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Difficulty: MediumProtective Devices

A metal oxide varistor (MOV) is installed in parallel across the incoming AC power lines of an industrial power supply to protect sensitive electronics against transient overvoltage spikes. When a high-voltage surge exceeds the clamping voltage rating of the MOV, how does this protective device respond?

  1. A
    Its internal resistance increases to infinite ohms, blocking the surge voltage from reaching the circuit.
  2. B
    A metallic element inside the device melts, permanently opening the main line conductors.
  3. Its electrical resistance drops sharply, shunting excess surge current away from downstream components and clamping line voltage.Answer
  4. D
    It reverses polarity to convert the incoming alternating current spike into direct current.

Answer

Its electrical resistance drops sharply, shunting excess surge current away from downstream components and clamping line voltage.
Metal oxide varistors (MOVs) are voltage-dependent protective devices that exhibit high electrical resistance under normal line voltages. When exposed to a transient voltage surge exceeding their clamping voltage rating, their resistance drops almost instantaneously to a very low value. This low-resistance path safely shunts the excess surge current away from sensitive downstream electronics, clamping the voltage across the load to a safe level.

Step-by-Step Solution

1
Identify the function and circuit configuration of a metal oxide varistor (MOV).
An MOV is a voltage-dependent, non-linear resistor connected in parallel across the power lines to protect against transient overvoltage spikes.
Recognizing that the MOV is wired in parallel confirms it serves as a voltage clamp rather than a series disconnect.
2
Determine the electrical behavior of the MOV when line voltage exceeds its clamping threshold.
The internal resistance of the MOV drops rapidly from very high resistance (megohms) to very low resistance (near zero ohms).
The semiconductor zinc-oxide material conducts heavily once the breakdown voltage is reached.
3
Evaluate the resulting protection provided to downstream components.
The low-resistance state diverts high surge currents away from the load, holding the voltage across the circuit to a safe clamped level.
By shunting the transient energy across the parallel branch, sensitive downstream circuits are shielded from damage.

Key Concept

Metal Oxide Varistor (MOV) transient overvoltage clamping mechanism
Estimated Time:1m 0s
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Topics

Protective Devices

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