Lightening arrester should be located

Question

Examoogle · Accepted Answer

Option C: Near the transformer

Explanation & Concepts:
A lightning arrester is placed as close to the transformer as possible to minimize the length of the conductor between them. This placement ensures that surge voltages resulting from traveling waves are clamped before reaching the transformer's sensitive insulation.

Working Principle: When a surge wave travels along a transmission line, it experiences reflection and refraction at impedance discontinuities, such as at a transformer terminal. If the arrester is far from the transformer, the inductive voltage drop $v = L \frac{di}{dt}$ across the lead wire increases the effective discharge voltage at the transformer, potentially exceeding its Basic Insulation Level (BIL). By minimizing the lead length, we reduce the additive voltage drop and ensure the transformer is protected by the arrester's protective characteristics.

Key Formulas: $V_{total} = V_{discharge} + L \frac{di}{dt}$ — Voltage experienced by the transformer; $BIL > V_{arrester}$ — Safety condition for transformer insulation

Key Points: The lightning arrester must be located as close to the equipment being protected as physically possible.; The lead wire length is critical; excessive length causes high inductive voltage drop during rapid surge currents.; The purpose is to shunt the surge current to the ground before it stresses the transformer windings.

Advantages: Prevents transformer winding insulation failure; Limits transient overvoltages effectively; Provides a low impedance path for lightning surges

Disadvantages: Increased system complexity near the transformer; Requires regular maintenance and inspection; Adds capacitive load if not properly designed

Applications: Power transformers in substations; Distribution transformer protection; High-voltage transmission line termination

Additional Info: Option A is incorrect because the arrester's primary purpose is to protect specific equipment like the transformer, not just the circuit breaker.. Option B and D are incorrect as increasing the distance allows reflected waves to increase the stress beyond the arrester's protection level.

Answer: C is correct — The lightning arrester is placed near the transformer to minimize the inductive voltage drop across the connecting leads and ensure the surge voltage remains below the transformer's insulation withstand level.

Exam Tip: Always remember that lead inductance $L$ acts as a low-pass filter for high-frequency transients; minimize lead length to keep the protection voltage $V$ low.

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Option A: Near the circuit breaker

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Option B: Away from the circuit breaker

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Option D: Away from the transformer

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