Heating Appliances and Thermal Controls
What the exam tests
This domain tests heating elements, nichrome, thermal insulation, bimetallic thermostats, simmerstats, thermal fuses and cut-outs, and the safety of electric heating appliances.
Core rules
- A heating element must have high resistivity, high melting point and oxidation resistance. Nichrome is standard; mica, ceramic and porcelain insulate it from the metal body.
- A bimetallic thermostat bends as temperature changes and opens or closes contacts. It controls temperature but may not be a safety cut-out.
- A thermal fuse is one-shot and opens at a fixed over-temperature; a thermal cut-out may reset. Neither should be bypassed.
- A simmerstat or energy regulator switches the element on and off in a duty cycle to control average power.
- Metal-bodied heating appliances must be earthed; double-insulated appliances are marked and must not be earthed to exposed metal.
Worked example
A 1500 W, 230 V heater should have R = V²/P = 230²/1500 = 35.3 Ω. The ohmmeter reads infinite across the element. The element is open. A zero reading would indicate a short or a closed thermostat path, not a good element. After replacing the element, check insulation resistance from live parts to the metal body before use.
Traps
Do not use a thermostat as a substitute for a thermal fuse. Do not fit a higher-wattage element without checking the cord, switch and cut-out rating. Do not test element resistance with the appliance connected to supply. Do not ignore a damaged soleplate or cracked insulation.