Fuse Terminal Block Symbol
Definition: The Fuse Terminal Block symbol represents a modular DIN-rail-mounted terminal block with an integrated replaceable fuse element, providing both circuit connection and overcurrent protection in a single wiring component, shown in electrical panel diagrams with an In (in) pin at the top and an Out (out) pin at the bottom.
Also known as: fuse terminal, fusable terminal block, fuse holder terminal, DIN fuse terminal, protected terminal block, terminal fuse block.
What the Fuse Terminal Block symbol means
The Fuse Terminal Block symbol denotes a DIN-rail-mounted wiring device that combines the wire-clamping function of a standard terminal block with an integrated fuse holder. Conductors are clamped at the In (in) and Out (out) screw or spring terminals, while the fuse cartridge (typically 5×20 mm or 6.3×32 mm glass or ceramic) sits in a holder between the two terminals, interrupting current if it exceeds the fuse's rated value.
Fuse terminal blocks are used in control panels, marshalling racks, and industrial cabinets wherever individual circuits branching from a main bus must be independently protected. The symbol appears in panel wiring diagrams and ladder diagrams to indicate both a mechanical connection point and a protection element, simplifying documentation by combining two functions into one symbol.
How to identify the Fuse Terminal Block symbol
The fuse terminal block symbol is drawn as a narrow rectangle or square (representing the terminal block body) with a small rectangle or dash inside the outline (representing the fuse element). The In (in) terminal emerges from the top of the symbol and the Out (out) terminal from the bottom, consistent with top-to-bottom current flow in panel wiring diagrams. The internal fuse element is drawn as a short bar or rectangle, differentiating it from a plain terminal block symbol.
Function in a circuit
The fuse terminal block performs two functions simultaneously: it mechanically clamps and electrically connects conductors on both sides (In and Out), and it limits the maximum current that can flow through the circuit branch by incorporating a fusible element. If circuit current exceeds the fuse rating, the fuse element melts and opens the circuit, protecting downstream wiring and equipment. The fuse can be replaced by opening the cover or extracting the fuse holder, restoring the circuit without removing any wiring.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-7-3 governs fuse terminal blocks (terminal blocks with integral fuse holders) and defines their test requirements, marking, and safety parameters. IEC 60269 governs the fuse links used in fuse terminal blocks. The schematic symbol for a fuse terminal follows IEC 60617-07 (fuse symbol: rectangle with leads) combined with the terminal block symbol. |
|---|---|
| ANSI/IEEE 315 | UL 508A (Industrial Control Panels) and UL 1953 (Fuse Holders) govern fuse terminal blocks used in North American control panels. The schematic symbol follows ANSI Y32.2 / IEEE 315 conventions, using a fuse symbol (S-curve or rectangle) with terminal connections. |
| Key difference | IEC 60617 represents the fuse as a filled rectangle within a rectangular outline (the cartridge fuse symbol). ANSI Y32.2 / IEEE 315 represents the fuse as an S-shaped or wavy line within a rectangular outline. The terminal block portion of the symbol looks identical under both standards. IEC fuse dimensions (5×20 mm, IEC 60127) differ from North American ATC/AGC types. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Fuse ratings: 0.063 A to 20 A (miniature cartridge, IEC 60127); voltage rating: 250 V AC or 500 V AC; fuse types: fast-acting (F), time-delay (T), super-fast (FF); cartridge sizes: 5×20 mm (IEC standard), 6.3×32 mm (North American); terminal wire gauge: AWG 24 to AWG 10 (0.14 mm² to 6 mm²); DIN rail: EN 50022 (TS 35).
Where the Fuse Terminal Block symbol is used
- Industrial control panels: individual fuse terminal blocks protect each 24 V DC sensor supply branch on a DIN rail
- Marshalling cabinets: fuse terminals protect field instrument cables entering a panel from external faults
- Motor control centres: fuse terminal blocks provide individual protection for each 120 V control circuit
- Building automation systems: fused terminal blocks protect low-voltage (24 V AC) control wiring for HVAC actuators
- PLC I/O wiring: a fuse terminal block on each output channel protects PLC output modules from field wiring shorts
- Solar and battery systems: fused terminals protect DC branch circuits in combiner boxes and battery distribution points
Example
In a PLC control panel, twelve fuse terminal blocks are mounted on a DIN rail between the 24 V DC supply bus and twelve individual sensor circuits. Each fuse terminal's In (in) terminal connects to the positive bus bar via a jumper bridge; the Out (out) terminal connects to the positive conductor of one sensor cable. A 0.5 A fast-acting fuse in each terminal block protects the sensor wiring, while the fuse terminal's spring-loaded cover allows fuse replacement without disturbing any wiring.
Key facts
- The Fuse Terminal Block symbol represents a DIN-rail-mounted terminal block with an integrated fuse holder, combining wire connection (In/Out) and overcurrent protection in one component per IEC 60947-7-3.
- The two terminals are In (in) at the top for the incoming line-side conductor and Out (out) at the bottom for the protected load-side conductor.
- IEC 60127 miniature cartridge fuses (5×20 mm) are the most common type used in fuse terminal blocks, with ratings from 63 mA to 20 A.
- Fuse terminal blocks allow individual circuit protection without a separate fuse holder, reducing panel space and simplifying wiring documentation.
- The IEC fuse symbol inside the terminal block outline is a small rectangle; the ANSI/IEEE symbol uses an S-curve or wavy line inside a rectangle.
- DIN-rail fuse terminal blocks typically accept EN 50022 (TS 35) 35 mm DIN rail and can be mechanically linked using jumper bridges for bus distribution.
- Fuse terminal blocks carry a dual designator: the terminal reference (X or XT followed by a number, e.g., XT1) and the fuse reference (FU followed by a number, e.g., FU1) in the same symbol in panel wiring documentation.
Frequently asked questions
What does the fuse terminal block symbol mean in a wiring diagram?
The fuse terminal block symbol represents a DIN-rail-mounted terminal block with an integrated fuse that both connects conductors and protects the circuit from overcurrent. It appears in panel wiring diagrams wherever individual circuit branches need both a wire connection point and overcurrent protection in a single modular component.
What does the fuse terminal block symbol look like?
The fuse terminal block symbol looks like a rectangular terminal body with a small rectangle or bar inside it representing the fuse element. The In (in) terminal connects at the top and the Out (out) terminal at the bottom, indicating top-to-bottom current flow through the protected connection.
What type of fuse is used in a fuse terminal block?
Most DIN-rail fuse terminal blocks use IEC 60127 miniature cartridge fuses in 5×20 mm or 5×25 mm sizes. Ratings range from 63 mA to 20 A in fast-acting (F), time-delay (T), and super-fast (FF) types. North American panels may use 6.3×32 mm or 1/4×1-1/4-inch AGC/AGX fuses depending on the terminal block manufacturer.
What is the difference between a fuse terminal block and a standard terminal block?
A standard terminal block only clamps and connects conductors electrically. A fuse terminal block adds an integral fuse element between the In and Out conductors, providing overcurrent protection for the downstream circuit without requiring a separate fuse holder. This reduces panel space and component count.
What standard governs fuse terminal blocks?
Fuse terminal blocks are governed by IEC 60947-7-3 (Low-voltage switchgear — terminal blocks with integral fuse holders). The fuse links used inside them are specified by IEC 60127 (miniature fuses) or IEC 60269 (low-voltage fuses). In North America, UL 1953 and UL 508A apply.
What is the designator letter for a fuse terminal block in a schematic?
In panel wiring documentation, fuse terminal blocks carry a dual designation: the terminal connection is referenced as XT (terminal block, fused) followed by a number (e.g., XT1), and the fuse element is referenced as FU followed by a number (e.g., FU1). Some documentation uses only the fuse designator F or FU when the terminal function is implied.
Can fuse terminal blocks be linked together with jumper bridges?
Yes. Most DIN-rail fuse terminal blocks include a comb-type jumper bridge connection point on the input (In) side, allowing multiple adjacent fuse terminals to share a common supply bus without individual wiring. The output (Out) terminal of each block remains separate to provide individual fused protection for each load circuit.
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