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10/04/2017

Current - Time Characteristic of Fuse

Current – Time Characteristics
  • The fast acting fuse is used to protect semi conductor device. 
  • The fuse is connected in series with semi conductor device and it open when the current exceeds its rated value. 
  • When fuse is not used in the circuit, the fault current increases up to point B. As the fuse current increases, the temperature also increases.
  • When fuse is used in the circuit, the fault current increases up to time t = tm. There is sperk across fuse when it opens at time t = tm
  • The fault current increases up to point A and it is called as Peak through let current. The peak through let current is shown in the figure point C.

fuse-characteristics.png


  • The arc resistance increases at point C therefore the fault current decreases. 
  • The arc diminishes at point D and fault current becomes zero at that time. The fault clearing time (tc) is sum of melting time (tm) and arcing time (ta) of the fuse.

          tc = tm + ta
  • The voltage across fuse during arcing time is known as Recovery voltage or Arcing voltage
  • It should be noted that the I2t rating of the fuse always less than the I2t rating of the SCR. The low rating SCR is protected by circuit breaker.

Current – time Curve
  • The current time characteristic of the semi conductor device is shown in the figure B. 
  • The RMS current rating of the fuse is always less than the RMS current rating of the semi conductor device in the low power converter protection.
protection-of-semiconductor-fuse.png
    

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