Real device: water heater, 220 V AC to hot water
A 15 L storage water heater: an RCD, a bimetal thermostat, a one-shot thermal fuse and a 1500 W sheathed element. The heat is the output.
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Parts list
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- Element draws its rating
- Thermostat stops it at 60 °C
- Welded thermostat: the thermal fuse opens below boiling
- Dry tank: the thermal fuse opens before the element burns
How it works
In everything else in this lab heat is waste. Here it is the product: the element is designed to turn all 1500 W into heat, so the Thermal view labels it "designed to heat" instead of flagging it. The engineering is all in stopping safely: one thing regulates, a second thing of a different kind stops it if the first fails, and a third watches for current going where it shouldn't.
A220 V AC at the wall.BWater at 60 °C in the tank.
The path, A to B
- 220 V AC wall (Mains). Point A: the wall, 220 V at 60 Hz.
- Earth leakage (RCD1). Leakage protection. Water and 220 V share this tank. If the element's sheath cracks, current escapes into the water and the earthed tank instead of returning on neutral; the RCD sees line and neutral no longer match and opens within 40 ms at 30 mA, below the current that can stop a heart.
- Thermostat (TH1). Regulation. A bimetal snap disc clipped to the tank opens at 60 °C and closes again near 54 °C. That is the whole control system: on, off, on, many thousands of times over its life.
- Thermal fuse (TF1). The last resort, of a different kind. A thermostat's contacts arc every time they break 7 A, and one day they may weld shut. Then the water keeps heating. At 95 °C, before the water boils and the pressure climbs, this fuse melts and opens for good. Try it: weld TH1's contacts in its inspector.
- Hot water (EL1). Point B: the heating element and the water. A nichrome coil packed in magnesium-oxide powder inside a metal sheath: the powder carries heat out but no current, so the water touches a cool, earthed tube, not a live wire. 32 Ω at 220 V is 1500 W, all of it heat, by design.
Every other part, and why
Why it lasts
- Two temperature limits that fail in different ways. The thermostat is mechanical and resettable; the thermal fuse is a one-shot pellet. Something that defeats one (welded contacts) does not defeat the other.
- The fuse is set below boiling (95 °C), so even the worst case never makes steam inside a closed tank.
- The RCD covers the fault no thermostat can see: current leaking out of the element into the water.
- The element is just a resistor in an insulated sheath: nothing to drift, nothing to wear but the thermostat's contacts. Kept under water, its sheath runs only about 30 °C hotter than the water. Running it dry (empty the tank in the inspector) is how elements die; the thermal fuse, clipped near the element, catches that too.
- Real tanks add a magnesium rod (sacrificial anode) that corrodes instead of the tank. Replace it every few years and the tank outlives the house's pipes.
On the mat: the amber line is the energy path from A to B. Click any part: why it is there shows at the top of the inspector. Turn the path off or on in the View menu (Real-device path & notes); the Thermal view shows where the heat goes.
Background
The bimetal thermostat (from John Harrison's 18th-century clock compensation) and the sheathed element (Calrod, 1920s) are the whole technology of electric water heating. A heater from the 1960s and a new one are wired the same way.