Overload Relays and Thermal Unit Selection – Catalog

Overload Relay Selection
Thermal Unit Selection
- Introduction
  - Procedure for Thermal Unit Selection
  - Selecting Slow Trip Thermal Units
- Selecting Thermal Units
- Trip Current Rating Calculation
  - Procedure for Calculating the Trip Current Rating
- Calculating the Trip Current for Ambient Temperatures Other than 40 °C
- Thermal Unit Selection on an Approximate Basis—Based on Horsepower and Voltage
- Mounting the Thermal Units
- Thermal Unit Selection Tables
  - Table 1
  - Table 2
  - Table 3
  - Table 4
  - Table 5
  - Table 9
  - Table 10
  - Table 12
  - Table 13
  - Table 14
  - Table 15
  - Table 16
  - Table 17
  - Table 18
  - Table 19
  - Table 20
  - Table 21
  - Table 22
  - Table 24
  - Table 26
  - Table 28
  - Table 31
  - Table 34
  - Table 40
  - Table 41
  - Table 43
  - Table 44
  - Table 49
  - Table 53
  - Table 54
  - Table 56
  - Table 58
  - Table 59
  - Table 61
  - Table 65
  - Table 66
  - Table 67
  - Table 68
  - Table 69
  - Table 72
  - Table 73
  - Table 74
  - Table 75
  - Table 76
  - Table 77
  - Table 78
  - Table 79
  - Table 80
  - Table 81
  - Table 82
  - Table 83
  - Table 84
  - Table 85
  - Table 86
  - Table 87
  - Table 88
  - Table 89
  - Table 90
  - Table 91
  - Table 92
  - Table 93
  - Table 94
  - Table 95
  - Table 96
  - Table 103
  - Table 104
  - Table 105
  - Table 109
  - Table 110
  - Table 111
  - Table 112
  - Table 113
  - Table 114
  - Table 115
  - Table 116
  - Table 127
  - Table 128
  - Table 129
  - Table 133
  - Table 134
  - Table 135
  - Table 136
  - Table 145
  - Table 146
  - Table 147
  - Table 148

9065CT9701R03/21

Introduction

Overload relays (OLRs) protect motors, controllers, and branch-circuit conductors against excessive heating due to prolonged motor overcurrents, up to and including locked rotor currents.

NOTE: Protection of the motor and the other branch-circuit components from higher currents—due to short circuits or ground faults—is a function of the branch-circuit fuses, circuit breakers, or motor short-circuit protectors.

Electric motors make up a substantial percentage of power system loads. Market demands for reduced downtime and increased productivity compel the motor control industry to continually evaluate motor protection technology. As technology advances, more options for motor protection become available.

This section provides a brief review of traditional motor protection technologies and discusses newer electronic motor protection options. This information should help you understand the available technologies and how to choose the right solution for a given application. Important factors to consider in determining the appropriate overload protection include the following:

Application requirements
Cost per feature of a given technology
Willingness and ability of all parts of your organization to embrace and implement newer technologies

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