Safety Considerations; Disconnecting Main Power; Safety Circuits; Power Distribution - Allen-Bradley SLC 500 User Manual

3-12 System Installation Recommendations

Safety Considerations

Publication 1747-UM011C-EN-P - December 2001
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.