A 220-240 V AC element is not a 24 V DC element with a different label on it — the resistance differs by the square of the ratio, and with it the conductor gauge and the whole circuit layout. This page sets out what 220-240 V AC does to the design of ksd9700 bimetal thermostats.
The Element at 220-240 V AC
Two equations do all the work: P = V²/R and I = P/V. At 220-240 V AC, 500 W is 96.8 Ω and 2.27 A.
| Worked example | 500 W at 220-240 V AC |
|---|---|
| Element resistance, R = V²/P | 96.8 Ω |
| Current, I = P/V | 2.27 A |
The span matters more than the example. At 220-240 V AC a 20 W element is 242 Ω and a 500 W element is 96.8 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 — 2.27 A for the worked example — but the element is a long, fine conductor, and a low-wattage heater at 220-240 V AC is the hardest thing in this catalogue to build: 20 W means 242 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
220-240 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 KSD9700 Bimetal Thermostats Is
The small snap-action bimetal disc fitted inside most of the heaters in this catalog: it opens at a fixed temperature and closes again when the part has cooled.
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 | 150 °C |
The continuous limit of 150 °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: Vulcanised inside the heater, bonded to the plate, or bolted.
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.
Which Constructions Are Wound for 220-240 V AC
220-240 V AC is not unique to this construction. In this catalogue it is also wound into:
The supply voltage is wound in at manufacture. A heater built for one voltage cannot be run on another and rated down by arithmetic — at half the voltage it makes a quarter of the power, and at twice the voltage it makes four times the power and destroys itself.
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.
KSD9700 Bimetal Thermostats — 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 | Snap-action bimetal disc, normally closed or normally open | Normally closed is the protective configuration |
| Open temperature | +40 u00b0C to +150 u00b0C in 5 u00b0C steps | Fixed at manufacture u2014 it is not adjustable |
| Tolerance | u00b15 u00b0C standard, u00b13 u00b0C selected | On the open point, measured rising |
| Reset differential | 15 u00b0C to 25 u00b0C below the open point | Wide differential is deliberate: it stops the contact chattering |
| Switching rating | 5 A at 250 V AC resistive | Resistive only u2014 a heater is a resistive load, which is why this rating applies |
| Life | 10,000 cycles at rated load | Far more at a fraction of the rating |
| Mounting | Vulcanised inside the heater, bonded to the plate, or bolted | It must read the part it is protecting, not the air near it |
| Certifications | UL, VDE, CQC on the component | Component approval carries into the heater assembly file |
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.
