5
1-3. Indicating Switch (refer to Figures 1-3 and 2-1)
Rotation of the DPU’s torque tube shaft is coupled through connecting linkage
within the switch case to move the pointer across the scale plate. An actuating
cam, directly connected to the torque tube shaft, rotates with the motion of the
shaft. Two cam follower roller/actuator arm assemblies, one for each switch,
respond to torque tube rotation by opening and closing the switches as they ride
on and off the cam. The levels of differential pressure at which the switches actu-
ate are adjustable with high and low alarm switch adjustments on the scale plate.
Standard models have two alarm switches - one or both may be used. Each switch
can be connected to operate normally-opened or normally-closed. The direct-set
switch contacts are adjustable over a scale range of 5-95% nominal.
100
0
50
Zero DP
100
0
50
Maximum DP
RED
Y
ELLO
W
B
LU
E
B
LA
C
K
W
H
IT
E
100
0
50
SWITCH "A"
(LEFT)
SWITCH "B"
(RIGHT)
CAM
TERMINAL
BLOCK IN
CASE
BOTH SWITCHES RELAXED
SWITCH
PLUNGER
SCREW
❶
❶
Note: Wire "Green" for 289A
and "Violet" for 291B.
Mid-point DP
HIGH
LOW
SWITCH "A" AC TUAT ED/SWITCH "B" RELAXED
SWITCH "A" RELAXED/SWITCH "B" AC TUAT ED
SWITCH
CLOSED
CONTACTS
OPEN
CONTACTS
GREEN/WHITE
GREEN/BLACK
A (LEFT)
B (RIGHT)
YELLOW/BLUE
RED/YELLOW
SWITCH
CLOSED
CONTACTS
OPEN
CONTACTS
GREEN/WHITE GREEN/BLACK
A (LEFT)
B (RIGHT)
RED/YELLOW
YELLOW/BLUE
SWITCH
CLOSED
CONTACTS
OPEN
CONTACTS
GREEN/WHITE
GREEN/BLACK
A (LEFT)
B (RIGHT)
YELLOW/BLUE
RED/YELLOW
SWITCH "A"
(LEFT)
SWITCH "B"
(RIGHT)
SWITCH "A"
(LEFT)
SWITCH "B"
(RIGHT)
figure 1-3. Switch actuation example
Notes: (1) Cam rotates counterclockwise with increased pressure; (2) Switches
shown with low switch set at 25% differential pressure (DP); High switch set at
75% differential pressure (DP); (3) To change setpoint: loosen lock screw, move
switch plate, tighten screw and test setpoint; (4) Internal wires use No. 22 AWG
and external wires use No. 18 AWG.
