System Design and Layout
GFK-0356Q
Chapter 12 System Design
12-9
12
circuits, etc. This type of circuit tends to fail “open,” which disables the equipment. For example,
if a wire breaks or a contact wears out, the circuit opens and the MCR drops out. If solid state
devices fail, they tend to fail “shorted,” which in the case of PLC output circuits would cause the
controlled device to turn on or stay on.
In the circuit below, the MCR is an electro-mechanical relay. It is energized by the application of
power to its solenoid coil which magnetically pulls the contacts to their energized states. When de-
energized, the contacts move to their normal, at rest states by the pull of a mechanical spring.
When the Reset pushbutton is pressed, and if all four of the E-Stop and Gate switches are closed,
the MCR relay will energize and “latch” in the energized state through its MCR contact in parallel
with the Reset pushbutton. Its other MCR contact applies power to the motor starter and PLC
output circuits. If any one of the E-Stop or Gate switches opens, or if a wire breaks in this circuit,
or if the MCR coil becomes defective, MCR will de-energize and open the circuit to the motor
starters and PLC output circuits.
As shown, the PLC’s main power and input circuits are not controlled by the MCR since they do
not directly control any outputs. Keeping these circuits energized is desirable because this allows
the PLC to continue gathering data, recording fault information, and controlling communications,
even if its Output module outputs are disabled by the MCR.
If an additional margin of safety were desired, two MCR relays could be used. Their coils would
be wired in parallel, their normally open contacts wired in series, and their normally closed contacts
wired in parallel. This would help guard against the possibility of a “welded contact” on a single
MCR relay.
MCR
MCR
Reset
E-Stop 1
E-Stop 2
Gate 1
Gate 2
MCR
To Motor Starters, PLC
Output Module Circuits, etc.
Master Control
Relay (MCR)
PLC Main Power and
Input Power Circuits
Figure 12-1. Hard-Wired MCR Circuit Example
Protection From Design Changes or Overrides
Only authorized personnel should be allowed to make changes that could impact the safe operation
of the equipment. Passwords and lockout circuits may be used to accomplish this. Some Series
90-30 CPUs have keylock switches to protect against program changes (see “CPUs” chapter for
keylock switch details).
