E-stop on a conveyor: hardwired or PLC?

Your intuition is correct and E-STOPS for system operate the main relay outside the PLC. It is also common to see a PLC OUTPUT contact in the relay circuit that allows the PLC to also enable and / or OPEN the main relay. Often pushing start provides an input to PLC that causes an output contact to act as to enable circuit to operate, if everything is OK per the PLC, See PLC hardware manuals as examples are often there and safty warnings, E-STOP must open relay, NO MATTER what and is always first in the power flow for the circuit.

Our standard makes the hard-wired emergency stop part of the switchgear safety circuit i.e. the switchgear drops out nevermind what. A second set of e-stop contacts goes to a digital input so the PLC knows why the conveyor tripped. That is the old method.

We are now looking at profisafe (profibus safety network). You could also consider a safety PLC, e.g. Pilz.

We installed some intelligent pullkey systems functioning on a frequency signal to effect an immediate trip should the circuit be opened (eg tripwire or e-stop) and then it uses a current summing technique to determine which station was tripped.

Hard-wired *fail-safe* relay logic is standard practice world-wide. The relay is wired to cut power to the conveyor drive and to release clutch brakes (where fitted). Tripping the power is required, since there is not really any way the control system can know why the e-stop was pressed.

Wiring an emergency stop system through a PLC (a non-safety-rated one, at any rate) is actually a criminal offence in some countries - but that doesn't mean it doesn't happen to save a buck or 3. Even Safety PLCs can be a problem if not programmed correctly...

Your local Occupational Heath and Safety regulations will govern what you can and can't legally do. I'm not familiar with all the Standards in your country, but you could start with NFPA 70 and go from there..

I hope this helps, Cameron:-)

You could not doubt find a few standards. I would cover myself with using industry practice.

Have the Emergency Stop Relay disconect contactor power to the drive motors while using an Auxiliary contact to signal the PLC. You can run the Emergency stop loop in series but use an auxiliary contact on the emergency stop to inform the PLC which emergency stop was actualy pressed. (this is a big issue on converyors)

Alternatively you could use a special safety PLC such as is supplied by PILZ or in some variants of the Siemens S7. These have duplicated cross checking inputs, outpusts, components and processors (and a special area for the safety related program) and are certified to directly opperate emergency stops or safety related start interlocks.

Be very careful here. If you are in the US, there are very specific standards, etc. that must be followed (as I am sure there are in other countries). Make sure that you look at the OSHA regs for Industrial Machinery. In there you will find specifics on the e-stop circuit and resetting of the circuit. I don't have the exact OSHA reg numbers, but you should be able to find it on their site.

My career path changed from electrical sales to Controls Engineer over the last few years. I've acquired most of what I know from looking at other's designs and applying what appeared to me as "best practices" from what I've seen. It has often occurred to me though, that there must be somewhere that spells out what these best practices are, or to a greater extent what is legal and what is not. Where do I go for this information? The NFPA 70 (National Electrical Code) doesn't cover controls or automation in any detail. If someone knows the publication number of an OSHA manual with useful, practical information on this topic, please post it here. Are there any professional organizations for controls people? Any other sources of knowledge (textbooks, publications, discussion groups, etc.) that would also help?

Have a look at NFPA 79 "Electrical Standard for Industrial Machinery". If I remember right, the OSHA regs refer to this. Seems like there are a lot of people who are not familar with NFPA 79, yet it provides a lot of useful information and is a requirement. You can access it at

Kevin and others,

Some OSHA and ANSI standards that relate to safety systems which must be "Control Reliable" as in emergency stops, safeguards etc.

OSHA 1910.217(b)(13) Control Reliability The control system shall be constructed so that a failure within the system does not prevent the normal stopping action from being applied to the equipment when required, but does prevent initiation of a successive stroke until the failure is corrected. The failure shall be detectable by a simple test, or indicated by the control system. This requirement does not apply to those elements of the control system which have no effect on the protection against point of operation injuries.

ANSI B11.19-1990 Section 2.12 Definitions Control reliability is defined as a method of ensuring the integrity of the performance of guards, devices or control systems.

ANSI B11.19-1990 Section 5.5.1 Control Reliability When required by the performance requirements of the safeguarding, the device system or interface shall be designed, constructed and installed so that a single componenet within the device, interface or system shall not prevent the normal stopping action from taking place, but shall prevent a successive machine cycle. This requirement does not apply to those components whose function does not affect the safe operation of the machine tool.

Further reference is made in ANSI B11.19 Section 11.1.1 (1996)

ANSI B11.20-1990 Section 6.13 Control Component Failure Protection The control system shall be designed, constructed and installed so that a single control component failure within the system does not prevent the sopping action from taking place, but will prevent successive system cycles until the failure has been corrected. This requirement only applies to those componenets whose failure can result in a hazardous condition.

There are also ANSI-RIA standards which have similar requirements for robot applications. I can post if requested.

These standards absolutley rule out using a standard PLC as the monitoring device for any safety circuit. An electromechanical relay circuit (classic control circuit) is better, but still doesn't meet the requirments for what the machine must do in the event of a failure (wiring fault, relay malfunction, etc.) A safety relay is what is needed in order to comply with these standards of "control reliability". Or, to take it a step further as some have suggested, a safety PLC also meets the requiremtents.

As a disclaimer, I work for Pilz, so I deal with these things daily. Pilz does offer a line of safety relays, programmable safety controllers and safety PLCs to meet any need I have come across thus far in the automation industry.

Hope this helps, Frank Ketchum

If someone knows the

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Bart Schaminee