How do door safety/Interconnect safety switches work?

[borges1]

So i am slowly working toward a completely revamped electrical enclosure/expansion for my PM 833tv mill. The enclosure i got for pretty cheap has a swinging door, which i expect will be convenient. Since i'm using this project as an exercise to begin learning more about machine control systems i figure i should incorporate a door shutoff/safety interconnect like on my industrial wood machines. After taking the thing apart and looking at the e-stop switch i cannot see a reason i cannot use a simple pressure switch in line with the e-stop so that when the door opens or the e-stop is pressed the machine shuts off. the stock e-stop switch lines are tiny, like 24ga, and run to a pinout on the stock board that says run/stop so it has to be some kind of relay. any reason this would not work and be safe? would it be simple to use the same interconnnect on the separate 120v circuit i am running to the box? that part seems beyond me right now.

in a more general way i had a hard time finding any info on how machine control interconnect safety switches work. can anyone explain it to a beginner? i see how if any of the safety inertconnects (which are switches in sequence?) or e-stop are hit the machine shuts down. Is this type of system generally designed to cut power, or go through a shut down sequence? like my big wood machiens have heavy duty elecric brakes, but i didnt test the e-swithes on those tonight. It would make sense to me they would shut down (i.e. brake) not just kill power (i.e. let giant cutters just spinn on the bearings until they stop).

The interconnect systems are obviously is not switches running the full voltage, which would be a bit scary on a motor of any size. so it has to be a control voltage in the interconnects all linked together? then what? to a relay that kills the power to everything? school me.

 

[mksj]

Typically they would interrupt the VFD low voltage run commands and sometimes activate a separate VFD input which is a fast stop command and requires the machine to have no active run commands to reset. If there is just a single set of wires, you would need to see if when the E-Stop is not activated if the switch is NO or NC state (would determine if the safety switch was in series or parallel). They may also break the voltage to a power relay when pushed, this would also would be something like 24V.

 

[mmcmdl]

I know more about dis-arming them than installing them . I carried the 2 prong key on my keychain and almost got reprimanded for doing so . We finally convinced management it was safer than climbing in the machines with doors closed and locked for troubleshooting .

 

[markba633csi]

You can put any number of normally closed switches in the same line as your E-stop, if the E-stop switch itself is a normally closed type.
On some older import systems you couldn't do that, if the system used an E-stop switch that stayed open when pushed.
But on most modern systems all it takes is a momentary breakage of the circuit to shut everything off, and stay off.

 

[RJSakowski]

The e Stop circuits that I have seen use the switch to break the circuit to a relay or contactor. The relay/contactor is energized by monumentally pressing the start button and an auxiliary set of contacts apply voltage to keep the relay/contactor energized when the start button is released. The e Stop is in that circuit so when it is pressed, it must be released and the start button pressed to restart. Normally, the e Stop would cut all power to the machine.

 

[GeneT45]

The architecture of the e-stop circuit varies depending on the assessed risk. Higher risk and/or complexity can add complexity to the safety circuit. At the top there are "smart" safety relays - which are more complicated than just a relay. They accept various inputs, all redundantly, and are pretty good at detecting most bypass attempts.

I have never seen a normally closed e-stop - because you want the device to fail in a safe manner. So if a wire in the safety circuit is disconnected, or broken, that also trips the e-stop circuit. Similarly if your saw or other machine with stored energy has a brake, then the brake should be active when power is removed, and deactivated by power being applied. A compliant safety system needs power to energize, but cannot require power to make the system safe, otherwise it may present an unsafe condition during a loss of power.

GsT

 

[borges1]

Typically they would interrupt the VFD low voltage run commands and sometimes activate a separate VFD input which is a fast stop command and requires the machine to have no active run commands to reset. If there is just a single set of wires, you would need to see if when the E-Stop is not activated if the switch is NO or NC state (would determine if the safety switch was in series or parallel). They may also break the voltage to a power relay when pushed, this would also would be something like 24V.

This makes sense. So for most modern machines the interconnects are linked to the programmable controls. Interrupting the safety circuit induces a programed shut down response. Older and/or simpler systems must just be relay based im guessing?

 

[markba633csi]

Yes

 

[tq60]

Repeating others...

Look for the e-stop or stop controls, either one should work.

Locate the actual switch and count the wires connected.

Wires should be smaller in size.

A "proper" configuration will be a normally closed switch that breaks the connection when activated.

This is to insure reliability, if a wire breaks, it will not operate.

With machine unplugged, disconnect one wire and check switch with ohmeter and it should read short until button pressed.

To test, make sure wire cannot touch anything and plug in machine then try to start it, should not go.

Unplug machine and reconnect until ready to connect.

If it is opposite measuring and machine starts, it is make contact, simple parallel switch.

Get the correct switch that can be mounted inside the door, this is control function so a 10 Amp switch should be fine.

The switch needs to be same as e-stop idle when door is closed.

If a closed circuit, the wire first removed is removed again, a pair of wires run to new switch.

One wire goes to the removed wire and other to where wire removed from.



Sent from my SM-G781V using Tapatalk

 

[den-den]

Lots of good information already posted, a few additional points:
Control systems, Alarm systems and Safety systems do similar things but are not the same.
Control systems are designed to start / stop machine(s). This may be as simple as a single start/stop push-button station or a single level switch to start / stop a pump automatically or complicated with multiple position switches, operator controls, level controls, pressure controls etc. They may be wired with a combination of "NO" and "NC" contacts. Sometimes they are 120 vac powered, sometimes 24 vac or 24 vdc, or 5 vdc.
Safety systems should be "fail-safe" and are wired in series to keep a relay(s) energized. When the relay drops out, the shut-down sequence starts (possibly as simple as "every thing stops"). Momentary push-buttons in a safety circuit would be wired to "NC" (normally closed) contacts (push the button - shutdown occurs. A door interlock switch would use "NO" (normally open) contacts (door opens, contacts open - shutdown occurs). Wire breaks somewhere in the circuit - shutdown occurs. These were all 24 vdc circuits where I worked (offshore oil & gas platforms) but could be otherwise.
I have seen alarm systems wired like a safety system (circuit opens - alarm goes off). I have also seen them wired so that a closed circuit sets off the alarm (these had a "end of line" device, usually a capacitor and electronics to monitor the circuit so that a broken wire triggered a "trouble alarm".

All of this is intended to point out that it is necessary to know / learn the equipment that you are working with and NOT assume that others are the same.