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

Application

Hand reset melting alloy overload relays use a eutectic alloy solder, which responds to the heat produced in a heater element by the motor current. When tripped, the overload relay can be reset manually after allowing a few minutes for the motor and overload relay to cool and the solder to solidify.

Schneider Electric one-piece thermal unit construction provides overload protection for the majority of motors. Repeated tripping does not affect the original calibration. Melting alloy thermal units are available in three designs:

Standard Trip (Class 20): All standard trip thermal units provide trip characteristics for normal motor acceleration up to approximately 7 seconds on a full voltage start.
Slow Trip (Class 30): Type SB thermal units provide trip characteristics for motor acceleration up to approximately 12 seconds on a full voltage start. The motor must be suitable for extended starting periods.
Quick Trip (Class 10): Type FB thermal units are used to protect hermetically sealed, submersible pump motors and other motors that can endure locked-rotor current for a very short time, or motors that have a low ratio of locked-rotor to full-load current.

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