Choosing a Circuit Protector

How to specify your Trimax thermal circuit protector:

Information Needed
  • 1
  • The maximum voltage of the circuit.
  • 2
  • The normal operating current for the circuit.
  • 3
  • The safety factor you wish to apply to ensure trouble-free operation by avoiding nuisance trips.
    NOTE: Generally, most engineers add at last 15% to the normal operating current as a safety factor. The actual amount to be added will depend on factors such as anticipated voltage fluctuations or surges, inductive versus resistive loads, manufacturing tolerances, etc.
  • 4
  • The ambient temperature to which the circuit protector will be exposed.
    NOTE: Most thermal circuit protectors require adjustment for ambient temperatures different than 25°C.
  • 5
  • The capacity correction factor for the ambient temperature to which the circuit protector will be exposed. Refer to the derating factors shown in the circuit protector’s data sheet.
    NOTE: Trimax MX22 and MX23 Series Expansion Type Circuit Protectors are not sensitive to ambient temperature and do not require uprating or derating.
  • 6
  • The maximum current at which the circuit protector must trip.
  • 7
  • The duration in seconds the system can withstand the maximum current without damage. (In the case of a motor application, this data may be obtained from the motor manufacturer.)

.: Example :.
Performance Requirements
  • 1
  • Maximum Voltage: 250VAC
  • 2
  • Normal Operating Current: 8.0 Amps
  • 3
  • Safety Factor: 15%
  • 4
  • Ambient Temperature: 40°C
  • 5
  • Derating Factor: 1.22
  • 6
  • Maximum Trip Current: 48 Amps
  • 7
  • Time duration the circuit can withstand the maximum trip current of 48 Amps: 10 seconds

Reference and Calculations
  • 1
  • Calculate the suggested circuit protector rating as follows:
Normal Operating Current X Safety Factor X Derating Factor = Suggested Protector Rating
8.0
 X
1.15
 X
1.22
 =
11.5 Amps
  • 2
  • Consult the appropriate circuit protector Options chart to find the nearest amperage rating (12 Amps is satisfactory).
  • 3
  • Determine the percentage overload using the following formula:
Maximum Trip Current / Selected Amperage Rating = Overload
48 Amps
 /
12 Amps
 =
4.00 or 400%
  • 4
  • Refer to Trip Time Curve to determine whether the trip time matches the specifications for the percentage overload calculated (400% in above example).
     
    NOTE: Derating factors to compensate for ambient temperature may vary between circuit protectors as well as between different amperage ratings within a given series. Please make sure that the derating factor used to calculate the full load current of the circuit protector ultimately selected matches the factor indicated for that product series.

Choosing Options

Refer to options selection charts to specify additional features.



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