FUSE, MCBs, MCCBs Sunil Bhat

Necessity of Fuse If designed properly, the current in a network is always within limits under normal working conditions If fault occurs in the network, The current exceeds normal limit This is mainly due to phase to phase short circuit fault/ phase to ground short circuit If not interrupted this high current may lead to permanent damage of instruments and wiring

A fuse is a part of the circuit which consists of conductor which melts easily and breaks the connection when current exceeds the predetermined value. An electrical fuse is a weakest part of an electrical circuit which breaks when more than predetermined current flows through it. The function of fuse wire is to carry the normal current without excessive heating but more than normal current when pass through fuse wire, it rapidly heats up and melts.

melting point and specific resistance of different metals used for fuse wire Aluminium 240oF 2.86 μ Ω – cm Copper 2000oF 1.72 μ Ω – cm Lead 624oF 21.0 μ Ω – cm Silver 1830oF 1.64 μ Ω – cm Tin 463oF 11.3 μ Ω – cm Zinc 787oF 6.1 μ Ω – cm

Important Terms need for Fuse Minimum Fusing Current : It is minimum value of current due to which fuse melts. Current Rating of Fuse : It is maximum value of current due to which fuse does not get melt. Fusing Factor : This is the ratio of minimum fusing current and current rating of fuse. The value of fusing factor is always more than 1.

Prospective Current in Fuse: Before melting, the fuse element has to carry the short circuit current through it.The prospective current is defined as the value ofcurrent which would flow through the fuse immediately after a short circuit occurs in the network. Melting Time of Fuse or Pre-arcing Time of Fuse: This is the time taken by an fuse wire to be broken by melting. It is counted from the instant, the over current starts to flow through fuse, to the instant when fuse wire is just broken by melting.

Arcing Time of Fuse: After breaking of fuse wire there will be an arcing between both melted tips of the wire which will be extinguished at the current zero. The time accounted from the instant of arc initiated to the instant of arc being extinguished is known as arcing time of fuse. Operating Time of Fuse : When ever over rated current starts to flow through a fuse wire, it takes a time to be melted and disconnected, and just after that the arcing stars between the melted tips of the fuse wire, which is finally extinguished. The operating time of fuse is the time gap between the instant when the over ratedcurrent just starts to flow through the fuse and the instant when the arc in fuse finally extinguished. That means operating time of fuse = melting time + arcing time of fuse.

Fuse Law Fuse law determines the current carrying capacity of a fuse wire. At steady state condition fuse carries normalcurrent without increasing its temperature to the melting limit. At this steady state condition, heat generated due to current through fuse wire is equal to heat dissipated from it.

Heat generated = I2.R  = 𝐼 2 𝜌𝑙 𝑎 = 𝐼 2 𝜌𝑙 𝜋 𝑑 2 4 = 𝐼 2 𝐾 1 1 𝑑 2 Heat lost ∝ surface area of fuse wire ∝ πd.l Heat lost = 𝐾 2 𝑑𝑙 At steady state heat lost=heat generated 𝐼 2 𝐾 1 1 𝑑 2 = 𝐾 2 𝑑𝑙 𝐼 2 =𝑘 𝑑 3 𝐼=𝑘 𝑑 1.5

METAL VALUE OF K WHEN D IS MEASURED IN MM Aluminium 59 Copper 80 Iron 24.6 Lead 10.8

Fuse Characteristics Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB body is the amount of current it will pass continuously. This is normally called the rated current or nominal current What if the current exceeds the nominal current? Will the device trip instantly?

if the rating is 30 amps, a current of 30 if the rating is 30 amps, a current of 30.00001 amps will trip it, right? This is not true! For 30 Amp Fuse, to be sure of tripping in 0.1 seconds, the fuse requires 300 amps. the fuse is marked `30 amps’, but it will actually stand 40 amps for over an hour, how can we justify calling it a `30 amp’ fuse?

answer is that the overload characteristics of fuses are designed to match the properties of modern cables a modern PVC-insulated cable will stand a 50% overload for an hour, so it seems reasonable that the fuse should as well.

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