Murphy A20 Series Switch User Manual


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Bulletin AT-95026B page 3 of 4
Pre-Alarm Using A20/A25TABS
Typical Internal Wiring Diagrams
The A20TABS and A25TABS features a standard limit contact for equipment
shutdown on high temperature. It also has an internal SPDT snap-switch to sig-
nal an alarm before shutting down. When the snap-switch trips (preset point), on
rising temperature, the switch completes a circuit to activate an alarm. If the
temperature continues to increase, the face-adjustable pointer contact will make
and the shutdown circuit will be completed (see the typical diagram below for
reference). The front contact shutdown limit setting (which is adjustable) and
the snap-switch are preset at the factory. Refer to “Temperature Ranges and
Factory Settings” table on opposite page for settings. For alternative alarm
before shutdown, see Magnetic Switch model 760A or 761APH.
_
+
Battery
A20TABS
SWICHGAGE
®
Alarm Before
Shutdown 
MINI-SIREN
®
Opens circuit to
shutdown engine.
Shutdown Device
or Ignition Circuit
518PH
TATTLETALE
®
HIGH
C
N. C.
N. O.
ABS
GNC
R
SW1 SW2
B
Magnetic Switch
GNC
R
SW1 SW2
B
_
+
Battery
A20P Pressure
SWICHGAGE
®
A20T Temperature
SWICHGAGE
®
518PH TATTLETALE
®
Energized To Run Devices
N.C. switches option
N.O. SWICHGAGE
®
instruments
C
LOW
N. C.
C
HIGH
N. O.
C
S
B
_
+
Battery
117 Magnetic Switch
Energized To Run Devices
A20T Temperature
SWICHGAGE
®
C
HIGH
N. O.
INDUCTIVE AND HIGH CURRENT LOADS REQUIRE THE USE OF A
MAGNETIC SWITCH. The SWICHGAGE
®
contacts are for light-duty elec-
trical switching to operate alarms or control devices. Murphy manufactures the
Magnetic Switch for protection of the pilot-duty SWICHGAGE
®
limit contacts.
TATTLETALE
®
Magnetic Switches show the cause of shutdown for applica-
tions that include: capacitor discharge or magneto ignitions, battery systems and
electric motor driven equipment. Typical wiring diagrams are shown below.
C
HIGH
N. C.
N. O.
C
HIGH
N. C.
N. O.
LOW
HIGH
C
N. C.
N. O.
ABS
P
L
H
C
N. C.
N. O.
P
L
H
C
N. C.
N. O.
P
L
H
C
N. C.
N. O.
RESET
SET
HIGH
HIGH
HIGH
SETRESETSETRESETSETRESET
LOW HIGH
LOW
C
N. C.
N. O.
P
L
H
C
N. C.
N. O.
P
L
H
C
N. C.
N. O.
A20T A20T-HL A20TABS A20TE
A25T
A25T-HL
A25TABS
A25TABS-HL A25TE
Pointer shown in the shelf position. Pointer type contact rating: pilot-duty 2 A @ 30 VAC/VDC resistive.
Snap-acting switch rating: 3 A @ 30 VDC inductive. 4 A @ 125 VAC inductive.
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