Why is spark produced in the switch of a fan when it is switched off

Why Does Sparking Occur Across a Switch When Switching Inductive Loads? Causes, Effects, and Protection Methods. Effect of Back-EMF explained.

A circuit breaker or switch that controls an inductive load has a spark across the contacts when they physically separate.

Sparking across the contact that controls the inductive load:

The reason is that when the switch becomes off, a falling magnetic field across the inductive load generates a high voltage pulse of back EMF that ionises the air the moment contact physically separates with a loud pop. Why, because ionised air temperature can be up to 4000 K. That temperature creates a violent expansion of the air that is heard as a pop.

This ionisation of the air is nothing but plasma (arc) that can conduct current through the air. It appears as a bright spark with pop, especially in a dark environment.

How to reduce sparking across the contact?

A snubber circuit parallel to the contact can reduce sparking. 

Lightning is nothing but the ionisation of air due to high voltage:

This phenomenon is similar to lightning.

FAQ Set:

Q1: Why does sparking occur across a switch when switching inductive loads?
Sparking happens because inductive loads resist sudden changes in current. When the switch opens, the collapsing magnetic field generates a high-voltage spike across the contacts, causing arcing.

Q2: What is back-EMF and how does it relate to sparking?
Back-EMF (Electromotive Force) is the voltage induced by a changing current in the inductor. This voltage opposes the applied voltage and can be high enough to cause sparks across switch contacts.

Q3: What are the effects of sparking on switches?

• Contact erosion or pitting

• Reduced switch lifespan

• Increased risk of fire or electrical hazards

• Circuit malfunction or voltage spikes are damaging other components

Q4: What protection methods can prevent sparking?

• Flyback diodes across DC inductive loads

• RC snubber circuits to absorb spikes

• Varistors or surge suppressors for AC circuits

• Using switches rated for inductive loads

Q5: Does the type of inductive load affect sparking?

Yes, motors, solenoids, relays, and transformers with high inductance generate larger back-EMF and are more prone to causing sparks.

Q6: How can you minimise back-EMF in practical circuits?

• Use diodes in parallel for DC loads

• Use properly rated contactors or relays

• Add snubber networks to dissipate energy safely

Q7: Why is understanding back-EMF important for circuit design?

It ensures safe operation, longer switch life, and protection of sensitive components, especially in circuits with inductive loads.

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