Terminal Block Symbol
Definition: The Terminal Block symbol represents a modular, insulated wire-termination device that provides a mechanically secure and electrically reliable connection point between wires or conductors, typically mounted on a DIN rail inside a control panel enclosure, featuring a Left and Right terminal as a through-connection point; it is standardised in IEC 60947-7-1 (terminal blocks for copper conductors) and represented per IEC 60617 and ANSI Y32.2 / IEEE 315 schematic notation.
Also known as: TB, terminal strip, screw terminal, DIN rail terminal, connection block, wiring terminal.
What the Terminal Block symbol means
The Terminal Block symbol denotes a connection point where conductors from different parts of a circuit are securely joined and individually accessible. Unlike a wire splice, a terminal block maintains each conductor as a discrete, numbered, and identifiable connection that can be inspected, tested, and replaced without disturbing other connections. Terminal blocks are used at the boundaries between sections of a control system — for example, between field wiring (sensors, actuators) and panel wiring (PLC, relays).
In wiring and control-panel diagrams, terminal block symbols appear in terminal strips or terminal block assemblies labelled with terminal numbers (TB1, TB2, etc.). The symbol signals that a formal, inspectable connection point exists at that location, and the terminal number provides a traceable reference for maintenance and troubleshooting.
How to identify the Terminal Block symbol
The Terminal Block symbol is drawn as a small rectangle with a Left terminal entering from the left and a Right terminal exiting from the right, representing the straight-through wire connection. Some representations show two small squares or rectangles side by side connected by a line, indicating the clamping mechanism on each side. The designator TB (with a terminal number) is placed above or beside the symbol. In terminal-strip diagrams, multiple terminal block symbols are shown in a horizontal row, each with unique number annotations.
Function in a circuit
A terminal block mechanically and electrically joins two conductors at a reliable, testable connection point. Its screw clamp, spring clamp, or push-in mechanism grips the wire without soldering, providing a gas-tight connection that resists vibration and corrosion. Terminal blocks allow panel wiring to be organised, numbered, and documented; they serve as test access points and enable quick disconnection of field wiring from panel equipment for maintenance.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-7-1 governs terminal blocks for copper conductors in low-voltage switchgear and controlgear. IEC 60947-7-2 covers terminal blocks for protective conductor (earth) circuits. IEC 60617 represents a terminal block as a rectangle with terminals on each side. DIN rail mounting is standardised by DIN EN 60715 (35 mm DIN rail). |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 represents a terminal block as a small rectangle or square with left and right connection points, labelled with the terminal designator (TB1, TB2, etc.). NEMA ICS 1 covers standards for industrial control equipment including terminal blocks used in panel wiring. |
| Key difference | IEC and ANSI use the same basic rectangle symbol for terminal blocks; the IEC symbol (per IEC 60947-7-1) emphasises the conductor cross-section range and clamping type in the bill of materials, while ANSI/IEEE practice focuses on the terminal designator and wiring number in the schematic. Both show a simple rectangular through-connection symbol. |
Terminals / pins
| Pin | Name |
|---|---|
| left | Left |
| right | Right |
Typical values
Conductor cross-section: 0.2 mm² to 95 mm² (product-dependent). Rated current: 10 A to 125 A. Rated voltage: 250 V to 1000 V AC/DC. Clamping types: screw clamp, spring/cage clamp, push-in (tool-free). DIN rail compatibility: 35 mm TS-35 rail per DIN EN 60715. Common pitch: 3.5 mm, 5 mm, 6 mm, 8 mm. Designator: TB.
Where the Terminal Block symbol is used
- Control panel field-wiring interface — connecting external sensor and actuator cables to internal panel wiring
- PLC I/O wiring — terminal blocks at each PLC input and output provide test points and replaceable field connections
- Motor control centres — terminal blocks terminate motor power and control cables at the panel boundary
- Building automation systems — terminal blocks connect field devices (thermostats, damper actuators, sensors) to the BAS controller
- Industrial machine control panels — providing numbered, documentable connection points for servo drives, safety relays, and I/O
- Electrical enclosures for HVAC equipment — separating compressor, fan, and control wiring at inspectable junction points
- Power distribution panels — terminal blocks for small-signal and control wiring where crimp or direct breaker connections are impractical
Example
In a PLC control panel wiring diagram, a row of Terminal Block symbols (TB1 through TB20) is drawn at the panel boundary between the field cable entry gland and the PLC input card. Each terminal block symbol shows the Left terminal receiving the field sensor wire and the Right terminal connecting to the PLC input terminal, providing a numbered test point for commissioning engineers to verify each input signal without disconnecting the PLC.
Key facts
- A Terminal Block (TB) is a modular, insulated through-connection device that provides a numbered, mechanical, and testable connection point between conductors; standardised by IEC 60947-7-1 for copper conductors.
- The symbol is a rectangle with a Left terminal on one side and a Right terminal on the other, representing the straight-through connection; designator is TB.
- Terminal blocks mount on 35 mm DIN rail (TS-35 per DIN EN 60715) inside control panel enclosures and are available with screw clamp, spring clamp, or push-in termination methods.
- Each terminal block provides an individual numbered connection point that can be tested, inspected, and rewired without disturbing adjacent terminals — essential for traceable industrial wiring.
- IEC 60947-7-1 specifies rated voltage (up to 1000 V), rated current (up to 125 A), and conductor cross-section range (0.2 to 95 mm²) for different terminal block types.
- Special-function terminal blocks include fused terminals (with integral fuse), disconnect terminals (with a lever to open the circuit), test terminals (with a test probe socket), and earth/ground terminals (with green/yellow housing).
- In control panel documentation, each terminal block is assigned a unique number in the terminal strip list, which cross-references the wiring diagrams, cable schedules, and test reports.
Diagrams that use this symbol
- 13 pin wiring diagram
- 4 pin wiring diagram
- junction box wiring diagram
- circuit block diagram
- wiring harness diagram
- 74ls47 pinout
- earphone connection diagram
- ev1 connector
Frequently asked questions
What does the terminal block symbol mean in a wiring diagram?
The Terminal Block symbol represents a modular, numbered wire-connection point where two conductors are mechanically joined in a testable, documentable interface. It indicates a formal connection boundary — typically between field wiring and panel equipment — where each wire is independently accessible for testing and maintenance without disturbing other connections.
What does the terminal block symbol look like?
The terminal block symbol is drawn as a small rectangle with a Left terminal on one side and a Right terminal on the other, representing the straight-through electrical connection. In terminal-strip diagrams, multiple terminal block symbols appear in a horizontal row, each labelled with a sequential terminal number (TB1, TB2, etc.). Some representations show the clamping mechanism as two small squares joined by a line.
What standard defines terminal blocks?
Terminal blocks for copper conductors are standardised by IEC 60947-7-1 (low-voltage switchgear and controlgear — terminal blocks for copper conductors). Earth terminal blocks are covered by IEC 60947-7-2. DIN rail mounting dimensions follow DIN EN 60715 (35 mm TS-35 rail). The schematic symbol follows IEC 60617 and ANSI Y32.2 / IEEE 315.
What types of terminal block clamping are there?
The three main clamping types are: screw clamp (conductor tightened by a screw — most common, reliable, requires a screwdriver), spring/cage clamp (conductor inserted and gripped by a spring — vibration-resistant, tool-free release available), and push-in/lever clamp (conductor inserted directly without tools for solid or ferrule-terminated wire — fastest termination). Spring and push-in types are increasingly preferred in automated assembly environments.
What is the designator letter for a terminal block in schematics?
The designator for a terminal block is TB (Terminal Block) followed by a number identifying the specific terminal position in the terminal strip (e.g. TB1, TB15, TB32). Some conventions use X or XT as the designator letter (IEC practice), with the terminal number following. The terminal number cross-references the cable schedule and commissioning documentation.
What is the difference between a terminal block and a wire nut?
A terminal block provides a numbered, individually-accessible, panel-mounted screw or spring connection that can be tested with a probe, rewired without cutting wire, and documented in a wiring schedule. A wire nut is an in-line splicing connector used inside junction boxes, providing no individual access, no numbering, and no test point. Terminal blocks are preferred in industrial and commercial panel wiring for traceability and maintainability.
What current and voltage ratings do terminal blocks have?
Terminal block ratings vary widely by size and design: small signal-wiring terminal blocks (for 0.5–4 mm² wire) are typically rated 10–25 A at 250–630 V AC. Medium terminal blocks (for 4–16 mm²) are rated 30–80 A at 600–1000 V. Large power terminal blocks (for 16–95 mm²) are rated up to 125 A. Specific ratings are defined by IEC 60947-7-1 and should be verified in the manufacturer's datasheet for each product.
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