Secondary protection ensures safe operation of downstream circuits by using properly sized breakers, relays, and protective devices to prevent overloads, short circuits, and equipment damage.

Purpose of Secondary Protection

Secondary protection in a distribution box or transformer system is designed to:

• Protect downstream equipment such as panelboards, motors, and switchgear.
• Ensure safe operation during normal and fault conditions.
• Maintain system reliability and prevent cascading failures in the network .

Key Components

A distribution box typically includes:

• MCBs, RCDs, RCBOs, and fuses for overcurrent and earth fault protection.
• Busbars, neutral bars, and earth bars for proper current distribution.
• Surge protective devices to guard against voltage spikes .
• Relays for monitoring and control, such as ABB Relion REF601 or REX615, which provide protection, measurement, and communication capabilities .

Steps to Use Secondary Protection

1. Calculate Full Load Current (FLC)
   • For a three-phase transformer: FLC = kVA × 1000 / (√3 × Voltage)
   • For a single-phase transformer: FLC = kVA × 1000 / Voltage
   • This value is the reference for selecting breakers and relay settings .
2. Select Proper Breakers
   • Choose breakers rated at approximately 125% of the full load current to avoid nuisance tripping during overload or inrush conditions.
   • Ensure the breaker's breaking capacity matches the maximum short-circuit current, which depends on transformer impedance .
3. Configure Relays
   • Use protection relays to detect overcurrent, short circuits, or abnormal conditions.
   • Set instantaneous and time-delayed trip settings according to downstream load characteristics and coordination with upstream protection .
4. Ensure Redundancy and Segregation
   • For critical systems, implement Group A and Group B protection, where two independent protection systems can fully protect the same equipment.
   • Avoid sharing CT cores between protection groups to prevent mis-operation and maintain redundancy .
5. Install and Label
   • Properly wire the secondary protection devices, ensuring clear separation of circuits.
   • Label breakers, relays, and disconnects for easy identification and maintenance .
6. Test and Commission
   • Perform functional testing of breakers and relays.
   • Verify coordination with upstream and downstream protection to ensure selective tripping and system safety .

Practical Considerations

• For short secondary conductors (<25 ft), overcurrent protection may be provided at the panel rather than at the transformer secondary .
• Include spare capacity in panels and cable trays for future expansion.
• Use DC systems for relays and control to maintain protection during AC faults . By following these steps, secondary protection in a distribution box can be effectively used to safeguard equipment, maintain reliability, and comply with electrical standards.