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
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,
email@example.com (Kurt Colvin) wrote in message
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 www.nfpa.org
On 11 Jun 2004 19:12:34 -0700, Kevin Spears wrote:
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
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
Hope this helps,
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