Megger indicate zero while both terminals are kept open.

Question

Examoogle · Accepted Answer

Option B: False

Explanation & Concepts:
A Megger (megohmmeter) measures insulation resistance by applying a high DC voltage across terminals. When terminals are open-circuited, the resistance is infinite ($\infty$), causing the pointer to deflect to the 'Infinity' mark on the scale, not zero.

Working Principle: The Megger consists of a hand-driven DC generator and a moving coil meter. In the 'Open' state, there is no conductive path between the probes, leading to infinite resistance ($R = V/I$, where $I \approx 0$). Consequently, the deflection of the moving coil remains at the maximum limit of the scale, which is labeled as '$\infty$'.

Key Formulas: $R = \frac{V}{I}$ — Ohm's law defining resistance as the ratio of voltage to current; $R_{insulation} = \frac{V}{I_{leakage}}$ — Insulation resistance measurement principle

Key Points: A Megger is designed to measure very high resistances (insulation).; Open terminals represent a break in the circuit, which corresponds to infinite resistance.; Short-circuited terminals represent a direct path with zero resistance, which would cause the pointer to read zero.; The scale on a Megger is calibrated such that 'Zero' is on the right and 'Infinity' is on the left.

Advantages: Portable and self-powered by a hand-crank generator; High voltage output allows testing of insulation integrity under stress

Disadvantages: Manual cranking can cause output voltage instability; Not suitable for measuring very low resistances (milli-ohms)

Applications: Testing winding insulation of transformers; Checking cable insulation resistance in power systems

Additional Info: Standard insulation resistance should be in the range of Mega-ohms ($M\Omega$).. Option A is incorrect because zero resistance indicates a short circuit, whereas an open circuit implies an open path and infinite resistance.

Answer: B is correct — When the terminals of a Megger are open, the resistance is infinite, so the instrument indicates infinity, not zero.

Exam Tip: Always remember that for a resistance meter: Open = $\infty$ ohms, Closed/Short = $0$ ohms.

Examoogle · Answer

Option A: True

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