Safety relays or just cutting power directly Estop

[LazyMan]

I'm setting up my controller and I am considering my options for Estoping. I considering just having my estop input and 220V power line to the control box and VFD running through the same stop obviously through different contacts. Is this a bad idea? I'm concerned about the high voltage line running right next to low voltage Estop input and EMI. Just seems like the cheapest and easiest way to guarantee cutting power. The alternative and more proper option would be to use a safety relay, but they are quite expensive. Anyone know of an inexpensive one? I will be running steppers or a servos but I want the machine to be as safe as possible without going overboard.

Ideas?

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[Al_The_Man]

The customary way without a Safety relay is to use the simple 3 wire circuit, any other e-stop items are placed in the series string, the relay has contacts that shut off power to the coils of any motive power contactor or relay, the e-stop relay can also have a contact that, say advises the PC an E-stop has taken place and/or disable drives as well as killing the main power to them.
See page 11 for e.g.
http://static.schneider-electric.us/...0140CT9201.pdf
Al.

[iraiam]

What you are talking about is commonly referred to as a Master Control Relay, it's a common and reliable practice. There are a couple finer points though, many times the MCR only supplies power to motor circuits (motor starters, VFDs, servo amplifiers, etc.) The E-Stop(s) are wired in series to power the coil on the MCR, if an e-stop is pressed the MCR opens and power is interrupted to all of the motor circuitry, usually, the control power is not supplied through the MCR, so any of the electronic controls are still powered during an E-stop condition such as a PLC or other electronic/software controller.

The power supply to discrete output modules is usually interrupted during an E-stop condition also, so even if somehow a motor start output is turned on, there will be no motion because the output has no power source and the motor control device it is connected to has no power.

A 3 wire start/stop circuit will work but lacks the ability to interrupt power to the motor starter/controller, so it is possible that a motor will continue to run during an E-stop condition if there is a specific kind of failure to a motor control device, such as contacts welded together on a contactor, or a specific electronics failure on a Servo amplifier (rare but I have seen it).

[Al_The_Man]

A 3 wire start/stop circuit will work but lacks the ability to interrupt power to the motor starter/controller, so it is possible that a motor will continue to run during an E-stop condition if there is a specific kind of failure to a motor control device, such as contacts welded together on a contactor, or a specific electronics failure on a Servo amplifier (rare but I have seen it).

That is not necessarily true, a contact on the E-Stop MCR is typically used to open the supply voltage to motor contactor coils or any motive power device supply, therefore dropping the contactors in a E-stop condition, this is the traditional method, if contact welding is a concern then a modern Safety Relay could be used.
Al.