Problem 14 main circuit — trace the two motor branches
This lesson teaches how to trace the supply to two separate motor output terminal blocks, using problem 14 in Q-Net's 2025-08-04 edition. MC1 controls M1 and MC2 controls M2. This is not one motor reversing direction; the earlier reversing description has been corrected.
Review MCCB/fuses, contactors and EOCR first. The exam work ends at the specified motor terminal blocks; the site's motor icons are learning aids for observing an output.
Separate three paths first
- Power: three phases pass through protection and the MC main contacts to the motor outputs.
- Control supply: L1 and L3 branch after the breaker and pass through fuses. L3 is not neutral N.
- Protective earth: PE links the designated earth terminals without passing through switching contacts.
Inspect the connections step by step
- Input terminal block: match the diagram's L1/L2/L3/PE order and orientation. Read labels rather than guessing from position.
- Breaker paths: in the site model, trace 1–4, 2–5 and 3–6 separately. Use the actual component diagram if its numbering differs.
- EOCR passage: the model's 1–7, 2–8 and 3–9 paths carry the corresponding phases. These are not its overload output contacts.
- Split to two contactors: each EOCR output phase feeds the corresponding input on both MCs. Do not cross phases or feed the second motor from the first motor's output.
- Output destinations: MC1 feeds M1's terminal block and MC2 feeds M2's. Tracing one phase at a time helps reveal accidentally joined nodes.
- Control branch: follow the protected L1/L3 nodes to the fuse inputs. Routing through another terminal on the same node must still respect terminal capacity and ratings.
- Earth path: trace input PE to each specified output PE. Do not use PE as L3 or as a coil return.
Connectivity matters more than wire count
A sample wire count is a construction aid, not proof of a correct circuit. Equivalent nodes can be routed in different orders. The referenced exam specifies brown/black/grey for the phases, yellow for the auxiliary circuit and green-yellow for PE. If three conductors meet at one terminal, investigate a suitable connection on the same electrical node; see terminal wiring.
Expected states
| State | Main contacts | Expected result |
|---|---|---|
| Breaker OFF | Supply interrupted | No load supply |
| Breaker ON, coils OFF | Both MC paths open | Neither motor runs |
| MC1 only ON | MC1 path closed | Power to M1 branch |
| MC2 only ON | MC2 path closed | Power to M2 branch |
| Stop or trip removes coil supply | MC contacts open | Corresponding motor supply removed |
Fault-finding exercise
| Symptom | Suspect | Inspection path |
|---|---|---|
| Contactor operates but motor does not | Missing phase in power circuit | Supply → breaker → EOCR → MC → output TB |
| MC1 command affects M2 | Output branches swapped | Follow all three MC1 output destinations |
| No control response | Missing fused branch or return | Trace both L1 and L3 and both sides of the fuses |
Question: Should switching both MCs off also disconnect PE? Answer: No. The protective conductor is independent of motor commands; the main contacts switch the power conductors.
Practice path and references
Open the practical project list →. Sign-in may be required. Create a project for public problem 14 and work through placement and wiring. This link does not automatically load a completed solution. Trace the supply first, then use the control lesson's state table.
Reference: Q-Net public-problem notice, 2025-08-04 edition, problem 14: page 11-1 for supply/work scope, 11-2 to 11-4 for wiring instructions, 11-7 for the circuit and 11-9 for internal connections. Download the official original alongside this explanation. The learning model is not a field design or an official grading decision.