1. Introduction

Electric Heat Tracing (EHT) systems are used to maintain the temperature of pipelines, tanks, and process equipment in industrial facilities such as:

• Oil & Gas
• Petrochemical plants
• Food processing industries
• Chemical plants

Heat tracing systems use electric heater cables installed along pipelines to maintain the required process temperature.

Modern EHT systems typically use the following control architecture:

RTD + PLC + PID Control + SSR

This configuration provides accurate temperature control, reliability, and long equipment lifetime.

Typical system structure:

RTD PT100
     │
Temperature Transmitter
     │
PLC / PID Controller
     │
Digital Output
     │
Solid State Relay (SSR)
     │
Heater Cable
     │
Pipeline Temperature

Functions of each component:

Component	Function
RTD PT100	Measures pipeline temperature
PLC	Processes control logic
PID	Calculates required heating output
SSR	Switches electrical power to the heater
Heater cable	Generates heat

What is an SSR?

A Solid State Relay (SSR) is an electronic switching device that replaces mechanical contacts with semiconductor components such as Triac or SCR.

Advantages of SSR:

• No mechanical contact wear
• Long service life
• Fast switching
• Silent operation
• Ideal for heater applications

Internal Structure of SSR

Control Signal
      │
Optocoupler (Electrical Isolation)
      │
Triac / SCR
      │
AC Load (Heater)

Most SSRs also include a protection circuit:

RC Snubber

This circuit protects against voltage spikes but also causes a small leakage current, which explains why voltage may still be measurable when the SSR is OFF.

4. PID Control in EHT Systems

PID is a control algorithm used to regulate temperature.

Basic PID equation:

Output = P + I + D

Where:

Term	Meaning
P	Proportional control
I	Integral control
D	Derivative control

The controller calculates the heating output required:

0 – 100 %

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Example Output Response

Process Temperature	Output
40°C	100%
45°C	70%
48°C	40%
50°C	10%
52°C	0%

5. How PID Controls the SSR

An SSR cannot reduce voltage level. It only has two states:

ON  = 220 VAC
OFF = 0 VAC

Therefore the PLC converts the PID output into a time-based switching control.

This method is known as:

Time Proportional Control
or Burst Firing Control

6. Burst Firing Control

Burst firing works by switching the heater ON and OFF in time intervals.

Example:

Cycle time = 20 seconds

Output = 50%

ON  10 seconds
OFF 10 seconds

Output = 25%

ON  5 seconds
OFF 15 seconds

Output = 75%

ON  15 seconds
OFF 5 seconds

The heater therefore receives average power proportional to the output percentage.

7. Voltage Waveform

Actual heater voltage:

220V ██████████        ██████████
     ██████████        ██████████0V            ██████████
              ██████████

When measured using a digital multimeter, the readings may fluctuate:

160 V
180 V
220 V

because the meter measures the average value of the switching waveform.

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8. Why EHT Uses Burst Firing

Heater cables behave as thermal loads.

Key characteristics:

• High thermal inertia
• Slow temperature response

Therefore:

ON 10 seconds
OFF 10 seconds

does not cause noticeable temperature fluctuation.

Advantages:

Advantage	Result
Low EMI	No interference with PLC or instrumentation
Low harmonic distortion	Stable electrical system
Longer SSR lifetime	Fewer switching cycles

9. Burst Firing vs Phase Angle Control

Feature	Burst Firing	Phase Angle
Control method	Full cycles ON/OFF	Partial sine wave control
EMI	Low	High
Harmonics	Low	High
Typical use	Industrial heaters	Light dimmers
Used in EHT	Yes	Rare

10. Common Issues in SSR + PID Systems

1. Voltage detected when SSR is OFF

Cause:

SSR leakage current

2. Unstable voltage readings

Cause:

Burst firing switching pattern

3. Frequent SSR failure

Common causes:

• Insufficient heat sink
• High operating temperature
• Heater inrush current

---

11. SSR Selection Guidelines for EHT

SSR current rating should typically be:

2 – 3 × heater current

Example:

Heater current = 12 A

Recommended SSR:

40 A SSR

Additional requirements:

Heat sink
Proper ventilation

12. Conclusion

The SSR + PID control system in Electric Heat Tracing operates according to the following sequence:

RTD → PLC → PID Calculation
           ↓
   Time Proportional Control
           ↓
          SSR
           ↓
      Heater Cable
           ↓
 Pipeline Temperature Stabilized

This control method provides:

• Accurate temperature control
• High reliability
• Long equipment lifetime
• Low electromagnetic interference

making it ideal for industrial heat tracing applications.