Fuses Cheatsheet

Cheatsheet Content

1. Fuses: Introduction & Basics Definition: A fuse is a short piece of metal inserted in a circuit, designed to melt and break the circuit when excessive current flows. Function: Interrupts circuits automatically on short-circuit faults or overloads. Key Principle: Joule's Law: $H = I^2 Rt$ (Heat generated is proportional to current squared, resistance, and time). Smaller fuse elements have higher resistance, heat up, and melt faster. Limits peak short-circuit current and reduces stress on electrical components. Time-Current Characteristics: Fuses have inverse time-current characteristics (Fig. 20.1) The greater the current, the smaller the time to blow out. Suitable for overcurrent protection. 2. Desirable Characteristics of Fuse Element Low melting point (e.g., tin, lead). High conductivity (e.g., silver, copper). Free from deterioration due to oxidation (e.g., silver). Oxidation increases resistance, leading to inefficient operation and premature failure. Low cost (e.g., lead, tin, copper). No single material possesses all characteristics; a compromise is made. 3. Fuse Element Materials Small Currents (up to 10 A): Tin or lead-tin alloy (37% lead, 63% tin). Larger Currents: Copper or silver. Copper is often tinned to prevent oxidation. Zinc (strip form): Used for fuses requiring considerable time-lag (slow to melt with small overload). Silver (Preferred due to): Comparatively free from oxidation. Does not deteriorate in dry air. Small coefficient of expansion, preventing critical fatigue. Very high conductivity: smaller mass needed for a given rating, minimizing vaporized material and permitting faster operation. Low specific heat: heats quickly to vaporization. Resistance increases abruptly at melting point, making transition to vaporization almost instantaneous. Vaporizes at a temperature much lower than its ionization temperature, ensuring high resistance in the arc path, leading to quick interruption. 4. Important Terms Current Rating of Fuse Element: Normal current a fuse can carry without overheating or melting. Depends on temperature rise of contacts, fuse material, and surroundings. Fusing Current: Minimum current at which the fuse element melts and disconnects the circuit. Always greater than the current rating. For a round wire: $I = k d^{3/2}$ (Ordinary fuse law, where $k$ is the fuse constant). Fusing Factor: Ratio of minimum fusing current to current rating. Value is always $>1$. Smaller fusing factor implies greater difficulty in avoiding deterioration. Semi-enclosed/rewirable fuses (copper wire): usually 2. Enclosed cartridge fuses (silver/bimetallic): lower values. Prospective Current: RMS value of the first loop of fault current if the fuse were replaced by a conductor (no fuse). Cut-off Current: Maximum fault current actually reached before the fuse melts (point 'a' in Fig. 20.2). Depends on current rating, prospective current, and asymmetry of short-circuit current. Fuses break the circuit before the fault current reaches its first peak, giving an advantage over circuit breakers by reducing thermal and electromagnetic effects. Pre-arcing Time: Time from fault commencement to arc initiation (fuse melts). Typically very small ($\approx 0.001$ sec). Arcing Time: Time from arc initiation to arc extinction. Total Operating Time: Sum of pre-arcing and arcing times. Generally very low ($\approx 0.002$ sec) compared to circuit breakers ($\approx 0.2$ sec). Breaking Capacity: RMS value of A.C. component of maximum prospective current a fuse can safely interrupt at rated service voltage without exploding or failing. 5. Low Voltage Fuses 5.1 Semi-Enclosed Rewireable Fuse (Kit-Kat Type) Construction: Porcelain base (fixed contacts for phase wires) and porcelain fuse carrier (holds tinned copper wire element). Operation: Fuse element blows on fault; carrier is removed, element replaced, carrier reinserted. Advantages: Replacement without danger of contact with live parts. Low replacement cost. Disadvantages: Possibility of using wrong wire size/material. Low breaking capacity; unsuitable for high fault levels. Deterioration due to oxidation from continuous heating, reducing current rating over time. Uncertain protective capacity due to ambient conditions. Inaccurate calibration due to dependence on element length. Application: Up to 500A rated current; breaking capacity $\approx 4000$A at 400V. Limited to domestic and lighting. 5.2 High Rupturing Capacity (H.R.C.) Cartridge Fuse Construction: Heat-resisting ceramic body with metal end-caps, silver current-carrying element, and a filling powder (chalk, plaster of paris, quartz, marble dust) for arc quenching/cooling. Operation: Current increases on fault, silver element melts before peak fault current, vaporizes, and reacts with filling powder to form high-resistance substance, quenching the arc. Advantages: Clears high and low fault currents. Does not deteriorate with age. High speed of operation. Reliable discrimination. Requires no maintenance. Cheaper than other interrupting devices of equal breaking capacity. Consistent performance. Disadvantages: Must be replaced after each operation. Heat from arc may affect associated switches. 5.3 H.R.C. Fuse with Tripping Device Construction: Ceramic body, metallic end-caps, multiple silver fuse elements, and a plunger connected via a fusible link and chemical charge. Operation: Silver elements blow first on fault, current transfers to tungsten wire, fusible link blows, detonating chemical charge, forcing plunger to operate circuit breaker. Advantages: Prevents "single phasing" in 3-phase systems by tripping all phases on a single-phase fault. Allows use of less expensive circuit breakers as the fuse clears the fault first, reducing stress on the CB. CB can handle smaller fault currents, reducing fuse replacement frequency. Application: Low voltage distribution systems (overload and short-circuit). Breaking capacity 16,000A to 30,000A at 440V. 6. High Voltage Fuses 6.1 Cartridge Type Similar to LV cartridge type but with special designs for HV. Fuse elements wound in a helix to avoid corona effects. Some designs have two parallel elements: low resistance (silver wire) for normal current, and high resistance (tungsten wire) for fault current reduction. Application: Up to 33 kV, breaking capacity $\approx 8700$A. 6.2 Liquid Type (e.g., Carbon Tetrachloride) Construction: Glass tube filled with carbon tetrachloride solution, sealed with brass caps. Fuse wire sealed at one end, other end held by a phosphor bronze spiral spring. Operation: On overcurrent, fuse wire blows, spring retracts part of it into liquid. Gas generated forces liquid into baffle, extinguishing arc. Application: Widest range for HV systems. Up to 100A rated current on systems up to 132 kV. Breaking capacity $\approx 6100$A. 6.3 Metal Clad Fuses Oil-immersed, provide substitute for oil circuit breakers. Application: Very high voltage circuits, operate well under short-circuit conditions near rated capacity. 7. Difference Between a Fuse and Circuit Breaker S. No. Particular Fuse Circuit Breaker 1. Function Performs both detection and interruption. Performs interruption only. Detection by relay system. 2. Operation Inherently completely automatic. Requires elaborate equipment (e.g., relays) for automatic action. 3. Breaking capacity Small Very large 4. Operating time Very small ($\approx 0.002$ sec). Comparatively large ($\approx 0.1$ to $0.2$ sec). 5. Replacement Requires replacement after every operation. No replacement after operation.