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Thermal device circuit breakers

The tripping element of thermal device circuit breakers is a thermal bimetal, a combination of bimetal and an electrical heating element. The bimetal consists of steel and zinc which is formed when affected by heat. If the thermal bimetal heats up when too high a current is reached, the switch-off mechanism is tripped.

Thermal device circuit breakers are a simple and cost-effective alternative for applications which do not necessarily require precise switch-off.

Structure (with switch lock)

Structure of a thermal circuit breaker  

Structure of a thermal circuit breaker

This particular design of circuit breakers works with a strip-thermal bimetal. Equipment is switched off by means of a spring-loaded contact mechanism. The switch enables protective devices to be switched on and off by hand.

A setting screw is used to adjust the switch-off time to the default settings. This also sets the pre-voltage for the thermal bimetal which activates the trip mechanism.

Nominal currents start in the milliamp range and extend up to the two digit amp range. They are suitable for use up to 230 V AC or 65 V DC.

Key:

  1. On/off switch
  2. Spring-loaded switching rod
  3. Switch contacts
  4. Bimetal
  5. Setting the pre-voltage
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Circuit diagram

Circuit diagram of a thermal circuit breaker

Circuit diagram of a thermal circuit breaker

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Structure (with operating lever)

Structure of a thermal miniature circuit breaker  

Structure of thermal miniature circuit breakers

Thermal miniature circuit breakers with a snap disc as the thermal bimetal are designed with extremely compact dimensions. The switch contact is directly secured to the washer. These versions feature a somewhat faster blowing tripping characteristic as those with bimetal strips.

They are essentially used for protecting integrated circuits in battery and board systems of up to 32 V nominal current. Nominal currents are in the single and two digit milliamp range.  Thermal miniature circuit breakers can be switched on again after tripping by means of a button. Lower nominal currents are protected by other protective devices.

Key:

  1. Switch contact
  2. On switch
  3. Bimetal

 

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Circuit diagram

Circuit of a thermal miniature circuit breaker

Circuit of a thermal miniature circuit breaker

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Tripping characteristics

Tripping characteristics of a thermal circuit breaker  

Tripping characteristics of a thermal circuit breaker

The tripping time of thermal device circuit breakers depends on the pending overload current and ambient temperature. With an increasing overload, the circuit breaker trips faster, whereas it trips slower with lower overload currents. For circuit breakers with different nominal currents, yet the same tripping characteristics, the tripping can also be presented in characteristic curve fields.

Thermal device circuit breakers obviously respond to the influence of heat. Even the ambient temperature influences the tripping time. The circuit breaker trips more easily at a high ambient temperature and in turn more slowly at a low ambient temperature.

Key:

t = switch time (in seconds)
xl = multiple of the nominal current/tripping factor
1 = current ranges of the characteristic field
2 = tripping characteristics of the lower temperature range (blue)
3 = tripping characteristics group 1
4 = tripping characteristics group 2
5 = tripping characteristics of the upper temperature range (red)

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