Page 217 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1
L2 Power
mains
L3
Main disconnect switch

Step-down
isolation transformer
L1 Fuse 120 L2
VAC PLC Control Panel
Machine Master
start/stop
Emergency stop switches control
buttons relay
MCR
Emergency Overtravel Stop Start
stop limit
switch
MCR
L1 L2 GND PLC power
supply

MCR
PLC input module
L1 1 2 3 4 5 6 7 8 L2

PLC output module
L1 1 2 3 4 5 6 7 8 L2

Figure 9-22 Safety wiring requirements for a PLC installation.
Source: Courtesy Minarik Automation & Control.

• A hardwired master control relay is included to pro- indicators on the processor module can still be
vide a convenient means for emergency controller observed.
shutdown. Because the master control relay allows • Note that the master control relay is not a substitute
the placement of several emergency-stop switches for a disconnect switch. When you are replacing
in different locations, its installation is important any module, replacing output fuses, or working on
from a safety standpoint. equipment, the main disconnect switch should be
• Overtravel limit switches or mushroom head emer- pulled and locked out.
gency stop pushbuttons are wired in series so that The master control relay must be able to inhibit all
when one of them opens, the master control is machine motion by removing power to the machine I/O
de-energized. devices when the relay is de-energized. This hardwired
• This removes power to input and output device electromechanical component must not be dependent
circuits. Power continues to be supplied to the on electronic components (hardware or software). Any
controller power supply so that any diagnostic part can fail, including the switches in a master control

198 Chapter 9 Program Control Instructions

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