## 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.

• 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.