A plating shop started its tank each morning by switching on a 5 kW PTFE immersion heater. Three or four mornings out of five, the breaker tripped within seconds of startup. The heater worked normally once the tank was warm. Maintenance replaced the breaker twice, then the heater once. The trips continued. The problem was not the breaker or the heater. It was the cold-start condition.
Cold start is the most electrically stressful moment in a heater's operating cycle. The resistance element is at ambient temperature, the internal insulation may have absorbed moisture overnight, and the inrush current can exceed the steady-state value by 10–30%. For PTFE immersion heaters, which are often installed in wet environments and operated intermittently, cold-start trips are a common and often misdiagnosed problem.
Why Cold Resistance Is Lower Than Hot Resistance
Heating alloys have a positive temperature coefficient of resistance. Resistance increases as the element heats up. A nichrome element that measures 14.4 Ω at 20°C may measure 15.5–16.0 Ω at operating temperature.
At the moment of startup, the element is at ambient temperature. Its resistance is at the lowest point in its operating range. For a fixed supply voltage, lower resistance means higher current. The inrush current at cold start can be 8–15% higher than the steady-state current at operating temperature. In some element designs with high temperature coefficients, the difference can reach 20–30%.
The higher current is normal and brief. The element heats up within seconds, resistance rises, and current falls to steady state. But if the breaker is sized close to the steady-state current, the inrush can exceed its instantaneous trip threshold.
| Element Temperature | Resistance | Current at 240V | Relative to Steady State |
|---|---|---|---|
| 20°C (cold) | 14.4 Ω | 16.7 A | 108% |
| 100°C | 14.9 Ω | 16.1 A | 104% |
| 200°C | 15.4 Ω | 15.6 A | 101% |
| 400°C (operating) | 15.6 Ω | 15.4 A | 100% |
Cold-start current characteristics for a typical 4 kW, 240V PTFE immersion heater
Moisture in the Internal Insulation
The second cold-start issue is moisture. Magnesium oxide and other internal insulation materials are hygroscopic. If the heater is idle overnight in a humid environment or if the terminal enclosure is not sealed, moisture can migrate into the insulation. This reduces the insulation resistance and can create a partial conductive path between the element and ground.
When power is applied, the moisture in the insulation begins to conduct. The leakage current adds to the normal load current, increasing the total current through the breaker. In severe cases, the leakage current alone is enough to trip a ground fault breaker.
As the heater warms up, the moisture is driven off. The insulation resistance recovers, and the leakage current falls. This is why the trips often occur only at cold start and disappear once the heater has been running for a few minutes.
| Insulation Condition | Insulation Resistance at 20°C | Leakage Current at 240V | Breaker Trip Risk |
|---|---|---|---|
| Dry, new heater | >500 MΩ | <1 μA | None |
| Dry, aged heater | 100–500 MΩ | 1–2 μA | Very low |
| Light moisture | 10–50 MΩ | 5–24 μA | Low–Moderate |
| Moderate moisture | 1–10 MΩ | 24–240 μA | Moderate–High |
| Heavy moisture | <1 MΩ | >240 μA | High |
Insulation resistance and leakage current at cold start for PTFE immersion heaters
Undersized Breakers and Long Cable Runs
The breaker must be sized for the heater's steady-state current plus a margin for inrush and voltage variation. For a 4 kW, 240V heater drawing 16.7 A at steady state, a 20 A breaker is the minimum. A 25 A breaker provides more margin for cold-start inrush.
Voltage drop in long cable runs reduces the voltage at the heater, which increases the current for the same power output. A 5% voltage drop increases current by about 5%. If the breaker was sized without accounting for voltage drop, the combination of low voltage and cold-start inrush can exceed the trip threshold.
| Heater Wattage | Steady-State Current at 240V | Minimum Breaker | Recommended Breaker | Notes |
|---|---|---|---|---|
| 2 kW | 8.3 A | 15 A | 15 A | Short cable runs |
| 3 kW | 12.5 A | 15 A | 20 A | Moderate cable runs |
| 4 kW | 16.7 A | 20 A | 25 A | Long cable runs |
| 5 kW | 20.8 A | 25 A | 30 A | Long cable runs |
| 6 kW | 25.0 A | 30 A | 35 A | Long cable runs |
Breaker sizing recommendations for single-phase PTFE immersion heaters at 240V
Distinguishing Cold-Start Trips From Other Faults
Not every cold-start trip is a sizing or moisture problem. The pattern of trips provides diagnostic information.
A trip that occurs every morning at startup and does not recur after reset is characteristic of cold-start inrush or moisture. The heater is likely healthy, but the breaker or the installation is marginal.
A trip that occurs at startup and recurs immediately after reset indicates a hard fault-a short circuit, a ground fault, or a failed element. The heater should be tested before further attempts to operate it.
A trip that occurs randomly during operation, not just at startup, indicates a developing fault-insulation degradation, a loose connection, or a failing controller. The heater should be inspected and tested.
Practical Solutions for Cold-Start Trips
Increase the breaker size to the next standard rating above the recommended value. A 25 A breaker for a 4 kW heater provides margin for cold-start inrush without compromising protection.
Verify that the supply voltage is within 5% of the nameplate value at the heater terminals. If voltage drop is excessive, increase the conductor size or reduce the cable length.
Dry the heater before startup if moisture is suspected. Applying low voltage-25–50% of rated-for 15–30 minutes drives moisture out of the internal insulation without generating full heat. This is particularly important for heaters that are idle for extended periods in humid environments.
Seal the terminal enclosure to prevent moisture ingress. A gasketed cover, sealed cable entry, and a desiccant pack inside the enclosure reduce the moisture that reaches the internal insulation.
When Cold-Start Trips Reveal a Deeper Issue
Repeated cold-start trips on a heater that tests healthy often indicate that the installation is marginal. Undersized breakers, excessive voltage drop, or chronic moisture ingress are installation problems, not heater problems. Correcting the installation is more effective than replacing the heater.
A custom PTFE immersion heater configuration with sealed terminal enclosures, moisture-resistant internal insulation, and an element design matched to the supply voltage addresses the root cause of cold-start trips. The specification process requires the supply voltage, breaker size, cable length, ambient conditions, and the heater's operating cycle.
