Supply voltage page for thermal cutoffs and one-shot fuses at 110-120 V AC. North American mains. Requires mains-rated insulation, a protective earth or double insulation, and a construction certified to the standard for the end product. The arithmetic, the safety class, the switching device and the capability limits are below.
The Element at 110-120 V AC
Two equations do all the work: P = V²/R and I = P/V. At 110-120 V AC, 500 W is 24.2 Ω and 4.55 A.
| Worked example | 500 W at 110-120 V AC |
|---|---|
| Element resistance, R = V²/P | 24.2 Ω |
| Current, I = P/V | 4.55 A |
The span matters more than the example. At 110-120 V AC a 20 W element is 605 Ω and a 500 W element is 24.2 Ω — a factor of twenty-five in resistance, met by changing the conductor cross-section and the length of the path, both of which are constrained by the outline.
At mains voltage the difficulty is the opposite one. Current is modest — 4.55 A for the worked example — but the element is a long, fine conductor, and a low-wattage heater at 110-120 V AC is the hardest thing in this catalogue to build: 20 W means 605 Ω, which is a very long track to fit inside a small outline without the pitch becoming so tight that adjacent runs interact.
Where the arithmetic will not fit the outline, the usual answers are a lower supply voltage with a small power supply behind it, several circuits in series, or a different construction entirely. What does not work is thinning the conductor further: the track becomes the fuse.
Isolation, Earthing and Safety Class
110-120 V AC is a mains supply, and the construction is built to match: mains-rated insulation, a protective earth connection or a double-insulated build, and creepage and clearance distances held between the element and any accessible surface.
Leakage current is the figure that surprises people. A large heated area behaves as a distributed capacitance to earth, so a big mains heater has a measurable leakage even when it is perfectly sound, and an RCD protecting several of them can trip on the sum. Where several elements share a circuit, allow for the total.
What a Thermal Cutoffs and One-Shot Fuses Is
The last line of protection: a fuse that opens once at a fixed temperature and never closes again, fitted where a resettable thermostat would let a fault repeat.
A bimetal disc that opens a contact at a fixed temperature. It is not an accurate controller and is not meant to be one: it is the device that decides whether the heater around it survives a fault, and it is specified by its open temperature, its reset behaviour and the current it can break.
The heat leaves through whatever the component is clamped or bonded to; a sensing component adds no heat of its own and simply has to be in thermal contact with the thing it is reporting on.
| Total thickness | Component |
|---|---|
| Maximum size, one piece | Component |
| Continuous working temperature | 240 °C |
The continuous limit of 240 °C is set by the insulation system. The element alloy would run far hotter; it is never the element that decides.
Switching an AC Element
AC switching has a zero crossing to hide behind. A zero-cross solid state relay makes and breaks at the crossing, which all but removes the switching transient and the conducted emissions that go with it — worth having on any installation that also has to pass an EMC test.
Control by burst firing — whole cycles on, whole cycles off, over a period of a second or more — rather than by phase angle. Burst firing generates almost no harmonics; phase angle control chops the waveform and is the reason a heater ends up in the EMC report at all. A heater has enough thermal inertia that a one-second period is invisible at the surface.
Size the relay for the inrush, not the running current, and heatsink it: a triac or SSR drops around one volt per amp of load, and that is real power to be got rid of inside the panel.
Attachment and Thermal Contact
A sensing component is fitted where it will read the temperature that matters: against the element for a limit device, against the load for a control device. The two positions give different numbers, and choosing the wrong one is the commonest reason a correctly built loop holds the wrong temperature.
From the capability table: In series with the element, inside the insulation.
A sensor reads its own junction, not the surface it is near. Bed it in thermal compound, clamp it, or pot it into the assembly: an air gap of a tenth of a millimetre between a thermistor and the plate it is supposed to be measuring is worth several degrees of error, and it is an error the controller cannot see.
Leads and Terminations
Termination is specified on the drawing: lead length, gauge, insulation and the connector or tag at the end of it.
The lead exit is a thermal discontinuity as well as an electrical one. The joint between a fine element and a stranded lead is the hottest point of many heaters, so it is kept out of the highest watt density zone, strain relieved, and — where the exit sits in a hot area — insulated in PTFE rather than in silicone.
Control and Protection
This heater has no way of limiting its own temperature. Resistance heating draws roughly the same power whatever the temperature, so with the load removed, the fluid gone, the fan stopped or the sensor open circuit, the element keeps producing watts until something stops it.
The design must therefore contain a control device and, separately, a protective one: a controller with a sensor to hold the set point, and a bimetal thermostat or a one-shot thermal cutoff wired in series with the element to open the circuit if the loop ever fails. The two are not interchangeable — a controller that has lost its sensor cannot protect anything, which is exactly the fault the cutoff exists for.
How This Construction Fails
- Loss of the control loop. An open-circuit sensor, a welded relay contact or a controller in manual leaves the element at full power with nothing watching it — the case the series thermal cutoff exists for, and the reason it must not be omitted to save a part.
- Measuring the wrong thing. A sensor in the air near a heated plate reports the air, and the loop holds the air where the specification wanted the plate. Nothing in the instrument can detect this.
How to Specify One
This is a made-to-drawing part, so there is no catalogue number to quote. A quotation is built from the following, and the more of it that arrives with the enquiry the fewer rounds it takes:
- The temperature to be sensed or switched, and the tolerance the application actually needs.
- Where the component sits — against the element, in the potting, on the load, or in the airflow.
- The electrical duty: for a switching device, the voltage, the current and whether the load is AC or DC.
- Lead length, insulation and termination, and whether the component is fitted by us into a heater assembly or supplied loose.
- The standard the end product is certified to, so the component chosen is one that fits inside that approval.
Lead time: Samples 7-10 days from approved drawing; production 15-20 days.
Thermal Cutoffs and One-Shot Fuses — Capability Table
These are process limits for the construction, not the specification of one stocked item. Every line is what the factory can hold; what your part holds is fixed on your drawing.
| Parameter | Specification | Limit / note |
|---|---|---|
| Type | One-shot thermal cutoff (TCO), non-resetting | Once it opens, the heater is dead and the fault has to be found |
| Function temperature | +72 u00b0C to +240 u00b0C, fixed | Chosen above the working temperature and below the damage temperature |
| Tolerance | 0 / -5 u00b0C on the function temperature | |
| Rating | 10 A at 250 V AC | |
| Holding temperature | Typically 15 u00b0C to 25 u00b0C below the function temperature | Continuous operation above it will eventually trip the fuse anyway |
| Mounting | In series with the element, inside the insulation | Outside the insulation it protects nothing |
| Why one-shot | A resettable thermostat can cycle on a fault indefinitely | Some faults must stop the product permanently, and this is the component that does it |
| Certifications | UL, VDE, CQC on the component |
Figures are the published tolerance and rating bands of the standard sensor and control components fitted to these heaters. Sensor tolerance classes follow IEC 60751 for RTDs and IEC 60584 for thermocouples; thermostat and thermal cutoff ratings are the component manufacturers' own.
