Class 958, 33-180 amp B Frame Instruction Sheet

Solid State Overload Relay
with Phase Loss Protection
Trip Curve “A” & “B”
Class 958
May, 2000
Supersedes Issue of
June, 1993
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 requiring
ambient insensitivity and quick trip response. The trip curves
(Fig. 1) 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 3 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 in motor must hold amps. Must trip amps are 112%
of the must hold setting. In addition to the markings on the dial
there are audibile 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. 2).
Hazardous voltage.
Can cause death, serious personal
injury, or property damage.
Disconnect power before working on
this equipment.
Cat No “A”Trip
Cat No “B”Trip
Current Range (Amps)
33 - 66
50 - 100
75 - 150
90 - 180
Figure 2
Figure 1
(For Engineering Reference Only - Rev. D)
Siemens Energy & Automation, Inc. 1000 McKee Street Batavia, Illinois 60510
Page 2
May, 2000
Supersedes June, 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 4.
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).
* The 90 - 180 Amp overload has an operating temperature range of
-22vF to 140vF (-30vC to 60vC)
Figure 3
Figure 4
Siemens Energy & Automation, Inc. 1000 McKee Street Batavia, Illinois 60510