1. The 4 Main Functions of a Terminal Box (Overview)
- Centralized transfer: Multiple control cables are terminated and cross-connected inside the terminal box, making management and maintenance easier.
- Signal isolation: Power and low-level signal circuits are routed through separate terminal boxes to reduce electromagnetic interference.
- Intrinsic safety isolation: Signals from hazardous areas pass through zener safety barriers before entering the terminal box.
- Easy maintenance: When a fault occurs, you can measure signals, disconnect wires, and troubleshoot inside the terminal box without touching the distribution box.
2. Why Does a Control Loop Need Its Own Terminal Box? (3 Reasons)
There are three reasons a control loop runs through its own terminal box instead of terminating in the distribution box:
| Why | What it should show |
|---|---|
| Function separation | The distribution box handles power distribution; the terminal box handles signal transfer. Neither interferes with the other. |
| Maintenance safety | The distribution box carries high voltage; the terminal box carries only control signals, so it can be safely isolated during maintenance. |
| Interference protection | Power cables and control signal cables are routed separately to prevent electromagnetic interference from corrupting signals. |
3. The Role of Terminal Boxes in DCS/PLC Systems (Signal Aggregation)
In DCS/PLC systems, signals from field sensors and actuatorsFirst, all field wiring is aggregated in the terminal box, then routed to the control room through a single multi-core cable. Benefits: (1) fewer cables entering the control room; (2) field maintenance is performed at the terminal box, with no need to enter the control room; (3) signal isolators / safety barriers can be installed inside the terminal box.
4. Terminal Box Selection: By Circuit Count (4 Tiers)
| Control circuits | Terminal block rows | Enclosure size | Terminal type |
|---|---|---|---|
| ≤ 20 circuits | 2 rows | 300×400×150 | UK-2.5 |
| 20–40 circuits | 4 rows | 400×500×200 | UK-2.5 |
| 40–60 circuits | 6 rows | 600×600×200 | UK-2.5 |
| 60–80 circuits | 8 rows | 800×800×250 | UK-2.5 |
5. Control Circuit Wiring Rules (3 Items)
GB 50054 Control circuit wiring requirements: (1) Route power and control cables in separate conduits, never in the same conduit; (2) control conductor cross-section ≥ 0.75 mm² (low voltage) or ≥ 1.5 mm² (power control); (3) inside the terminal box, keep power and low-voltage terminal blocks inseparate zones, with an insulating partition between them.
FAQ FAQ
Why can't control circuits be wired into a distribution box?
A distribution box handles power distribution (energized at high voltage), and routing control circuits through itmakes maintenance unsafe and exposes them to heavy electromagnetic interference. A dedicated terminal box provides safe isolation.
Can switches be installed inside a terminal box?
A standard terminal box carries terminal blocks only, no switches. If switching is required, use a distribution box or control box.
Why do DCS signals go to the terminal box first?
Signals are firstaggregatedat the terminal box, then sent to the control room as a group — fewer cables, easier field maintenance, and space inside the box for isolators / safety barriers.
Can a signal terminal box and a power terminal box share one enclosure?
Not recommended. Power circuits interfere with low-voltage signals, so they should be run in separate conduits and housed in separate boxes. If they must share an enclosure, add a metal partition.
Related pages:Technical notes · Products · Industries · Resource center · FAQ · Get a quote