1. Where the Three-Tier Distribution Box Fits in the Three-Tier System (Final Stage · Equipment Connection)
Three-tier box = switch box,directly connected to a single piece of equipment, and it is the tier closest to personnel, so the RCD must be ≤ 30mA(personnel safety current). JGJ 46 Clause 8.2.10: the final-stage switch box must be equipped with an RCD, with rated residual operating current not exceeding 30mA and operating time ≤ 0.1s.
2. Three-Tier Box Standard Configuration Checklist (5 Required Components)
- Final-stage disconnect switch(QS): visible break point, physical disconnection during maintenance.
- Final-stage residual current device(RCD): ≤ 30mA, ≤ 0.1s,dedicated to personnel protection.
- Final-stage circuit breaker(QF): miniature circuit breaker MCB or molded case, selected by load current.
- Outgoing sockets/terminals: three-hole/four-hole sockets with PE, or wiring terminals.
- Enclosure grounding: PE terminal connected to the sheet metal enclosure, grounding resistance ≤ 4Ω.
3. How to Select the 30mA End-of-Line RCD (3 parameters)
| Parameter | Value | Why |
|---|---|---|
| Rated residual operating current IΔn | ≤ 30mA | Personnel protection current (30mA is the ventricular fibrillation threshold) |
| Rated residual non-operating current | ≤ 15mA | Prevents nuisance tripping from normal leakage current |
| Trip time | ≤ 0.1s | Fast disconnection limits electric shock energy |
4. Common Errors with Tertiary Distribution Boxes (4 red lines)
① One tertiary box feeding multiple pieces of equipment → Wrong, one machine, one disconnect, one RCD, one box — each piece of equipment gets its own switch box; ② 100mA RCD used → Wrong, end-of-line must be ≤ 30mA; ③ Three-hole socket with no PE connection → Wrong, enclosure becomes live with no protection; ④ Enclosure not grounded → Wrong, if the RCD fails the live enclosure can be fatal.
5. Tertiary Box Selection: By Load Current (4 load categories compared)
| Equipment type | Typical current | Circuit breaker | RCD |
|---|---|---|---|
| Handheld power tools | ≤ 16A | MCN C16 | 30mA/30ms |
| Small motors | 16–32A | MCN C32 | 30mA/0.1s |
| Welding machines | 32–63A | MCCB 63A | 30mA time-delayed |
| Tower cranes / hoists | 63–100A | MCCB 100A | 30mA + overload |
FAQ FAQ
Can a tertiary box use a 100mA RCD?
No. The end-of-line switch box RCD must be ≤ 30mA for personnel protection; 100mA is only for primary/secondary line protection.
What does "one machine, one disconnect" mean?
Every piece of equipment must have anindependent switch box, with its own disconnect switch and RCD inside — multiple pieces of equipment cannot share one tertiary box.
Can a tertiary box have sockets?
Yes, but they must be 3-hole/4-hole sockets with PE, and the socket circuit must have its own RCD.
Are there requirements for tertiary box enclosure material?
For outdoor use, ≥ IP65 stainless steel or powder-coated galvanized sheet; for indoor use, powder-coated steel sheet is acceptable. Enclosure thickness ≥ 1.5mm.
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