Not sure what to even google... Safety lock out relay?

Thread Starter


Joined May 9, 2018
Electronics newbie, so please bear with me... Thanks in advance!

I'm designing a control system for some solenoid-actuated hydraulic valves on my tractor to run a brush cutter. If you're not familiar, brush cutters can be pretty dangerous implements, so I want to reduce the likelihood of the brush cutter coming on unexpectedly as much as possible.

My design so far has a control activation switch and a brush cutter control switch. The brush cutter control switch doesn't work unless the control activation switch is closed, and the control activation switch must be OPEN in order for the tractor to start. The next issue I'm trying to solve for is where the operator pushes the control activation switch (to close it) thinking the brush cutter will be off, but turns it on because the brush cutter control switch (which is a latched switch) was left on.

I'm thinking this would be some kind of sensing device where, if it sensed a 0V to ~12V input from the control activation switch AND current getting past the brush cutter control switch, it would instantly interrupt the flow of electricity from the brush cutter control switch until it sensed 0 current. I suppose it would be sort of like what a GFCI does...?

What is such a device called? Or, if it's not a device, how do you do this? What's the circuit arrangement called? Just need something to go on so I can google and educate myself. This is a very simple, 12VDC vehicle electrical system.

Again, thanks!


Joined Oct 2, 2009
A relay or solenoid switch and two push buttons, one for RUN and the second for STOP.

1) When the power is off the relay is disengaged.
2) The relay is only engaged when the tractor is running AND a momentary push button (normally open - NO) is pressed.
3) Relay contacts are wired to make the relay self latching.
4) A second momentary push button (normally closed - NC) interrupts power to the relay and disengages the relay.


Joined Jun 7, 2009
Lets be clear, if you enter into the operational envelope of the equipment, it needs to be off, and blocked as required. Periods following repair can be especially dangerous due to entrained air.

adding 5) add field maintained contacts in series to lockout


Joined Sep 11, 2014
...................What is such a device called? Or, if it's not a device, how do you do this? What's the circuit arrangement called? Just need something to go on so I can google and educate myself. This is a very simple, 12VDC vehicle electrical system.................
Safety standards deem it mandatory to protect against automatic restarting of equipment, upon restoration of electrical power following an unscheduled interruption.

The device / circuit is known as an 'anti-restart relay' that prevents equipment from restarting, until a 'start' button is depressed. It's a standard electromagnetic relay that is energized with the press of a button and remains so till the stop button is depressed or power is interrupted.

In your application the device should energize the hydraulic valve only after the 'start' button is depressed.

Last edited:

William Ketel

Joined Jan 23, 2018
An arrangement that is often required with production machines is "two handed anti-tiedown, anti repeat". That function requires that two "start" buttons be pressed at close to the same time in order to initiate some action. A second thing is a latching action stop button, which must then be manually pulled back out to enable starting again. That assures that some fault within the system does not lead to an automatic restart when the stop button is released, which can happen if a contact sticks shut on a relay.
Having another stop button that is accessible from where somebody would be to service the brush cutter is also a good idea.


Joined Jun 8, 2013

Here is an example. I've shown relays instead of switches but you could replace the relay contacts with corresponding switch contacts for analysis purposes. The "Lockout" is shown in dotted lines and is a circuit configuration rather than a type of relay.
The lockout function prevents one function from operating if an conflicting function has already been selected. In the lockout circuit below, if the TSR relay has already been selected, then the CAR relay is prevented from energizing, and visa versa. The stop button resets/stops the entire circuit. This is just an example and other things, like current capacity, etc., would have to be considered in a real circuit.