Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 26)

Chapter 1
Installing Your Controller
1–10
!
ATTENTION: Explosion Hazard — Do not connect or
disconnect while circuit is live unless area is known to be
non-hazardous.
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 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.
•
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 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 out, it no longer provides any safety protection. These switches
should be tested periodically to assure they will stop machine motion when
needed.
The following explains power considerations for the micro controllers.
Isolation Transformers
You may want to use an isolation transformer in the ac line to the controller.
This type of transformer provides isolation from your power distribution
system and is often used as a step down transformer to reduce line voltage.
Any transformer used with the controller must have a sufficient power rating
for its load. The power rating is expressed in volt-amperes (VA).
Power Considerations