I'm currently looking at installing a new controls cabinet for a conveyor system. Many of our existing units have a safety realy running the e-stop circuits. Is there something in wiring code says it needs to be this way or can it be incorporated into the PLC. Any refernces would be appreciated as well

ANSI B11.19: 12.9 STOP AND EMERGENCY STOP DEVICES

Stop and emergency stop devices shall meet the requirements of NFPA 79

This squared document is helpful as well http://www.squared.com/us/products/machine...S_Standards.pdf

Bottom Line - Emergency Stops must be Control Reliabile and PLC's usually do not meet this criteria unless designed for it. Hence Safety PLC's.

Thank you Bob

Beckhoff has released a line of safety controller modules that plug into their DIN-rail modular controller line. My previous employer had setup a number of systems, but I did not have the chance to get my fingers in the code. In this scenario, you save panel build time by having less wiring, but the trade off is more engineering time getting the code written.

NFPA 79. It depends on the application. It doesn't always require control reliable. But you can't just use an ordinary PLC input/output. Reason being that if the PLC faults, what happens? If a PLC output relay burns up and shorts, what happens? Further more, NFPA 79 doesn't REQUIRE an E-Stop. It just specifies what you have to do if you have one. Your risk assessment defines whether you actually need it or not. Quite often the risk assessments end up dumping the E-Stop if you take them seriously and don't have an "all other problems" category (which pretty much invalidates the risk assessment).

I've done it several times. You CAN use a safety PLC but they are fairly expensive. You get to use everything in the same environment, and your safety code must be written in a separate and slightly different ladder logic, but it at least exists in the same PLC. The difference between a "safety" PLC and an ordinary one is that there's a bit of extra hardware logic in it and a second complete PLC. Both PLC's run lock-step with each other. If there is a fault or if the outputs of one PLC ever disagree with the second one, then the hardware logic in between goes into fail-safe mode (shut everything down). That's why the cost is a little over twice that of an ordinary PLC...the double redundancy is necessary to achieve control reliable status.

If you implement everything using ordinary safety relays, you can then monitor outputs from the safety relay. Generally you just interpose the safety relay in series with the appropriate PLC outputs where necessary to make things work OK. You NEED to monitor the safety relay status outputs though because you need to know when to shut down the PLC activity (and vice versa) so that reset can proceed correctly.