Distribution Hierarchy: MDB, SDB & FDB

Architecture Overview

The 3-Tier Power Distribution Hierarchy

Modern facility electrical networks require a systematic three-tier distribution topology to ensure high power reliability, operational safety, and full fault discrimination. Power cascades progressively through Main Distribution Boards (MDB), Sub-Distribution Boards (SDB), down to Final Distribution Boards (FDB).

This architecture ensures that high prospective short-circuit currents are handled safely at primary cubicles, while intermediate and branch circuits benefit from targeted isolation, zone metering, and touch-safe maintenance.

Tier Architecture

The Three Distribution Tiers

Tier 1 Main Distribution Switchboard (MDB) Cubicle Assembly

Tier 1 • Primary Intake

Tier 1: Main Distribution Switchboards (MDB)

Primary intake from utility transformers or standby gensets (800 A – 6300 A). Fully design verified to IEC 61439-1 & 2 for 50°C ambient operation with Form 4b physical segregation and 50 kA / 3s to 100 kA / 1s ASTA fault ratings.

Intake800 A – 6300 A

SegregationForm 2b to Form 4b

Fault Level50 kA 3s • 100 kA 1s

Tier 2 Sub-Distribution Switchboard (SDB) Loadbank 250

Tier 2 • Intermediate Zone

Tier 2: Sub-Distribution Switchboards (SDB)

Floor or zone-level power splitting (125 A – 800 A). Wall-mounting and pedestal sheet steel assemblies providing 4 to 18 triple-pole outgoing ways, side cableways, and add-on DIN96 digital energy metering.

Capacity125 A – 800 A

SegregationForm 2 & Form 4

Fault Level25 kA – 50 kA / 1s

Tier 3 Final Distribution Board (FDB) LoadPro

Tier 3 • Final Branch

Tier 3: Final Distribution Boards (FDB)

End-point consumer units and row-type boards supplying lighting circuits, socket outlets, and small appliances (10 A – 250 A) with 30 mA RCBO/MCB shock protection and split-busbar configurations.

Capacity10 A – 250 A

Protection30 mA RCBOs / MCBs

Fault Level6 kA – 17 kA

Tier Comparison

Hierarchy Tier Specifications

Hierarchy Level	Role & Intake	Current Rating	Fault Withstand	Separation
Tier 1: MDB Main Distribution Switchboard	Primary intake from utility transformers or standby gensets. Feeds major plant and SDB risers.	800 A – 6300 A	50 kA – 100 kA / 1s (50 kA / 3s)	Form 2b to Form 4b (Type 2 to 7)
Tier 2: SDB Sub-Distribution Switchboard	Intermediate floor, departmental, or wing level. Isolates local zones and balances phases.	125 A – 800 A	25 kA – 50 kA / 1s	Form 2 standard, Form 4 available
Tier 3: FDB Final Distribution Board	End-point consumer unit feeding lighting, power sockets, HVAC terminal units and appliances.	10 A – 250 A	6 kA – 17 kA / 250ms	Form 1 to Form 2 (IP41 / IP54)

Tier 1 Details

Tier 1 — Main Distribution Boards (MDB)

Parameter	MDB Technical Specification
Standards & Verification	IEC / BS EN 61439-1 & 2, certified by ASTA / KEMA
Ambient Temperature	Fully design verified for operation at 50 °C ambient
Enclosure Construction	2.0 mm – 2.5 mm welded/bolted frame, 1.6 mm – 2.0 mm covers, RAL 7035 finish
Busbar System	4-pole with fully rated neutral, HDHC tin-plated copper (1250 A – 6300 A)
Incoming Devices	Withdrawable / Fixed 3-pole and 4-pole Air Circuit Breakers (ACB) up to 6300 A
Outgoing Devices	Fixed / Plug-in MCCBs up to 800 A, Fuse-Switch Combination Units (AC23A to 80 kA)
Power Monitoring	True RMS multi-function digital power analysers, THD monitoring, Modbus/Ethernet

Tier 2 Details

Tier 2 — Sub-Distribution Boards (SDB)

Parameter	SDB Technical Specification
Standards & Verification	IEC / BS EN 61439-2, ASTA certified
Mounting & Enclosure	Wall mounting and pedestal sheet steel assemblies (1.2 mm – 1.6 mm steel)
Incomer Capacities	250 A, 400 A, and 800 A MCCB or Switch Disconnector
Outgoing Ways	4 to 18 triple-pole outgoing ways accommodating MCCB frames up to 250 A
Modular Add-Ons	Cable spreader boxes, side cableways, metering door kits, and service centers
Surge Protection	Integrated Type 1 Class I and Type 2 Class II surge suppression kits

Tier 3 Details

Tier 3 — Final Distribution Boards (FDB)

Parameter	FDB Technical Specification
Standards & Verification	IEC / BS EN 61439-3, ASTA certified
Configurations	SP&N Consumer Units, TPN Vertical Boards (4–16 way), Horizontal Row Type DBs (1–6 rows)
Incomer Options	63 A – 250 A double-pole/four-pole Switch Disconnectors, RCCBs, or MCCBs
Outgoing Devices	1P, 2P, 3P MCBs (B, C, D curves 6 A – 63 A), 30 mA RCBOs and RCCBs
Busbar Construction	100 A – 250 A hard-drawn high-conductivity tin-plated copper (ETP Grade C101)

Engineering Benefits

Advantages of Coordinated Hierarchy

Total Discrimination: Downstream faults at lighting or socket outlets trip only the localized MCB/RCBO without knocking out entire floors.
Safety & Segregation: High energy busbars stay isolated in primary plant rooms under Form 4 barriers, while final user boards are touch-safe.
Scalable Expansion: Add-on cableways, spare outgoing ways, and modular enclosures allow plant expansion without redesigning main switchboards.
Granular Metering: Energy sub-metering at SDB and FDB levels enables ISO 50001 energy accounting and departmental billing.

Applications

Typical Applications

Multi-storey commercial offices Hospitals and critical healthcare wings Data centres and server halls Manufacturing and packaging plants Shopping malls and retail chains Educational campuses Hotels and resorts Residential apartment towers

FAQs

Directly cabling all circuits from an MDB causes massive cable congestion, huge voltage drop on long runs, and catastrophic single-point failure risks. Distributing via SDBs to localized FDBs reduces cable costs, improves voltage regulation, and allows isolated maintenance without facility-wide blackouts.

We perform a comprehensive discrimination and cascading study using manufacturer time-current curves. Incomers at MDB level use electronic trip units with short-time delay (Category B), intermediate SDB MCCBs operate faster, and final FDB MCBs/RCBOs clear instantaneous branch faults in under 20 milliseconds.

Yes. Our 250 A and 400 A SDB panel boards are purpose-designed for wall mounting with common 700 mm or 900 mm widths and slim depths, making them ideal for electrical risers and plant cupboards where floor space is limited.

Standard engineering practice at NPE provides a minimum of 20% to 25% spare unequipped ways and blanking covers on all SDB and FDB assemblies, allowing future circuit additions without replacing the busbar chassis or enclosure.