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

Motor Failure and Protection

Motor failure can result from electrical or mechanical factors. A study commissioned by the Electrical Research Associates (ERA) of the United Kingdom in 1986 indicated that the most common causes of motor failure are:

Overcurrent: 30%
Contamination: 18%
Single Phasing: 15%
Bearing Failure: 12%
Aging (natural wear): 10%
Rotor Fault: 5%
Miscellaneous: 7%

Failure modes 1, 3 and 7 are attributable to electrical issues. Failure modes 2, 4, 5 and 6 are the result of mechanical (and some manufacturing) issues.

Historically, motor protection provided with the controller was only able to address the electrical causes of motor failure. These electrical issues account for at least 45% of the most common causes of motor failure.

Motor branch circuits are protected against short-circuits (instantaneous overload currents) and steady-state or low-level, sustained overloads. In the U.S., this protection is provided by the short-circuit protective device (SCPD) and the motor overload relay, when they are applied according to the National Electrical Code^® (NEC^®).

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