Automatic Transfer Switch (ATS) and Settings Calculator – IEEE, IEC

Automatic Transfer Switches (ATS) ensure seamless power transfer between sources, maintaining uninterrupted electrical supply. Calculating ATS settings accurately is critical for system reliability and safety.

This article explores ATS settings calculation based on IEEE and IEC standards, providing formulas, tables, and real-world examples. Learn how to optimize ATS configurations for diverse applications and standards compliance.

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• Calculate ATS transfer time for a 400 V, 100 A system under IEC standards.
• Determine the minimum ATS rating for a 250 kW generator with IEEE guidelines.
• Compute the ATS breaker trip settings for a 480 V, 200 A load per IEC 60947-6-1.
• Find the optimal ATS delay settings for a 3-phase, 600 V system following IEEE 446.

Common Values and Parameters for Automatic Transfer Switch (ATS) Settings – IEEE and IEC Standards

Essential Formulas for ATS Settings Calculation Based on IEEE and IEC Standards

Accurate ATS settings calculation requires understanding key electrical parameters and their relationships. Below are the fundamental formulas used in ATS design and settings, with detailed explanations.

1. Rated Current Calculation (Ir)

The rated current of the ATS must be equal to or greater than the maximum load current.

• Ir: Rated current of ATS (Amperes)
• I_load: Maximum continuous load current (Amperes)

2. Transfer Time (t_transfer)

Transfer time is the duration for the ATS to switch from the primary to the secondary source. It depends on mechanical and control system characteristics.

• t_delay: Voltage sensing delay (seconds)
• t_mechanical: Mechanical switching time (seconds)

3. Voltage Drop During Transfer (ΔV)

Voltage drop must be within acceptable limits to avoid equipment damage or malfunction.

• V_nominal: Nominal system voltage (Volts)
• V_transfer: Voltage during transfer (Volts)

4. Overcurrent Protection Setting (I_oc)

Overcurrent protection is set as a multiple of the rated current to prevent nuisance tripping.

• I_oc: Overcurrent setting (Amperes)
• k: Safety factor, typically 1.25 to 1.5
• Ir: Rated current (Amperes)

5. Short-Circuit Current Rating (I_sc)

The ATS must withstand the maximum prospective short-circuit current without damage.

• I_sc: ATS short-circuit current rating (Amperes)
• I_fault: Maximum prospective fault current at ATS location (Amperes)

6. Make-Before-Break Time (t_mbb)

To avoid power interruption, the ATS may use a make-before-break mechanism with a controlled overlap time.

• t_mbb: Time overlap during switching (seconds)

Real-World Application Examples of ATS Settings Calculation

Example 1: Calculating ATS Settings for a 400 V, 100 A Industrial Load (IEC 60947-6-1)

An industrial facility requires an ATS to switch between utility power and a backup generator. The load is rated at 400 V, 100 A. The maximum fault current at the ATS location is 25,000 A. The mechanical switching time is 0.2 seconds, and the voltage sensing delay is set to 1 second.

Step 1: Determine Rated Current (Ir)

The ATS rated current must be at least equal to the load current.

Select a standard ATS rating of 125 A (next standard size above 100 A).

Step 2: Calculate Transfer Time (t_transfer)

Step 3: Set Overcurrent Protection (I_oc)

Using a safety factor k = 1.3:

Set overcurrent protection at 160 A (nearest standard setting).

Step 4: Verify Short-Circuit Current Rating (I_sc)

The ATS must withstand 25,000 A fault current.

Select an ATS with a short-circuit rating of 30,000 A.

Step 5: Confirm Make-Before-Break Time (t_mbb)

Set t_mbb to 0.2 seconds to ensure no power interruption during transfer.

Example 2: ATS Settings for a 480 V, 200 A Commercial Building Load (IEEE Std 446)

A commercial building uses an ATS to switch between utility and diesel generator power. The load is 480 V, 200 A. The maximum fault current is 40,000 A. The voltage sensing delay is 0.8 seconds, and mechanical switching time is 0.15 seconds.

Step 1: Rated Current (Ir)

Select a 250 A rated ATS.

Step 2: Transfer Time (t_transfer)

Step 3: Overcurrent Setting (I_oc)

Using k = 1.4:

Step 4: Short-Circuit Rating (I_sc)

ATS must handle 40,000 A fault current.

Select an ATS rated for 50,000 A short-circuit current.

Step 5: Make-Before-Break Time (t_mbb)

Set t_mbb to 0.15 seconds for smooth transfer.

Additional Technical Considerations for ATS Settings

• Voltage Sensing Accuracy: IEEE Std 446 recommends voltage sensing accuracy within ±5% to avoid false transfers.
• Load Characteristics: Inductive loads require longer transfer times to prevent inrush current damage.
• Environmental Conditions: Temperature and humidity affect ATS mechanical components and should be considered in settings.
• Communication Interfaces: Modern ATS units support remote monitoring and control per IEC 61850 standards.
• Maintenance Intervals: Regular testing per IEEE Std 446 ensures ATS reliability and correct settings.

Authoritative External Resources

• IEEE Std 446-1995 – IEEE Recommended Practice for Emergency and Standby Power Systems
• IEC 60947-6-1 – Low-voltage switchgear and controlgear – Part 6-1: Multiple function equipment – Transfer switching equipment
• IEEE Xplore Digital Library – ATS Design and Settings Papers
• NFPA 70 – National Electrical Code (NEC) for ATS Installation