1. What Exactly Is a Primary Distribution Box? Where Does It Sit in the Three-Tier Distribution System? (JGJ 46 Three-Tier Distribution System)
JGJ 46-2005requires construction sites to usethree-tier distribution, two-tier protection: primary (main distribution box) → secondary (distribution box) → tertiary (switch box). The primary box is thefirst distribution point after the transformer low-voltage side, handling four functions: main isolation, main ground fault protection, main metering, and branch outgoing circuits. It is not just any large enclosure.
| Tier | Location | Primary Function | Ground Fault Protection Configuration |
|---|---|---|---|
| Primary — Main Distribution Box | Transformer low-voltage outgoing side | Main isolation + main metering + branch outgoing circuits | Main ground fault protection ≥ 300mA |
| Secondary — Distribution Box | Downstream of primary / equipment area | Branch isolation + branch outgoing circuits | Branch ground fault protection 100–300mA |
| Tertiary — Switch Box | End of single equipment | Final-stage isolation + final-stage ground fault protection | Final-stage RCD ≤ 30mA |
2. Standard configuration checklist for a primary distribution box (6 mandatory components)
Inside the primary boxAt least 6 components, one missing and it fails inspection:
- Incoming line isolation switch(QS): knife switch or isolation switch fuse group,with a visible break point, visible disconnection during maintenance.
- Main residual current device(RCD): rated residual operating current ≥ 300mA(JGJ 46 Clause 8.2.10), time-delay type 0.2s, for line protection not personnel protection.
- Main circuit breaker(QF): molded case circuit breaker MCCB, setting current selected according to transformer capacity or incoming cable ampacity.
- Ammeter/voltmeter + kWh meter: metering circuit, voltmeter with selector switch for three-phase measurement, ammeter with current transformers.
- Branch outgoing circuit breakers: 3–6 outgoing circuits, each with a branch isolation switch + branch RCD.
- N busbar + PE busbar: neutral busbar and protective busbarmust be separate, PE busbar with repeated grounding (grounding resistance ≤ 4Ω).
3. How are incoming/metering/outgoing arranged? How to read a single-line diagram (5-step diagram reading)
When you receive the single-line diagram for a primary box, read from left to right in5 steps:
- Check the incoming line: incoming specifications (cable cross-section + ampacity), incoming method (top entry/bottom entry), whether an incoming isolation switch is provided.
- Check the main switch: main circuit breaker model + setting current In + breaking capacity Icu (≥ 35kA).
- Check metering: current transformer ratio (e.g. 600/5), voltage transformer (e.g. 380/100, only with a PT cabinet), kWh meter.
- Check outgoing circuits: number of branches, each branch circuit breaker model + setting + outgoing cross-section + destination (which secondary box).
- Check protection: main RCD parameters, branch RCD parameters, PE busbar grounding.
4. Common configuration errors in primary distribution boxes (4 items to verify at acceptance)
① Using a 30mA residual current device on the main incoming breaker for personnel protection → Wrong, the main residual current device on the primary distribution box must be ≥ 300mA for circuit protection; only the final stage uses 30mA; ② Shorting the N bar to the PE bar → Wrong, they must be kept separate, with the PE bar grounded independently; ③ Feeding equipment directly from a final-stage socket inside the primary distribution box → Wrong, supply must go through the secondary and tertiary distribution stages, no skipping levels; ④ Incoming isolating switch without a visible break point → Wrong, a visible disconnection is required.
5. Primary Distribution Box Selection: How to Calculate Capacity (Incoming cable → main breaker → branch circuits)
Three steps for selection:calculate incoming current → size the main breaker → size the branch circuits. For a 500kVA transformer, the low-voltage side rated current is about 720A; select 2×(3×185+1×95) copper-core cables in parallel for the incoming line, an MCCB 800A for the main breaker, and 4–6 branch circuits at 250A each. Zhejiang Golden Unicorn builds to drawing,busbar ≤ 2000A, voltage 380/400V, up to 8 units in parallel per section; beyond that, use sectionalized busbars.
FAQ FAQ
Does a primary distribution box require a residual current device?
Yes, it is required. JGJ 46 Clause 8.2.10: the main distribution box shall be fitted with a main residual current device, with a rated residual operating current of not less than 300mA, time-delayed type. Residual current devices in final-stage switch boxes shall be ≤ 30mA for personnel protection.
Can the primary distribution box and secondary distribution box be combined?
No. Three-stage distribution is mandatory: primary (main), secondary (branch), and tertiary (final) must be physically separate, each with its own enclosure; they cannot be combined to save cost.
What is the grounding resistance requirement for a primary distribution box?
PE bar with repeated grounding, grounding resistance ≤ 4Ω(JGJ 46 Clause 5.3.2). If this cannot be met, add a resistance-reducing agent or additional ground electrodes.
Can a primary distribution box be installed outdoors?
Yes, but the ingress protection (IP) rating ≥ IP65, with a rain hood, bottom at least 1.2m above ground, and it must not sit directly on the ground.
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