1. Introduction

In Electric Heat Tracing (EHT) systems, heater cables are used to maintain the temperature of pipelines, vessels, and process equipment.
When a heater circuit is energized suddenly at full power, the inrush current and rapid temperature rise may cause:

• Electrical stress on the heater cable
• Sudden load on the power system
• Reduced lifetime of SSR and circuit components
• Thermal shock to the pipeline

To prevent these issues, many modern EHT control panels implement a Soft Start function using Solid State Relays (SSR) and controller logic.

Soft start gradually increases the heating power instead of applying full power instantly.

A typical EHT control architecture includes:

Component	Function
RTD (PT100)	Measures pipeline temperature
PLC / Controller	Executes control logic
PID Algorithm	Calculates required heating output
SSR	Switches AC power to heater
Heater Cable	Generates heat along pipeline

Control sequence:

RTD → Controller → PID Calculation → SSR Switching → Heater Cable → Pipeline Temperature

2. What is Soft Start in EHT?

Soft start is a control strategy that gradually increases heater power when the system starts.

Instead of applying:

0% → 100% instantly

the system ramps power slowly:

0% → 20% → 40% → 60% → 80% → 100%

This reduces electrical and thermal stress.

3. Soft Start Principle with SSR

Because SSR is a switching device, it cannot directly reduce voltage.

SSR has only two states:

ON = Full AC Voltage (220/230 VAC)
OFF = 0 V

Therefore, soft start is implemented using time-proportional control (burst firing).

Example with a 20-second control cycle:

Step	SSR Operation	Effective Power
Start	ON 2 s / OFF 18 s	10%
Step 2	ON 5 s / OFF 15 s	25%
Step 3	ON 10 s / OFF 10 s	50%
Step 4	ON 15 s / OFF 5 s	75%
Normal	ON continuously	100%

The heater power increases gradually.

4. Soft Start Temperature Response

Soft start results in a smooth temperature rise.

Without soft start:

• Temperature rises rapidly
• High electrical current occurs at startup

With soft start:

• Temperature increases gradually
• System stability improves

5. Advantages of Soft Start in EHT

Advantage	Description
Reduced Inrush Current	Prevents sudden current spikes
Extended Heater Lifetime	Avoids thermal shock
Protection for SSR	Lower switching stress
Stable Power System	Prevents voltage dip
Improved Control Stability	Smooth temperature ramp

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6. Example Soft Start Logic in PLC

Typical startup sequence:

System Start
      ↓
Check RTD signal
      ↓
Enable Soft Start
      ↓
Increase Output gradually
      ↓
Reach Normal PID Control

Example ramp table:

Time	Output
0 min	10%
2 min	30%
5 min	60%
10 min	100%

After reaching full output, the system switches to normal PID temperature control.

7. Why Soft Start is Important in Heat Tracing

Heat tracing cables can have significant cold-start current when energized.

Particularly:

• Self-regulating heater cables
• Long heater circuits
• Low ambient temperature conditions

Soft start helps prevent:

• Circuit breaker trips
• Excessive cable heating
• Power system disturbance

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8. SSR Soft Start vs Contactor Start

Feature	SSR Soft Start	Contactor Start
Startup stress	Low	High
Power control	Gradual	Instant
Temperature control	Smooth	Step change
Equipment lifetime	Longer	Shorter
Electrical noise	Low	Medium

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9. Conclusion

Soft start control using SSR and PID logic is widely used in modern Electric Heat Tracing control panels.

The method works by:

Gradually increasing SSR duty cycle
→ increasing heater power
→ preventing thermal and electrical stress

Benefits include:

• Improved heater reliability
• Reduced electrical disturbances
• Better temperature control stability

For large industrial heat tracing systems, soft start is considered an essential feature for safe and reliable operation.