Honeywell HC900 Installation And User Manual page 55

System Monitor Function Blocks
The HC900 Controller includes function blocks that enable the user to monitor the status of system
functions. When constructing a control configuration, add the following function blocks to the control
strategy:
• ASYS - System Monitor
• FSYS - Fast System Monitor
• RK - Rack Monitor
These function blocks are described in the HC900 Function Block Reference Guide.
Master Control Relay
A Master Control Relay (MCR) structure is a safety mechanism for shutting down the process control
system in emergency conditions. This mechanism, which is hard-wired (provided and installed by the
User) can include several Emergency Stop switches., strategically located near process equipment. An
example of an MCR structure is given in Figure 22.
Operating any of the Emergency-Stop switches opens the holding path for the MCR. When the MCR
de-energizes, the MCR contact opens, disconnecting all AC power that is supplied to AC Input Modules
and to AC Output Modules. Notice that AC power is disconnected only from the AC input/output modules.
Power is still available to Power Supplies at the Controller Rack and at each I/O expansion rack. The
Controller Module and the Scanner Modules in the racks continue to execute diagnostics and other
programs.
The Master Control Relay does not remove power from the Controller rack or from any of the I/O
expansion racks.
• Before performing service tasks such as installation of terminal connections or fuse replacement, use the
appropriate switch(s) to disconnect power from the power supply at each module.
• Ensure that wiring design precludes over-riding of the MCR by operator actions.
Failure to comply with these instructions could result in death or serious injury.
• DO NOT REMOVE OR REPLACE MODULES WHILE CIRCUIT IS LIVE UNLESS THE AREA IS
KNOWN NOT TO CONTAIN FLAMMABLE VAPORS.
Revision 5
9/03
Class 1, Division 2 Installations
HC900 Hybrid Controller Installation and User Guide
Pre-Installation Planning - Electrical Considerations
45