1. What's on a Distribution Box System Diagram? (4 Categories of Elements)
On a system diagram,4 categories of elementsmust all be present — if any one is missing, the diagram is non-compliant:
| Element | Symbol Example | Function |
|---|---|---|
| Incoming/Outgoing Lines | L1 L2 L3 N PE | Mark phase sequence and conductors |
| Switching Devices | QS QF KM FU | Isolating switch / circuit breaker / contactor / fuse |
| Measurement | TA PA PV PJ | Current transformer / ammeter / voltmeter / kWh meter |
| Busbar | WB | Main busbar and neutral busbar |
2. Reading the System Diagram in 5 Steps (Left to Right)
- Step 1: Check the incoming line: where the power comes from (transformer / upstream distribution cabinet), cable specification, and whether it enters from the top or bottom.
- Step 2: Check the main breaker: Isolating switch and circuit breaker model, current setting, breaking capacity.
- Third: check the busbars: Main busbar spec (e.g., 100×10 copper bar), whether it is sectioned, and whether the N bar and PE bar are separate.
- Fourth: check the outgoing circuits: Number of branches, breaker model and destination for each circuit, and whether ground fault protection is included.
- Fifth: check metering and protection: CT ratio, meter configuration, surge protective device (SPD).
3. Quick reference: common symbols in system diagrams (12 high-frequency items)
| Symbol | Meaning | Symbol | Meaning |
|---|---|---|---|
QS | Isolating switch | QF | Circuit breaker |
KM | AC contactor | FU | Fuse |
TA | Current transformer | TV | Voltage transformer |
PA | Ammeter | PV | Voltmeter |
PJ | Electricity meter | SPD | Surge protective device |
WB | Busbar | PE | Protective ground bar |
4. The 3 most common mistakes when reading diagrams (required reading for beginners)
① Treating a system diagram as a wiring diagram— A system diagram shows topology only; it does not mark specific terminal numbers. For wiring, refer to the secondary wiring diagram and terminal block diagram; ② Missing the CT ratio— The ammeter reading must be multiplied by the ratio to get the actual current (e.g., with a 600/5 CT, a 3A reading means 360A actual); ③ Ignoring the PE bar— In the system diagram the PE bar is drawn separately from the N bar. Combining them is a safety hazard.
5. After receiving a system diagram, how do you verify it? (3-step self-check)
3-step self-check: ① Verify the incoming breaker setting ≤ incoming cable ampacity; ② verify each outgoing breaker setting ≤ outgoing cable ampacity; ③ verify the main ground fault protection ≥ 300mA and the final-stage ground fault protection ≤ 30mA. If you are not sure how to verify it, send us the diagram for a free review.
FAQ FAQ
What is the difference between a system diagram and a wiring diagram?
A system diagram showstopology(what connects to what); a wiring diagram showsDetailed wiring connections(which wire goes to which terminal). Quotes are based on the system diagram; production is based on the wiring diagram.
What if I can't read the system diagram?
Read from the incoming supply left to right: find the main breaker first, then the branch circuits. For any symbol you don't recognize, check the GB/T 6988 symbol table. You can also send it to us for a free review.
What does the setting current on the system diagram mean?
The setting current is the trip currentset on the circuit breaker; the breaker trips when current exceeds this value. When selecting, keep the setting ≤ cable ampacity — otherwise the cable will burn before the breaker trips.
Must the system diagram be delivered with the cabinet?
Yes, it is required. GB/T 7251.1 requires that assembled equipment ship with its documentation, including the single-line diagram, layout drawing and terminal block diagram.
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