NFPA 79 - 9.2.7.5 Use of More Than One Operator Control Station

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katan840df

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Grand Rapids, MI
I'm in the design phase of a piece of equipment for a customer that would like two operator stations for their equipment. It's a simple application that has a rotating bucket, but the customer would like to move the equipment around and have the operator work from either side.

So, my design includes a Micrologix 1400 (customer spec.) and an AB 440R safety relay.

I'm checking to see if I'm allowed to duplicate the controls. Here are the operators that I have on each side:
  1. Fault Light for MSP trip
  2. E-stop
  3. MCR Reset/Power On Ill. Pushbutton
  4. Press To Enable / Hold to Run Ill. Pushbutton
  5. 3 position spring to center Rotate/Off/Return Selector Switch
My sequence for the operator would be (if the E-stop is not pressed and the MCR is reset) to come up to one side or the other, Press the White Illuminated Enable PB, white light illuminates meaning this side has control, and then for the operator to hold this button with one hand and rotate or return the equipment with the other hand. As soon as either operator is released, the equipment stops.

Equipment is electrotonically interlocked through the PLC. But can I have an MCR reset button on both sides, since the reset doesn't enable motion, it just resets the MCR relay?


Code snippet.
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Thanks for any feedback!
 
You can duplicate the controls as long as a control is only active from one location simultaneously.

IMO, how you chose to create the interlock(s) to prevent simultaneous operation at more than one location is up to you.

I try to avoid such things but often it is can be handy or even necessary.

Keep in mind you will need a hard wired estop at any location where there are operator controls.
 
A simple way is to put a key switch in each station, but only have one key. Use a "captive" key switch meaning the key can only be removed when the switch is in the Off position.

I've also done it with an two latching relays. In each station, you have 2 buttons labeled "Control" and "Release" that latches or unlatches a relay to give that station the ability to control, disabling the the Control button on the other side along with the other controls (except for the E-Stops). So to allow the other side to be used for control, the operator must UNLATCH the relay for the side he is on first by pressing the "Release" button, then go to the other side and press "Control" for that side. Hitting an E-stop unlatches both stations. I used this on a remote control system for a gravel loader. The operator could run the rock crusher from a local control station or from a radio controller on the loader, but only one at a time.
 
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