Rockwell Automation 1747-L40 SLC 500 Fixed Hardware Style - Manual (Page 48)

Chapter 2
System Installation Recommendations
2–9
Wiring Safety Circuits
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.
Distributing Power
There are some points about power distribution that you should be aware of.
First, 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.
Second, 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–on and 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 is connected through a set of master control relay contacts.
Testing the 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.
The printed circuit boards of the controller must be protected from dirt, oil,
moisture, and other airborne contaminants. To protect these boards, the
controller must be installed in an enclosure suitable for the environment.
The interior of the enclosure should be kept clean and the enclosure door
should be kept closed whenever possible.
Regularly inspect your terminal connections for tightness. Loose
connections may cause improper functioning of the controller or damage the
components of the system.
!
ATTENTION: To ensure personal safety and to guard against
damaging equipment, inspect connections with incoming power
off.
The National Fire Protection Association (NFPA) provides recommendations
for electrical equipment maintenance. Refer to article 70B of the NFPA for
general requirements regarding safety related work practices.
Preventive Maintenance