The heater element of a bimetallic overload relay is sized to the motor's full-load current.

• A. True
• B. False
• C. It is sized to twice the full-load current
• D. It is independent of motor current

Answer: (A)

In a thermal overload relay, the trip action comes from heat produced by current flowing through a heater element. That heater is sized to the motor’s full-load current so that at normal full-load the heat generated maintains the bimetallic element at its safe temperature and no trip occurs. When current exceeds full-load due to overload, the extra heat builds up faster, causing the bimetal strip to bend and trip after a short time. If the heater were sized differently (for example, to twice the full-load current) or kept independent of motor current, the protection would either trip too soon, too late, or not respond accurately to actual motor loading. So matching the heater to full-load current is what makes the protection reliable.