealersuppo ... 031.r2.pdf

[Richards]

Keith,
I agree that a mechanical disconnect, like a knife switch, might be the best choice if you needed to kill all of the power; but, a Master Control Relay typically does not kill all the power. Depending on how it is wired, it kills the power to selected devices while leaving other devices activated. For instance, on a CNC router using a vacuum table, you might need to stop the servos while keeping the vacuum going. Or, it could be used as a fail-safe device to cut power to a spindle so that the operator can safely change cutters. If the E-Stop is punched, the switch's knob would normally have to be twisted to release and then the Start button would have to be pressed to re-enable the circuit.

In the PLC world, where I spend a lot of time, a Master Control Relay is used to cut power to output devices without stopping the PLC program. By doing that, the PLC can continue to send MQTT, MODBUS and Ethernet IP data to the supervisory computers while technicians fix whatever needs to be fixed before the outputs are turned back on. I know that I'm comparing apples to oranges, but an MCR is an option for those who need a partial shutdown in the middle of a job. (Of course, in that case, the MCR's E-Stop button would not be wired into the Acorn.)