A cartridge heater that stops heating is not always the actual source of the failure. Checking resistance, insulation resistance, applied voltage, current, bore condition, controls, and sensor placement helps technicians identify whether the problem is inside the heater or elsewhere in the thermal system.
Table of Contents
Work Safely Before Testing Identify the Failure Symptom Perform a Visual Inspection Test Heater Resistance Check Insulation Resistance Verify Voltage and Current Inspect Bore Fit and Heat Transfer Check Controls and Sensors Troubleshoot Lead-Wire Failures Removing a Stuck Cartridge Heater Symptoms and Likely Causes Preventing Repeat Failures Frequently Asked Questions Cartridge Heater Support from Big ChiefWork Safely Before Testing
Cartridge heaters are commonly installed in molds, dies, platens, sealing bars, packaging equipment, and other metal components that may remain hot after electrical power is removed. Before inspecting or testing a heater, follow the facility's electrical-safety and lockout/tagout procedures.
Confirm that:
- Electrical power has been disconnected and locked out.
- Stored electrical energy has been discharged.
- The heater circuit has been verified de-energized with properly rated test equipment.
- The machine and heated component have cooled to a safe working temperature.
- Pneumatic, hydraulic, and mechanical movement has been isolated.
- The heater leads have been identified before they are disconnected.
Resistance and insulation-resistance measurements must be taken with the heater isolated from controllers, relays, power controllers, and parallel heating circuits. Testing through connected equipment can produce misleading readings or damage sensitive electronics.
Identify the Failure Symptom
Record what the machine is doing before removing components. The symptom often narrows the investigation and helps distinguish a heater failure from a control or power-delivery problem.
Common symptoms include:
- The heating zone does not warm at all.
- The zone heats more slowly than normal.
- The temperature cannot reach its setpoint.
- The zone overheats or continues heating after output should stop.
- A fuse, breaker, or ground-fault device trips.
- The temperature reading is unstable or clearly incorrect.
- One heater fails repeatedly in the same location.
- The heater works intermittently when the leads move.
- The controller indicates output, but no current reaches the heater.
Record the controller display, alarms, process temperature, setpoint, output indication, line voltage, and current before cycling power. Intermittent faults may disappear temporarily after the equipment is restarted.
Perform a Visual Inspection
Begin with the heater, wiring, termination area, and heated component. Visible damage can reveal the likely failure mode before electrical testing begins.
Look for:
- Burned, brittle, pinched, or frayed lead wires
- Loose terminals or discolored connectors
- Oil, plastic, moisture, or contamination around the lead exit
- Sharp lead bends near the heater termination
- Dents, cuts, or tool marks on the heater sheath
- Swelling, dark spots, or localized sheath discoloration
- Evidence that the heater was hammered or forced into the bore
- A heater protruding farther from the bore than designed
- Oxidation or residue around the bore opening
Mechanical damage during installation is a frequent source of open circuits and ground faults. Pliers, hammers, set screws, and prying tools can damage the sheath, compacted insulation, resistance wire, or lead connection even when the exterior damage appears minor.
Test Heater Resistance
A resistance test determines whether the heater circuit is open, shorted, or reasonably close to its expected value. Isolate the heater leads and use an ohmmeter to measure resistance across the heating circuit.
The approximate expected resistance of a resistive heater can be calculated using:
Resistance = voltage² ÷ wattage
For example, the expected resistance of a 500-watt heater rated for 240 volts is:
240² ÷ 500 = 115.2 ohms
Interpret the result as follows:
- Infinite or open resistance: The resistance wire, internal connection, lead, or terminal is broken.
- Resistance substantially below the expected value: The heater may be internally shorted, incorrectly identified, or designed for a different voltage or wattage.
- Resistance reasonably close to the expected value: The heating circuit is continuous, but additional testing is still required.
- Reading changes when the lead is moved: A conductor or lead transition may be broken internally.
Cold resistance may differ from the calculated value because of manufacturing tolerances, conductor temperature, meter accuracy, and heater construction. Use the manufacturer's specification or drawing when available.
Check Insulation Resistance
A cartridge heater can pass a continuity test and still be unsafe or unreliable. Insulation-resistance testing checks for electrical leakage between the energized resistance circuit and the metal sheath.
With the heater isolated, test between each electrical lead and the sheath using an insulation-resistance tester and the voltage specified by the heater manufacturer or facility procedure.
Low insulation resistance may result from:
- Moisture absorbed during storage
- Process fluid entering the termination
- Damaged lead seals
- A punctured or crushed sheath
- Internal insulation breakdown
- Carbonized contamination around exposed terminals
- Excessive operating temperature
A moisture-affected heater may sometimes be restored through a controlled drying or bakeout procedure approved by the manufacturer. Do not apply full line voltage simply to dry a heater unless that method is specifically permitted and properly controlled.
Verify Voltage and Current
If resistance and insulation resistance are acceptable, check whether the machine is delivering the correct power to the heater. Perform energized measurements only when required, using appropriate safety procedures and test equipment.
Verify:
- Line voltage at the heater circuit
- Voltage while the controller is calling for heat
- Current through the heater
- Fuse and breaker condition
- Contactor or relay operation
- Solid-state relay or SCR output
- Loose terminals and damaged conductors
- Correct wiring for series or parallel heater circuits
For a single-phase resistive load, expected current can be approximated using:
Current = watts ÷ volts
A 500-watt heater rated for 240 volts should draw approximately:
500 ÷ 240 = 2.08 amps
Voltage significantly above the heater rating increases wattage and can cause rapid overheating. Low voltage reduces output and may produce slow heat-up even when the heater is undamaged.
If multiple heaters share one circuit, compare current among equivalent zones. A lower-than-normal circuit current may indicate one open heater within a parallel group.
Inspect Bore Fit and Heat Transfer
Cartridge heaters transfer heat primarily by conduction from the sheath into the surrounding metal. An excessive gap between the heater and bore traps air, reduces heat transfer, and raises the heater's internal operating temperature.
Inspect the bore for:
- Oversized diameter
- Taper or out-of-round conditions
- Rough surfaces and machining marks
- Rust, oxidation, or carbon buildup
- Plastic, oil, or process contamination
- Burrs at the bore opening
- Insufficient depth
- Damage from previous heater removal
The heater should fit closely enough to transfer heat effectively without requiring force during installation. The acceptable clearance depends on heater diameter, watt density, application temperature, and manufacturer recommendations.
A bore that is too loose can cause repeated heater burnout even when voltage, controls, and wattage are correct. A bore that is too tight can damage the sheath during installation and make later removal difficult.
Check Controls and Sensors
A failed sensor or switching device can damage a working cartridge heater. Do not assume the heater caused the over-temperature condition simply because it failed afterward.
Check:
- Temperature-sensor type and controller configuration
- Sensor polarity and wiring
- Sensor contact with the heated component
- Distance between the heater and sensing point
- PID and cycle-time settings
- Output indication from the controller
- Solid-state relays for a shorted-on condition
- Contactors for welded contacts
- Independent over-temperature protection
A sensor located too far from the heater may respond slowly while the heater and nearby metal overheat. A loose or withdrawn sensor can report a temperature lower than the actual tool temperature, causing the controller to apply continuous power.
Sensor position should represent the temperature that matters to the process while responding quickly enough to protect the heater and equipment.
Troubleshoot Lead-Wire Failures
The lead transition is one of the most mechanically vulnerable parts of a cartridge heater. Repeated bending, vibration, pulling, contamination, and excessive ambient heat can break the conductor or damage its insulation.
Common lead-related problems include:
- Sharp bends directly at the heater exit
- Leads pulled tight during machine movement
- Wires rubbing against sheet metal or moving components
- Lead insulation exposed above its temperature rating
- Plastic leakage surrounding the termination
- Oil or moisture entering through damaged seals
- Unsupported leads vibrating during machine operation
Route and support leads so movement occurs away from the heater transition. Use suitable sleeving, armor, strain relief, right-angle exits, or terminal configurations where the operating environment requires additional protection.
A heater that operates intermittently when its leads are repositioned should be removed from service. An internal conductor that reconnects temporarily can arc, overheat, or fail without warning.
Removing a Stuck Cartridge Heater
A cartridge heater may become stuck because of oxidation, contamination, bore distortion, thermal cycling, or inadequate installation clearance. Allow the assembly to cool and follow the equipment manufacturer's removal procedure before applying force.
Avoid:
- Pulling directly on the electrical leads
- Hammering the heater deeper into a blind bore
- Crushing the sheath with locking pliers
- Drilling into the heater while it remains electrically connected
- Applying uncontrolled heat near wiring or combustible material
If the bore passes completely through the component, the heater may be driven out from the opposite side using a correctly sized, non-damaging tool after power is isolated and the equipment has cooled. Blind-hole installations may require a puller, threaded extraction feature, split-sheath design, or controlled machining procedure.
After removal, clean and inspect the bore before installing a replacement. Forcing a new heater into a damaged or contaminated hole can immediately recreate the original problem.
Symptoms and Likely Causes
Heater Has No Continuity
An open resistance reading usually indicates a broken resistance wire, failed internal connection, damaged lead, or severe overheating. Replace the heater and investigate why the circuit opened.
Heater Has Continuity but Produces No Heat
Check applied voltage, current, fuses, wiring, controller output, relays, contactors, and power controllers. The heater may be functional but receiving no electrical power.
Heater Trips Ground-Fault Protection
Test insulation resistance between the heater circuit and sheath. Moisture, contamination, sheath damage, or internal insulation breakdown may be creating leakage to ground.
Heater Warms Too Slowly
Check for low voltage, incorrect wattage, an open heater in a parallel circuit, excessive heat loss, poor contact with the bore, or a process load greater than the original design.
Heater Fails Repeatedly in the Same Bore
Inspect bore diameter, straightness, contamination, sensor location, watt density, applied voltage, controller operation, and heat removal from that section of the tool.
Lead Wires Burn Near the Heater
Check ambient temperature, lead insulation rating, strain relief, contamination, terminal tightness, and whether the lead end is positioned inside an excessively hot area.
Temperature Overshoots or Oscillates
Review sensor contact and location, controller tuning, cycle time, heater wattage, switching-device operation, and the thermal response of the tool.
Preventing Repeat Failures
Installing an identical replacement without identifying the failure cause can produce the same result. Preserve the failed heater until the process, controls, electrical system, and bore have been evaluated.
Useful preventive practices include:
- Recording heater voltage, wattage, dimensions, and part numbers
- Measuring bore diameter and condition during scheduled maintenance
- Keeping bores clean and free of oxidation or process residue
- Protecting leads from heat, movement, fluids, and sharp edges
- Checking controller output and switching devices
- Confirming temperature sensors remain fully seated
- Monitoring current for changes among equivalent heating zones
- Using independent high-limit protection where overheating could cause damage
- Selecting watt density for the available heat transfer and operating temperature
- Following the heater manufacturer's installation and removal instructions
Trend data can help identify deterioration before a complete failure occurs. Increasing heat-up time, declining current, unstable temperature, or recurring control alarms may indicate a developing heater, sensor, connection, or heat-transfer problem.
Frequently Asked Questions
How do you test a cartridge heater with a multimeter?
Disconnect and isolate the heater, then measure resistance across its leads. Compare the result with the manufacturer's specification or the approximate value calculated by dividing voltage squared by wattage.
Why does a cartridge heater keep burning out?
Common causes include poor bore fit, excessive watt density, incorrect voltage, inadequate heat transfer, over-temperature conditions, sensor problems, failed switching devices, contamination, and lead damage.
What does low insulation resistance mean on a cartridge heater?
It indicates electrical leakage between the heating circuit and metal sheath. Moisture, contamination, sheath damage, overheating, or internal insulation breakdown may be responsible.
Can a cartridge heater test correctly and still be faulty?
Yes. A heater may show normal resistance when cold but fail as it expands during heating. It may also have intermittent lead damage or insulation resistance that deteriorates at operating temperature.
Can a failed cartridge heater be repaired?
Cartridge heaters are normally replaced rather than repaired because the resistance wire and compacted insulation are sealed inside the sheath.
Cartridge Heater Support from Big Chief
Big Chief can help maintenance teams determine whether repeated cartridge-heater failures are related to bore fit, watt density, operating temperature, voltage, lead configuration, controls, or application conditions. Photographs of the failed heater and bore, resistance readings, equipment information, and the original heater specifications can make cross-referencing and root-cause review more effective.
Big Chief supplies standard, high-density, split-sheath, square, metric, and custom cartridge heaters for molds, dies, platens, sealing equipment, packaging machinery, and OEM systems. When an exact replacement is unavailable, the heater dimensions, heated length, wattage, voltage, termination, sensor requirements, and installation environment can be reviewed to identify an appropriate alternative.
