Allen-Bradley 2080-LC20-20QBB User Manual page 33

Rockwell Automation Publication 2080-UM005E-EN-E - March 2018
Safety Circuits
WARNING: Explosion Hazard
Do not connect or disconnect connectors while circuit is live.
Circuits installed on the machine for safety reasons, like overtravel limit switches,
stop push buttons, and interlocks, should always be hard-wired directly to the
master control relay. These devices must be wired in series so that when any one
device opens, the master control relay is de-energized, thereby removing power to
the machine. Never alter these circuits to defeat their function. Serious injury or
machine damage could result.
Power Distribution
There are some points about power distribution that you should know:
• The master control relay must be able to inhibit all machine motion by
removing power to the machine I/O devices when the relay is de-
energized. It is recommended that the controller remain powered even
when the master control relay is de-energized.
• If you are using a DC power supply, interrupt the load side rather than the
AC line power. This avoids the additional delay of power supply turn-off.
The DC power supply should be powered directly from the fused
secondary of the transformer. Power to the DC input and output circuits
should be connected through a set of master control relay contacts.
Periodic Tests of Master Control Relay Circuit
Any part can fail, including the switches in a master control relay circuit. The
failure of one of these switches would most likely cause an open circuit, which
would be a safe power-off failure. However, if one of these switches shorts out, it
no longer provides any safety protection. These switches should be tested
periodically to assure they will stop machine motion when needed.
About Your Controller Chapter 2
23