Silicon carbide (SiC) heating elements are the workhorse of industrial high-temperature furnaces up to roughly…
MoSi2 Element Cold-Start Procedure (1/3 Voltage Rule)
The cold-start rule that protects every MoSi2 element
The single most important operating rule for MoSi2 is the reduced-voltage (often 1/3 supply) cold start. Skip it and you can destroy a new element on the first switch-on. Here is the physics and the procedure.
Why cold MoSi2 draws a huge current
MoSi2 has a strongly positive temperature coefficient: at room temperature its resistance is only a fraction of its operating resistance (often 1/8 to 1/10). The result is a very large inrush current the instant you apply full voltage to a cold furnace. As the element heats past ~1000°C its resistance climbs and the current falls to normal running levels.
This behaviour and the load limits behind it are in surface load & current limits and the full spec sheet.
The 1/3-voltage procedure
- Start at reduced voltage. Use a transformer tap or SCR/thyristor controller to apply roughly one-third of the rated supply voltage to the cold elements.
- Hold through warm-up. Keep the reduced voltage until the element temperature passes ~1000°C and resistance has risen — typically the furnace is already glowing.
- Ramp to full voltage. Once resistance is up, bring voltage to 100% and continue to setpoint. The current now stays within the rated band.
What happens without it
| Start method | Result |
|---|---|
| Full voltage, cold | Massive inrush; element or supply damage |
| 1/3 voltage ramp | Controlled current; long element life |
Two linked cautions
- Do not dwell in the 400–700°C band during warm-up — it accelerates low-temperature oxidation. Our pesting guide explains why you should ramp through it.
- Size the transformer and leads for the running load plus margin, using the grade limits in the temperature-grade guide.
Not sure your controller ramps correctly? Send us your electrical schematic and we will check the start sequence.
Why Specify Molybdenum Disilicide (MoSi2) heating elements with HeatingDriver
HeatingDriver is a manufacturer and exporter of Molybdenum Disilicide (MoSi2) heating elements with nearly 20 years of experience supplying furnace builders, labs, and integrators worldwide. We stock standard grades and build custom sizes from your drawings, and we review your element drawings free of charge before production. Browse our Molybdenum Disilicide (MoSi2) heating elements catalog or contact our engineers for a quote.
Frequently Asked Questions
Q: What exactly is the 1/3-voltage cold-start procedure?
Apply roughly one third of the rated supply voltage to the cold elements using a transformer tap or an SCR/thyristor controller, hold that reduced voltage until the element passes about 1000°C and its resistance has risen — by which point the furnace is typically already glowing — then ramp to full voltage and continue to setpoint.
Q: Why does a cold MoSi2 element draw such a large current?
MoSi2 has a strongly positive temperature coefficient, so at room temperature its resistance is only a fraction of the operating value, often between one eighth and one tenth. Applying full voltage to a cold furnace therefore produces a very large inrush. As the element heats past about 1000°C its resistance climbs and the current falls back to normal running levels.
Q: What happens if the reduced-voltage start is skipped?
A full-voltage start into cold elements produces a massive inrush that can damage the element or the supply, and it can destroy a new element on the very first switch-on. A 1/3-voltage ramp keeps the current controlled and gives long element life.
Q: What else should be watched during warm-up?
Do not dwell in the 400–700°C band, because that accelerates low-temperature oxidation, known as pesting — ramp through it instead. Also size the transformer and leads for the running load plus margin, using the limits for your element grade.
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