A mechanical relay rated at 10,text A resistive load is used to switch a 5,text A inductive AC load. The relay contact rating should be de-rated because:

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Electronics Engineering QUESTION #11725

Question 1

A mechanical relay rated at $10\,\text{A}$ resistive load is used to switch a $5\,\text{A}$ inductive AC load. The relay contact rating should be de-rated because:

Inductive loads have higher steady-state current
Inductive loads cause contact bounce on make
Arc energy at break is higher due to energy stored in inductance $E = \frac{1}{2}LI^2$ extending the arc✔️
Inductive loads cause coil voltage spikes in the relay

Correct Answer Explanation

When a relay breaks an inductive circuit, the magnetic energy $E = \frac{1}{2}LI^2$ stored in the inductor must be dissipated. This sustains an arc across opening contacts well beyond the natural current zero, causing excessive contact erosion, pitting, and welding. Inductive load relay contacts must be de-rated to typically 20–40% of resistive ratings. RC snubbers or MOV suppressors across contacts mitigate arc damage.

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