Pre-Installation Planning - Electrical Considerations
52 HC900 Process Controller Installation and User Guide Revision 19
06/14
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 32.
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.
Class 1, Division 2 Installations
• DO NOT REMOVE OR REPLACE MODULES WHILE CIRCUIT IS LIVE UNLESS
THE AREA IS KNOWN NOT TO CONTAIN FLAMMABLE VAPORS.
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