Screw Terminal (3-pos) Symbol
Definition: The Screw Terminal (3-pos) symbol represents a 3-position screw-clamp connector in circuit and wiring diagrams, denoting a three-conductor mechanical connection block where each conductor is independently secured by a set screw, as defined under IEC 60947-7-1 terminal block standards and ANSI Y32.2 / IEEE 315 connector conventions.
Also known as: 3-way screw terminal, 3-position terminal block, TB3, 3-pole screw connector, 3-wire terminal strip.
What the Screw Terminal (3-pos) symbol means
The 3-position screw terminal symbol identifies a three-conductor interface point at which field wiring enters or leaves a PCB, control panel, or junction box. The three positions P1, P2, and P3 represent three independent and electrically isolated connection nodes within a single mechanical housing.
In wiring diagrams this symbol commonly marks an L (line), N (neutral), and PE (protective earth) entry point for mains equipment, or a three-conductor signal interface such as an RTD's three-wire connection. The 3-pos format is the minimum block size for many 3-phase and protective-earth wiring schemes.
How to identify the Screw Terminal (3-pos) symbol
The 3-position screw terminal symbol appears as an elongated rectangular block with three equally-spaced internal divisions, each representing a screw clamp position. Three short wire-stub leads extend from the block (or from one side), and pins are labelled P1, P2, and P3 left-to-right. The block is slightly wider than the 2-pos symbol and uses the same rectangular outline style.
Function in a circuit
A 3-position screw terminal block provides three independently clamped electrical connection points in a single body, allowing three separate conductors to be connected and disconnected individually with a flat-blade or cross-head screwdriver. The three positions are mechanically held apart by insulating barriers, meeting creepage and clearance requirements defined in IEC 60947-7-1.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-7-1 governs low-voltage terminal blocks; IEC 60617 uses a rectangular connector block outline for terminal strip symbols. Positions are labelled 1–3 and the block designator prefix is TB. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 represents terminal blocks as rectangular connector bodies with pin stubs. Designator convention is TB or J followed by a reference number. |
| Key difference | IEC and ANSI graphical representations are functionally identical. IEC documentation typically specifies terminal block characteristics per IEC 60947-7-1 (industrial) or IEC 60998 (household); ANSI references UL 508 for industrial control equipment. |
Terminals / pins
| Pin | Name |
|---|---|
| p1 | P1 |
| p2 | P2 |
| p3 | P3 |
Typical values
Standard pitch: 2.54 mm, 3.5 mm, 5 mm, 5.08 mm, 7.5 mm. Current ratings: 10 A–32 A at 300 V–600 V (PCB mount); up to 125 A at 1000 V (DIN-rail industrial). Wire gauge: AWG 28–AWG 8 (0.08 mm²–10 mm²) depending on product series.
Where the Screw Terminal (3-pos) symbol is used
- Mains L/N/PE entry termination on power supplies, motor drives, and panel meters
- 3-wire RTD temperature sensor connections (Ref+, RTD+, RTD-) eliminating lead resistance errors
- 3-phase signal monitoring: sampling one phase line, neutral, and ground for meter input
- Control panel field wiring for 3-wire proximity sensors (Brown VCC, Blue GND, Black Signal)
- AC motor single-phase connections: line, neutral, and protective earth to the motor terminal box
- DIN-rail terminal strip rows in industrial control panels for organised field wiring
Example
In an industrial panel meter wiring diagram, TB1 is drawn as a 3-pos screw terminal: P1 accepts the 230 V AC line conductor, P2 accepts the neutral, and P3 is bonded to the protective earth bar, completing the IEC 60364 mains connection to the meter's input terminals.
Key facts
- The 3-position screw terminal symbol designator is TB (terminal block) followed by a reference number; pins are individually identified as P1, P2, and P3 in the schematic netlist.
- IEC 60947-7-1 is the primary international standard governing the electrical and mechanical requirements of low-voltage terminal blocks including 3-position units.
- The 3-pos screw terminal is the minimum size for a line/neutral/earth (L/N/PE) mains termination compliant with IEC 60364 and EN 60439 wiring practice.
- A 3-wire RTD connection requires exactly three terminals: one reference, one supply, and one signal; the 3-pos screw terminal is the standard field interface for this sensor type.
- Creepage distances between adjacent positions are specified in IEC 60947-7-1 and vary with pollution degree and rated insulation voltage — this determines the minimum physical pitch for a given voltage class.
- Phoenix Contact, Weidmuller, and WAGO are the three dominant manufacturers of DIN-rail 3-position terminal blocks; all use the same TB schematic symbol regardless of internal clamping mechanism.
- PCB-mount 3-pos screw terminals at 5.08 mm pitch are mechanically compatible with 0.1-inch header grid designs, allowing hybrid connector layouts on the same board.
Frequently asked questions
What does the 3-position screw terminal symbol look like?
The 3-pos screw terminal symbol is a rectangular block with three equal internal divisions (representing the three screw positions), three short wire leads exiting the block, and pin labels P1, P2, and P3. It looks like a slightly wider version of the 2-pos symbol with an extra position added.
What does the screw terminal 3-pos symbol mean in a wiring diagram?
It marks a physical 3-conductor connection block where field wiring attaches to a circuit board or control panel. Each of the three positions is an independent, removable electrical connection secured by a screw clamp.
What standard governs 3-position screw terminal blocks?
IEC 60947-7-1 covers low-voltage terminal blocks for industrial use, specifying current ratings, voltage ratings, creepage distances, and mechanical endurance. ANSI/UL 508 covers equivalent requirements for the North American market.
What is the designator for a 3-position terminal block?
The standard designator is TB (terminal block) followed by a sequential number, for example TB1 or TB3. In older ANSI drawings the designator J (jack/connector) may be used instead of TB.
Why is a 3-pos screw terminal used instead of a 2-pos for mains wiring?
Mains equipment requires connections for Line (L), Neutral (N), and Protective Earth (PE) — three separate conductors. A 3-pos terminal block provides all three in one housing, keeping the wiring compact, organised, and code-compliant per IEC 60364.
What is the difference between a 3-pos screw terminal and a 3-pos spring-clamp terminal in a schematic?
In a schematic diagram both types use the same TB rectangular block symbol. The distinction between screw-clamp and spring-clamp (WAGO style) is a physical manufacturing difference that does not change the schematic representation or the electrical function.
Can a 3-position screw terminal handle 3-phase power?
A 3-pos screw terminal block can carry three-phase conductors only if each position is rated for the full line voltage and current. For 3-phase 400 V industrial use, dedicated 3-phase terminal blocks rated to IEC 60947-7-1 at 690 V are typically used.
Place the Screw Terminal (3-pos) symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.