AutomationDirect DL105 - Three Levels of Protection; Emergency Stops

Installation, Wiring,
and Specifications
2--3
Installation, Wiring, and Specifications
DL105 PLC User Manual, 3rd Edition
The publications mentioned provide many ideas and requirements for system
safety. At a minimum, you should follow these regulations. Also, you should use the
following techniques, which provide three levels of system control.
S Orderly system shutdown sequence in the PLC control program
S Mechanical disconnect for output module power
S Emergency s top switch for disconnecting system power
It is recommended that emergency stop circuits be incorporated into the system for
every machine controlled by a PLC. For maximum safety in a PLC system, these
circuits must not be wired into the controller, but should be hardwired external to the
PLC. The emergency stop switches should be easily accessed by the operator and
are generally wired into a master control relay (MCR) or a safety control relay (SCR)
that will remove power from the PLC I/O system in an emergency.
MCRs and SCRs provide a convenient means for removing power from the I/O
system during an emergency situation. by de--energizing an MCR (or SCR) coil,
power to the input (optional) and output devices is removed. This event occurs when
any emergency stop switch opens. However, the PLC continues to receive power
and operate even though all its inputs and outputs are disabled.
The MCR circuit could be extended by placing a PLC fault relay (closed during
normal PLC operation) in series with any other emergency stop conditions. This
would cause the MCR circuit to drop the PLC I/O power in case of a PLC failure
(memory e rror, I/O communications error. etc.).
Saw
Arbor
ESTOP
Master
Control
Relay
MCR
Emergency
Stop
Power On
Use E-Stop and Master Relay
Guard
Limit
Guard Limit Switch
MCR
L1 to Output Supply
L1 to Input Supply
(optional)
MCR
Hot
Neutral
Three Levels of
Protection
Emergency Stops