Class 958, 15-44 amp A Frame Instruction Sheet

Class 958, 15-44 amp A Frame Instruction Sheet
June, 2000
Supersedes Issue of
September, 1993
Solid-State Overload Relay with
Phase Loss Protection Self-Reset Version
Trip Curve “A” & “B”
Class 958
Class 958 overload relays are self-powered solid-state overload
relays (i.e. they generate their own running power and do not
need a separate source of 120V to power the circuit board).
These overload relays provide excellent protection of hermetically sealed compressors and artificially cooled motors which
require ambient insensitivity and quick trip response. The trip
curves (Fig. 2) have been custom tailored to provide proper
overload protection on such loads without nuisance tripping.
The overload also provides phase loss protection for the motor
by tripping within three seconds when there is a complete loss
of one phase of the three phase motor branch circuit. Each
overload has a 2:1 current adjustment range with the adjustment dial reading out must hold amps. Must trip amps are
112% of the must hold setting. In addition to the markings on
the dial there are audible clicks which allow for extremely fine
tuning. Note that while thermal overloads require a heater
selection based on a relatively wide range, these overloads have
many clicks covering the same ampere range (Fig. 1).
Hazardous voltage.
Can cause death, serious personal
injury, or property damage.
Disconnect power before working
on this equipment.
Cat. No.
“A” Trip Curve
Cat. No
“B” Trip Curve
Current Range
Note: (*) Hot trip times will vary depending on previous running condition, duty
cycle and length of “OFF” time.
Figure 1
Figure 2
(For Engineering Reference Only - Rev. F)
Siemens Energy & Automation, Inc. 1000 McKee Street Batavia, Illinois 60510
Page 2
June, 2000
Supersedes September, 1993
There are mounting hole locations in the housing of the overload
so that they may be directly attached to a mounting panel. All
of the mounting locations should be used to prevent vibration
and/or breakage of the housing during use.
These overloads are not connected to the motor branch circuits
in any way. Proper installation simply requires that the motor
leads be passed through the “windows” in the overload relay
(Fig. 3), and the normally closed contact be wired into the control circuit. A typical wiring diagram is shown in Figure 5.
Class 958 overload relays are suitable for use with 50 or 60 Hz
motors and have an operating range from -22vF to +159vF (-30vC
to 70vC). The motor voltage can exceed 600 Volts, but the control circuit contact is rated NEMA A600 and NEMA P600 (10
Amps, 600 VAC maximum and 5 Amps, 600 VDC maximum).
In applications involving use of 12 - 14 gauge wire, usually
motors of 5 HP or less, the wires may be looped and passed a
second time through the windows before wiring to the contactor (Fig. 4). By looping the motor leads through the windows in
the overload, the overload will read twice the current actually
going to the motor. Hence the overload can protect a motor
needing half the must hold current capability of the overload: An
overload with a current range of 15 to 30 can be used to protect
a motor requiring an overload range of 7.5 to 15 must hold amps.
Similarly, motors that require only one-third, one-fourth, or onefifth the overload’s current capability can use the same size
overload by looping 2, 3, or 4 times. The table in Figure 6
demonstrates how the looping process reduces the current setting of the overload by the number times the wires pass through
the windows of the overload.
Figure 5
Looping Option
Figure 3
Figure 4
Figure 6
(Note: If used with CT’s make sure to use consistent polarity in regard to the direction that the wires pass thru the overload.)
Siemens Energy & Automation, Inc. 1000 McKee Street Batavia, Illinois 60510
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