Snap Acting Relay; Lockout Relay - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

PNEUMATIC CONTROL FUNDAMENTALS
ROOM
THERMOSTAT
DA WINTER
RA SUMMER
M B
RESTRICTOR
M
M
VALVE
COIL
Fig. 35. Typical Switching Relay for Application.
During the cooling cycle, the 90 kPa mainline pressure at
the pilot port (P) is below the minimum switching pressure of
the preset limits. The common port (C) connects to the normally
connected port (O), which is capped. The normally disconnected
port (X) is closed and removes the low-limit controller from
the system.
In a dpdt model, the common, normally connected, and
normally disconnected ports are duplicated in the second
switch section.

SNAP ACTING RELAY

The snap acting relay is a spdt switch that provides two-
position switching action from a modulating signal and has an
adjustable switching point. The switching differential is less
than 7.0 kPa. The switching pressure is manually adjustable
for 20 to 100 kPa operation.
Figure 36 shows a snap acting relay application. Operation
is similar to the switching relay. When the branchline pressure
from the outdoor air thermostat equals or exceeds the preset
switchover pressure, the relay connects the normally
disconnected port (X) and blocks the normally connected port
(O) to deliver main air to the normally open heating valve
and provide positive close off. When the outdoor air thermostat
pressure drops below the relay setpoint, the normally
disconnected port (X) is blocked and the normally connected
port (O) connects to the common port (C) to connect the valve
actuator to the room thermostat.
SWITCHING
RELAY
CAP
C O
P X
LIMIT
CONTROLLER
DISCHARGE
AIR
C2379
DA OUTDOOR AIR
THERMOSTAT
M
B
M
SNAP ACTING
RELAY
N.O. HEATING
VALVE
Fig. 36. Typical Application for Snap Acting Relay.

LOCKOUT RELAY

The lockout relay is a three-port relay that closes off one
pressure signal when a second signal is higher. Figure 37 shows
a typical application in which mixed air control becomes
disabled when outdoor air temperature is higher than return air
temperature. To prevent air from being trapped in the line
between the lockout relay and the snap acting relay, a small
bleed must be present either in the pilot chamber of the snap
acting relay or in the line.
MIXED AIR
TEMPERATURE
CONTROLLER
SNAP ACTING
M B
RELAY
X
M
O
EXH
RETURN AIR
THERMOSTAT
M B
M
Fig. 37. Lockout Relay in Economizer Cycle.
Figure 38 shows the lockout relay used as a repeater. This
application provides circuit isolation by repeating the pilot
signal with a second air source.
P 1
EXH
M
B
RESTRICTOR
LOCKOUT
RELAY
OUTPUT
Fig. 38. Lockout Relay as Repeater.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
78
DA ROOM
THERMOSTAT
P X M B
M
C O
VAV TERMINAL UNIT
DAMPER ACTUATOR
C2360
OUTDOOR AIR
DAMPER ACTUATOR
C
P
OUTDOOR AIR
THERMOSTAT
BLEED
LOCKOUT
RELAY
M B
P 1
P 2 M
B
C2362
THERMOSTAT
RESTRICTOR
P M
2
TO
ACTUATOR
C2355