A 380-415 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 380-415 V AC does to the design of alumina ceramic thick film heaters.
The Element at 380-415 V AC
Two equations do all the work: P = V²/R and I = P/V. At 380-415 V AC, 3000 W is 48.1 Ω and 7.89 A single phase, or 4.56 A per phase balanced across three (P = √3 · V · I).
| Worked example | 3000 W at 380-415 V AC |
|---|---|
| Element resistance, R = V²/P | 48.1 Ω |
| Current, I = P/V | 7.89 A |
The span matters more than the example. At 380-415 V AC a 20 W element is 722 Ω and a 500 W element is 288.8 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 — 7.89 A for the worked example — but the element is a long, fine conductor, and a low-wattage heater at 380-415 V AC is the hardest thing in this catalogue to build: 20 W means 722 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.
Resistance tolerance is u00b110% standard, u00b15% selected, and power follows resistance inversely: a u00b110% standard, u00b15% selected tolerance band is the same band on the delivered wattage. Where the wattage matters — a warm-up to a deadline, a pack of heaters that must match — say so on the enquiry and the tighter band is selected and tested.
Isolation, Earthing and Safety Class
380-415 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.
| Dielectric withstand | 2500 V AC, 1 minute |
|---|---|
| Insulation resistance | greater than 100 Mu03a9 at 500 V DC |
Every heater is tested to these before it ships, element to any exposed surface. A dielectric test is a pass or a fail, not a measurement — a part that passes at 2500 V is not one that may be run at 2500 V.
What an Alumina Ceramic Thick Film Heaters Is
The same printed construction on 96% alumina, for higher temperature and for applications that need the element electrically isolated from everything around it.
Dielectric and resistor pastes screen printed onto stainless steel or alumina and fired at around 850 °C, then laser trimmed to resistance. There is no separate element and no interface: the heated surface is the substrate itself. On alumina the dielectric is the substrate, so the isolation is excellent and the temperature capability is the highest here — at the cost of a brittle part that must be clamped evenly and never asked to take a bending load.
The heat leaves through a clamped joint, and a clamped joint is only as good as the pressure and the flatness behind it. A thermal interface material or a thin film of thermal grease fills the gaps that machining leaves; without one, contact happens at the high spots and the element runs hotter than the plate it is bolted to.
| Element | fired resistor paste, laser trimmed |
|---|---|
| Insulation system | fired glass dielectric on the substrate |
| Total thickness | 1 mm to 3 mm total |
| Maximum size, one piece | 150 x 200 mm |
| Continuous working temperature | 400 °C |
The continuous limit of 400 °C is set by fired glass dielectric on the substrate. 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.
Watt Density: the Two Figures
Two watt densities are quoted because there are two thermal situations. With nothing conducting heat away, 10 W/cm² is what the insulation survives. Bonded to a heat sink over the full outline, 80 W/cm² is. Anything in between depends on the quality of the joint, and the joint is the customer's to make.
The failure is silent and quick: the element does not know how it is mounted, so at 80 W/cm² in free air the heat simply accumulates in the insulation until it degrades. This is the single most common way these heaters are destroyed, and it happens on the first run rather than after a year.
Thick film reaches these densities because the heated surface is the substrate — there is no interface between element and load to conduct across. That is also why dry-run protection is not optional: with the fluid gone, so is the entire thermal path, and the fired layers reach their limit in seconds rather than minutes.
Leads and Terminations
Termination is specified on the drawing: lead length, gauge, insulation and the connector or tag at the end of it.
Connector and termination options: Welded tabs, crimp terminals or soldered leads depending on temperature. A keyed housing removes a soldering operation from the customer's line and removes the possibility of the assembly being connected backwards; on a heater with a separate sensor circuit it also stops the sensor pair being mistaken for the power pair.
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.
On a fluid heater the limit device belongs on the substrate, close to the resistor, not downstream in the water. Downstream is where the temperature has already gone wrong; the substrate is where it goes wrong first, and at these watt densities the interval between the two is short.
Sensors can be built into the part: NTC thermistor, PT100 or PT1000 RTD, or a K-type thermocouple, fitted under the same insulation and tested with the heater as one assembly. A sensor laminated into the heater reads the heater; a sensor on the load reads the load. Both are legitimate, and the drawing has to say which is wanted.
Which Constructions Are Wound for 380-415 V AC
380-415 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:
- An outline drawing, or a sketch with dimensions: overall size, holes, cut-outs, radii and the lead exit position.
- The supply voltage, and whether it is AC or DC. The element is wound for one voltage; it is not a switchable range.
- The power required, or — better — the temperature to be reached and the mass or flow it has to be reached in. Given the second, the wattage can be calculated instead of guessed.
- What the heater is attached to, and how. This is what decides which watt density figure applies, and it is the question a quotation cannot proceed without.
- The ambient, and the lowest temperature the assembly must start from. A heater sized for a 20 °C start is undersized for a -30 °C one.
- Duty cycle: continuous, intermittent, or a warm-up to a deadline. Time to temperature is a specification, not a by-product.
- Sensor and protection: which sensor, where it goes, and which limit device is fitted in series with the element.
- The certification the end product is built to, named on the enquiry rather than after approval.
- Quantity and target price, because the construction that is right for fifty pieces is often not the one that is right for fifty thousand.
- The fluid, its flow rate and its inlet temperature, and how a dry-run condition is detected in your system.
Lead time: Samples 7-10 days from approved drawing; production 15-20 days.
Alumina Ceramic Thick Film Heaters — 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 |
|---|---|---|
| Construction | Glass dielectric and resistor pastes screen printed and fired onto the substrate, then laser trimmed to resistance | Fired, bonded or encapsulated as a single assembly |
| Substrate | 96% alumina ceramic, 0.6 mm to 2 mm | The substrate carries the heat and the mechanical load |
| Thickness | 1 mm to 3 mm total | Substrate plus element system |
| Maximum size, one piece | 150 x 200 mm | Limited by the firing or curing fixture |
| Dimensional tolerance | u00b10.2 mm on the substrate, u00b10.1 mm on printed features | On the machined substrate |
| Supply voltage | 12 V DC to 400 V AC | Circuit is designed for the ordered voltage |
| Resistance tolerance | u00b110% standard, u00b15% selected | Thick film resistors are laser trimmed where a tighter band is required |
| Watt density | 10 W/cmu00b2 typical, 80 W/cmu00b2 maximum in a wetted application | Achievable because the heat is generated in the surface being cooled |
| Maximum working temperature | +400 u00b0C at the element system | Of the element system; the substrate itself tolerates more |
| Dielectric strength | 2500 V AC, 1 minute | One minute, circuit to substrate |
| Insulation resistance | greater than 100 Mu03a9 at 500 V DC | At 500 V DC |
| Terminations | Welded tabs, crimp terminals or soldered leads depending on temperature | Soldered, welded or crimped as the temperature allows |
| Mounting | Bolted, brazed or welded into the assembly; flatness of the mating face is critical | Flatness of the mating face matters more than clamping force |
| Sensor and control options | Printed PTC or NTC track, or a bonded thermocouple | Integrated into the same firing or bonded to the substrate |
| Certifications available | UL 499, IEC/EN 60335, RoHS, REACH, CE | Construction level |
| Isolation | Substrate is the insulator u2014 no separate dielectric needed | Which is what allows the highest watt densities here |
| Handling | Brittle; support fully and never clamp on a point | Nearly every ceramic heater failure in the field is mechanical, not electrical |
Capability figures are this factory's process limits for fired thick film construction. Watt densities far above any flexible heater are achievable because the resistor is printed directly onto the surface being cooled — there is no interface and no adhesive in the heat path.
