What Are DIN 44081 and DIN 44082 Standards?

DIN 44081 and DIN 44082 are German industrial standards defining PTC (Positive Temperature Coefficient) thermistor specifications for motor overload protection, established by Deutsches Institut für Normung (DIN) in 1980 and 1985 respectively.

DIN 44081 covers single-element sensors with cold resistance of 30-250Ω at 25°C, while DIN 44082 specifies triple-element sensors for three-phase motors with standardized color coding from 60°C to 180°C. Both standards mandate critical resistance thresholds: <550Ω at TROT-5K, >1,330Ω at TROT+5K, and >4,000Ω at TROT+15K. As of 2020, both standards have been consolidated into DIN VDE V 0898-1-401:2020-03, aligned with international equivalent IEC 60738-1.

The Critical Knowledge Gap

However, 90% of engineers overlook why the ROT±5K and ROT±15K temperature points are critical — they stem directly from the Curie temperature physics of BaTiO₃ (barium titanate) ceramic. More critically, using a standard multimeter with >10mA test current creates self-heating errors that elevate resistance readings by 30-50Ω, making measurements unreliable for DIN compliance verification.

This guide covers the 4-step DIN-compliant test procedure, complete color code matrix (60°C-180°C), protection relay compatibility, and application guidance for EV traction motors, refrigeration compressors, and industrial equipment.

Standard Evolution and International Alignment

Historical Timeline

1980 - DIN 44081:1980-06 published for single-element PTC thermistors in motor windings
1985 - DIN 44082:1985-06 introduced triple-element configuration with standardized color coding
2016 - DIN VDE V 0898-1-401:2016-03 consolidated both standards
2020 - Current version DIN VDE V 0898-1-401:2020-03 published with minor test clarifications

Why Triple-Element Design Matters

DIN 44082 addresses a critical reality: three-phase motors frequently experience load imbalances causing one phase to overheat while others remain normal. The series connection of three sensors (one per U/V/W phase) ensures any single phase reaching trip temperature triggers protection, preventing winding burnout and fire hazards.

International Standard Equivalents

DIN 44081 vs DIN 44082 Quick Comparison

Understanding R-T Characteristics: The Physics Behind ROT±5K and ROT±15K

The resistance-temperature specifications in DIN 44081/44082 directly reflect Curie temperature transition physics of BaTiO₃ ceramic. Understanding why specific resistance thresholds exist requires examining the crystallographic phase transformation.

The Curie Temperature Phenomenon

Barium titanate (BaTiO₃) undergoes a paraelectric-to-ferroelectric phase transition at its Curie temperature (Tc):

• Below Tc: Tetragonal crystal structure, free electron mobility, low resistance (30-250Ω)
• Above Tc: Cubic structure, grain boundary barriers, resistance increases 3-4 orders of magnitude within 10-15°C

According to IEC 60738-1:2022, Annex B, minimum temperature coefficient is ≥15%/K near TROT. High-quality sensors achieve 20-35%/K for faster protection response.

Critical Test Points Explained

TROT-5K (<550Ω): Pre-Warning Zone

Why 550Ω maximum? According to DIN VDE V 0898-1-401:2020-03, Clause 5.3.2:

• Protection relays use 2.5-4.5kΩ trip thresholds
• 550Ω provides 5-8× safety margin below trip points
• Enables predictive maintenance: resistance drift from 150-250Ω baseline toward 550Ω indicates impending protection event

TROT+5K (>1,330Ω): Critical Trip Point

Why 1,330Ω minimum? According to IEC 60034-11-2:2010, Table 2:

• Industry standard requires ≥2.4× resistance increase from TROT-5K to TROT+5K
• Calculation: 550Ω × 2.4 = 1,320Ω (1,330Ω specification confirms minimum transition sharpness)
• Ensures universal relay compatibility and irreversible high-resistance state until cooling

TROT+15K (>4,000Ω): High-Resistance Lock

Why 4,000Ω minimum? According to DIN 44080:1980-04:

• Confirms material stability (sensors failing to reach 4kΩ may have degraded ceramic)
• Enables diagnostic distinction: 1.3-3kΩ = normal trip; >4kΩ = severe overload requiring inspection
• Guarantees fail-safe circuit opening even if relay components age

Complete Resistance-Temperature Specification Matrix

Data Sources: DIN VDE V 0898-1-401:2020-03 Clause 5.3; IEC 60738-1:2022 Table 4; Focusens Technical Datasheet 2025

Why Test Voltage Matters: The Self-Heating Trap

According to DIN 44080:1980-04, Clause 6.2, test currents must be <1mA to avoid self-heating. For a 130°C PTC sensor with R₂₅ = 150Ω:

• Standard multimeter (10mA): Power = 15mW → Temperature rise 5-15°C → Resistance error +30 to +50Ω
• Compliant meter (<1mA): Power = 0.15mW → Temperature rise <0.5°C → Resistance error ±2Ω

Critical Impact: When verifying TROT-5K (<550Ω), a sensor reading 580Ω on a standard multimeter might actually be 540Ω (compliant), but incorrectly rejected due to self-heating error.

Professional Equipment (per IEC 60738-1:2022, Annex C):

• Keysight 34461A (6½ digit, <0.5mA) — $1,500
• Fluke 8846A (6½ digit, selectable current) — $2,200
• Focusens FS-PTC-TESTER (portable, <0.5mA, auto temp compensation) — $250 with ±3% accuracy for field testing

DIN 44082 Color Coding Standard: Complete 60°C-180°C Matrix

DIN 44082:1985-06, Annex A standardized wire colors to prevent installation errors. Without color coding, technicians might install incorrect ratings, causing either nuisance trips (under-protection) or dangerous winding burnout (over-protection).

Complete Color Code Reference

Data Sources: DIN 44082:1985 Annex A; VDE 0898-1-401:2020 Table 6; Focusens Catalog 2025; Motors & Drives Magazine 2024 Survey

Why 190-210°C Lacks Standard Colors

According to IEC 60738-1:2022, Clause 4.3, standard BaTiO₃ formulations exhibit instability above 200°C due to:

• Dopant diffusion: Rare earth ions (La³⁺, Nd³⁺) migrate, shifting Tc by ±10-15°C over 1,000-2,000 cycles
• Oxidation reactions: Oxygen vacancy concentration changes at >200°C, altering resistivity
• Proprietary formulations: Each manufacturer uses custom doping, preventing color standardization

Manufacturing Cost: Achieving stable 200°C+ operation requires modified perovskite structures, nano-grain engineering, and protective atmosphere sintering — increasing costs 3-5× vs. standard grades.

Recommended Solution: For >180°C applications, use hybrid monitoring: DIN 44082 PTC at 155-160°C (primary protection) + Type K thermocouple or PT100 RTD (continuous monitoring). This provides reliable overload protection while avoiding >190°C PTC stability issues.

Testing Procedures: 4-Step DIN-Compliant Protocol

According to DIN 44080:1980-04 and IEC 60738-1:2022, Clause 7, proper PTC verification requires progressive testing. Most field technicians skip critical steps, leading to false pass/fail conclusions.

Common Testing Mistakes to Avoid

Data Sources: DIN 44080:1980 Clause 6; IEC 60738-1:2022 Annex C; Focusens Field Service Reports 2020-2025

Step 1: Cold Resistance Verification (25°C Baseline)

Equipment: Low-current ohmmeter (<1mA test current, ≤0.1Ω resolution), insulated test clips, 25°C environment

Procedure:

1. Stabilize sensor 30 minutes at 25°C ±2°C
2. Clean lead wires with isopropanol (removes oxidation)
3. Set test voltage to ≤2.5V DC
4. Record resistance after 5-10 second stabilization

Acceptance Criteria:

• Single-element (DIN 44081): R₂₅ = 30-250Ω (check datasheet for specific grade)
• Triple-element (DIN 44082): R₂₅ = 90-750Ω (3× single value)
• Repeatability: Three readings within ±5%

Interpretation:

• R₂₅ < 10Ω → Sensor shorted (ceramic cracked)
• R₂₅ > 500Ω (single) / >1,500Ω (triple) → Poor contact or lead break
• Note: Normal R₂₅ does NOT confirm functionality. According to Focusens failure analysis (2020-2025), 23% of failed PTCs showed normal cold resistance but degraded Curie transitions. Steps 2-3 are mandatory.

Step 2: Trip Temperature Verification (TROT Confirmation)

Equipment: Temperature-controlled oil bath (silicone oil, 0-200°C), programmable controller (1°C/min ramp), low-current ohmmeter, Type K thermocouple (±0.5°C)

Procedure:

1. Immerse PTC in 25°C silicone oil (minimum 50mm depth)
2. Position thermocouple within 10mm of sensor body
3. Ramp temperature at 1°C/min (per DIN 44080:1980 Clause 6.3)
4. Record resistance every 1°C or continuously

Data Analysis:

• Identify T₁ (onset): Resistance crosses 550Ω
• Identify T₂ (completion): Resistance exceeds 1,330Ω
• Calculate TROT: Midpoint between T₁ and T₂

Acceptance Criteria:

• TROT tolerance: Within ±5°C of rated value (e.g., 110°C sensor: 105-115°C acceptable)
• Temperature coefficient: dR/dT ≥15%/K in TROT±5K range
• Transition sharpness: T₂ - T₁ ≤15°C

Example (110°C sensor):

• At 105°C: R = 480Ω | At 110°C: R = 850Ω | At 115°C: R = 1,520Ω
• Temp coefficient: (1520-480)/480 × 1/(115-105) = 21.7%/K ✓ Pass (exceeds 15%/K)

Expert Tip: Focusens provides pre-calibrated R-T curves for each batch (±2°C TROT accuracy) via QR code on packaging. Field measurements can be compared against batch data to quickly identify outliers.

Step 3: Response Time Testing (Thermal Lag)

Equipment: Two baths (25°C and TROT+20°C), high-speed data logger (≥10Hz sampling)

Procedure (per DIN 44080:1980 Clause 6.4):

1. Stabilize sensor in 25°C bath for 15 minutes
2. Rapidly transfer to TROT+20°C bath (<2 seconds transfer time)
3. Record resistance continuously
4. Measure time until resistance exceeds 2,000Ω

Acceptance Criteria:

• Single-element (DIN 44081): Response time (ta) <3 seconds
• Triple-element (DIN 44082): Response time (ta) <5 seconds

Interpretation:

• ta > 10s → Unsuitable for motor protection (damage may occur before trip)
• ta = 3-5s → Acceptable for most applications
• ta < 2s → Premium grade (Focusens FS-PTC-FAST series with optimized thermal path)

Step 4: Insulation Integrity (Dielectric Strength)

Equipment: High-voltage tester (0-5kV AC), insulated fixture

Procedure (per DIN 44080:1980 Clause 6.5):

1. Short-circuit sensor leads together
2. Apply 2.5kV AC between shorted leads and sensor housing
3. Maintain voltage for 60 seconds
4. Monitor leakage current

Acceptance: No breakdown/arcing, leakage current <1mA. Test before and after temperature cycling to verify thermal stress hasn't compromised insulation.

For High-Voltage Motors (>690V): Specify 5kV-rated sensors. Focusens FS-PTC-HV series uses triple-insulated wire (silicone/fiberglass/silicone) and ceramic-sealed lead exit for 5kV withstand, suitable for medium-voltage motors up to 3.3kV line-to-line.

Key Takeaways: Essential DIN 44081/44082 Parameters

• DIN 44081 defines single-element PTC thermistors (30-250Ω at 25°C) for single-phase motors and DC applications
• DIN 44082 specifies triple-element sensors for three-phase motors with standardized color coding (60-180°C)
• Critical resistance thresholds: <550Ω at TROT-5K, >1,330Ω at TROT+5K, >4,000Ω at TROT+15K per DIN VDE V 0898-1-401:2020
• Test voltage limits: ≤2.5V DC for room temperature, ≤7.5V DC for TROT+15K to avoid self-heating errors
• Temperature coefficient requirement: Minimum 15%/K resistance change rate near Curie temperature (TROT±5K range)
• Current standard: Both DIN 44081/44082 superseded by DIN VDE V 0898-1-401:2020-03, equivalent to IEC 60738-1
• Color code examples: 110°C = Brown/Brown (AC compressors - #1 most specified); 155°C = Blue/Black (EV traction motors)
• Protection relay compatibility: KRIWAN INT69 (4.5kΩ trip), ABB CM-MSS.32 (2.83kΩ trip), max series 1-9 PTCs
• Common test mistake: Standard multimeters with >10mA test current cause 30-50Ω self-heating error; use <1mA low-current ohmmeter
• Selection rule: Choose DIN 44082 triple for motors >5kW or three-phase; DIN 44081 single sufficient for <1kW single-phase
• Response time standard: <3 seconds (single-element), <5 seconds (triple-element) per DIN 44080 thermal shock test
• Installation location: Single-element embeds in winding center; triple-element distributes across U/V/W phase end-turns

Frequently Asked Questions

Q1: Is DIN 44081 still valid in 2026?
A: DIN 44081:1980 and DIN 44082:1985 were superseded by DIN VDE V 0898-1-401:2020-03, but specifications remain unchanged. The newer standard consolidates both into a single document aligned with IEC 60738-1.

Q2: Can I use a regular multimeter to test PTC thermistors?
A: No. Standard multimeters use >10mA test current, causing self-heating that elevates resistance by 30-50Ω. Use a low-current (<1mA) ohmmeter or bridge meter per DIN 44080. Focusens FS-PTC-TESTER ($250) provides field-portable DIN-compliant testing.

Q3: What's the difference between TROT+5K and TROT+15K test points?
A: TROT+5K (>1,330Ω) verifies the sensor has entered high-resistance protection after crossing Curie temperature. TROT+15K (>4,000Ω) confirms material quality and long-term stability under sustained heat.

Q4: Why do 190-210°C PTC sensors lack standard color codes?
A: DIN 44082 only standardizes colors up to 180°C. Higher-temperature sensors use custom BaTiO₃ doping formulations that vary by manufacturer, making color standardization impractical. Recommended solution: hybrid monitoring with 155-160°C PTC + Type K thermocouple.

Q5: How many DIN 44082 triple sensors can I connect to one KRIWAN INT69 relay?
A: Up to 3 triple sets (9 individual PTCs) as long as total cold resistance stays <1.8kΩ. Always consult relay datasheet for exact limits based on your specific model.