Limited RTD for Temperature Measurement and Limitation in Electric Heat Tracing Systems
In Electric Heat Tracing (EHT) systems, accurate temperature control alone is not sufficient.
Equally important is overtemperature protection, especially in industrial and hazardous environments.
This protection is provided by a critical device known as the Limited RTD.
What Is a Limited RTD?
A Limited RTD (Temperature Limiting RTD) is a resistance temperature detector used to monitor and limit the maximum allowable temperature of a pipe or equipment in an electric heat tracing system.
Unlike a control RTD, a limited RTD is not used for temperature regulation,
but for safety protection against overheating.
Limited RTD vs Control RTD
| Item | Control RTD | Limited RTD |
|---|---|---|
| Function | Temperature control | Temperature limitation |
| Role | Process control | Safety protection |
| Response to high temperature | Adjusts heater output | Trips / cuts power |
| Priority | Process | Safety |
Diagram: Electric Heat Tracing System with Limited RTD
Power Supply
│
▼
+——————+
| Controller / PLC |
+——————+
│
├── Control RTD (Temperature Control)
│
└── Limited RTD (High-Temperature Limit)
│
▼
+——————+
| Heater Cable |──────► Pipe / Equipment
+——————+
Diagram Explanation
- The control RTD maintains the required process temperature.
- The limited RTD monitors the maximum surface temperature.
- If the temperature exceeds the limit, the system trips the heater circuit.
Typical Installation Location of a Limited RTD
Insulation
┌─────────────────┐
│ Heater Cable │
│ ● Limited RTD │ ← Hot Spot Location
│ │
│ Pipe Surface │
└─────────────────┘
Common Locations
- Pipe surface
- Valves and flanges
- Heater overlap areas
- Known hot spots
Why Is Limited RTD Important?
1. Equipment Protection
- Prevents heater cable damage
- Protects thermal insulation
- Reduces thermal stress on piping
2. Hazardous Area Safety
- Ensures surface temperature remains within T-rating
- Prevents ignition risks
3. Standards Compliance
- IEEE 515
- IEC 60079-30-1
- NEC 427
Limited RTD Requirement by Heater Cable Type
| Heater Cable Type | Limited RTD Requirement |
|---|---|
| Self-Regulating | Recommended |
| Constant Wattage | Required |
| MI Cable | Highly required |
Conclusion
A limited RTD is a critical safety component in electric heat tracing systems.
A properly designed EHT system must not only provide sufficient heat,
but must also ensure that temperatures never exceed safe limits.