Rockwell Automation 1747-L5xx SLC 500 Modular Hardware Style User Manual - Manual (Page 72)

Publication 1747-UM011G-EN-P - June 2008
72 System Installation Recommendations
Safety Considerations Safety considerations are an important element of proper system installation.
Actively thinking about the safety of yourself and others, as well as the
condition of your equipment, is of primary importance.
Disconnecting Main Power
Locate the main power disconnect switch where operators and maintenance
personnel have quick and easy access to it. Ideally, the disconnect switch is
mounted on the outside of the enclosure, so that it can be accessed without
opening the enclosure. In addition to disconnecting electrical power,
de-energize all other sources of power (pneumatic and hydraulic) before
working on a machine or process controlled by an SLC controller.
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.
Power Distribution
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.
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