Question

(a) Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse. (6)

(b) Evaluate for a frequency of 15 kHz, the amplification and the phase difference between input and output signals of a voltage amplifier using a triode having an amplification factor of 48 and a mutual conductance of 1.2 mA/V with an anode-load resistant of 160 kΩ. The output p.d. is fed by a coupling capacitor of negligible reactance to a subsequent circuit of resistance 480 kΩ and the total shunt capacitance is 90 μF. (10)

Answer 1

Part (a)

Both Current Limiting Circuit Breakers (CLCBs) and High Rupturing Capacity (HRC) fuses are protective devices designed to interrupt fault currents and limit damage in electrical systems. Their effectiveness can be compared as follows:

Feature	Current Limiting Circuit Breaker (CLCB)	High Rupturing Capacity (HRC) Fuse
Mechanism	Detects overcurrent/short circuit and trips rapidly, limiting peak current by contact separation and arc quenching.	Metallic element melts and vaporizes under fault conditions, forming a high-resistance arc to interrupt current.
Current Limiting	Limits current by rapid contact opening and arc control, preventing the prospective peak fault current.	Inherently provides excellent “cut-off” characteristics via very rapid melting and arc formation; often superior for high fault currents.
Speed of Operation	Very fast (milliseconds).	Extremely fast; often faster than CLCBs for very high prospective faults.
Reusability	Resettable; allows quick restoration of power.	One-time use; requires replacement after operation.
Cost	Higher initial cost due to complex mechanism.	Lower initial cost, but replacement costs accumulate.
Maintenance	Requires periodic testing and calibration.	No maintenance; replace when blown.
Discrimination / Coordination	Adjustable trip characteristics allow easier selective coordination.	Coordination can be difficult; upstream fuses may blow prematurely.
Arcing	Arc formed between contacts; controlled by arc chutes and design.	Arc contained within fuse body, usually filled with sand.
Protection	Excellent for overloads and short circuits; may include advanced features like ground-fault protection.	Excellent for very high short-circuit currents due to rapid cut-off and low let-through energy.
Downtime	Very low—simply reset.	Higher—requires physical replacement.

✓ Verified Model Answer (Text Solution)

Both Current Limiting Circuit Breakers (CLCBs) and High Rupturing Capacity (HRC) fuses are protective devices designed to interrupt fault currents and limit damage in electrical systems. Their effectiveness can be compared as follows: