Auxiliary distribution powers the protection and control functions that make the MV assembly operable and safe. Source architecture, protective-device selectivity and loss-of-supply monitoring must be designed before individual loads are connected.
Learning objectives
Separate trip, close, motor, heater and electronic loads, allocate protective devices and preserve essential functions after a single fault.
Core engineering principles
Loads have different criticality and duty
Trip coils are short-time safety loads; relay electronics are continuous; spring motors are intermittent; heaters are thermal loads. Combining them under one device can create unnecessary common failure.
DC systems require polarity and earth-fault philosophy
Ungrounded, high-resistance or grounded DC networks use different monitoring and isolation practices. Both poles may require protection and isolation according to the system design.
Selectivity limits outage scope
Branch MCBs or fuses should clear local faults before upstream devices where achievable. DC time constants and device characteristics differ from AC behaviour.
Redundancy must avoid hidden common points
Dual batteries are ineffective if joined through one terminal, test switch or trip output. Separation should continue to the intended redundant loads.
Loss of supply must be actionable
Each critical branch needs suitable healthy indication or alarm. Alarm grouping should identify the failed function, not merely “DC fault.”
Engineering application method
- Step 1: Create a load schedule with continuous, peak and simultaneous demand.
- Step 2: Classify essential and non-essential branches.
- Step 3: Select sources, protective devices and conductor sizes.
- Step 4: Perform DC voltage-drop and selectivity assessment.
- Step 5: Define isolation, monitoring and FAT tests for each branch.
Practical example
Placing relay, trip coil and spring motor on one MCB can cause a motor fault to remove protection and tripping. Separate coordinated branches reduce common consequence.
Common mistakes
- Sizing only from total steady watts.
- Using AC MCB data for DC without rating check.
- Combining redundant supplies on one terminal.
- Omitting branch healthy alarms.
- Ignoring coil and motor peak current.
Design and review checklist
- Are loads classified by duty and criticality?
- Is DC voltage drop acceptable?
- Are protective devices DC-rated and selective?
- Are redundant paths separated?
- Can each branch be isolated and supervised?
Standards basis and official sources
- IEC 61439-1:2020 — General rules for low-voltage switchgear and controlgear assemblies where applicable.
- IEC 62271-200:2021 + AMD1:2024 — AC metal-enclosed switchgear assemblies above 1 kV and up to 52 kV.
- IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.