Which Terminal Block Do You Actually Need?
In this guide
What is a Terminal Block?
A terminal block (also called a terminal connector or rail terminal) is a modular insulated block that secures two or more wires together. Instead of twisting wires and wrapping them in tape — a method frowned upon in any professional or permanent installation — a terminal block gives each wire its own dedicated, inspectable, re-openable connection point.
Terminal blocks snap onto a standard DIN 35 mm mounting rail inside a distribution board, control panel, or enclosure. They can be placed side by side to build a tidy "terminal strip" of any length. ONKA terminals from pelek.uk are designed and manufactured in Turkey to European standards, and every product is CE marked and rated to 1000 V for maximum versatility.
Why use terminal blocks at all? Terminals make your wiring safe to inspect, easy to fault-find, and simple to modify later. They are required by IET Wiring Regulations (BS 7671) in most permanent fixed installations. A terminal block cannot work loose on its own the way a wire nut can — making them the professional's choice in every sector from home automation to heavy industry.
Screw-Type Terminal Blocks
The screw terminal is the most traditional and most widely used type worldwide. A steel or brass clamping screw bears directly on the wire (or on a clamping plate above the wire) and is tightened with a flat-blade or Pozidriv screwdriver. The connection holds as long as the screw remains tight.
Who should use screw terminals? They are perfect for anyone who already owns a screwdriver and wants a proven, universally understood connection. Electricians, panel builders, and confident DIYers all reach for screw terminals first. They accept solid wire, stranded wire with or without a ferrule, and are easy to re-terminate in the field.
ONKA Screw Terminal Blocks — up to 35 mm².
The blue, grey or camouflage screw terminal is the workhorse of DIN rail wiring. It covers the overwhelming majority of domestic and light commercial circuits: lighting, sockets, 3-phase motor controls (up to ~125 A), sensor wiring, PLC I/O, and junction boxes.
The transparent housing lets you check wire seating without a tool. The cage clamp design protects stranded wire from damage during tightening. Available for 1.5 mm² , 2.5 mm², 4mm², 6mm², 10mm², 16mm², 25mm² and 35 mm² wires.
Shop Screw Terminals →Pro tip: Always tighten screw terminals to the manufacturer's specified torque. Under-tightening causes heat and fire risk; over-tightening can cut stranded wire strands. Check the product page on pelek.uk for torque specs on each size.
Spring Clamp (Push-In) Terminal Blocks
Spring clamp terminals replace the tightening screw with a stainless-steel spring that holds the wire under constant, self-adjusting pressure. To insert a wire, you depress the spring lever with a screwdriver, push in the wire, and release. The spring clamps down instantly.
Who should use spring clamp terminals? Spring terminals are the preferred choice of professional panel builders and any installer who values speed and long-term reliability under vibration. They are ideal for:
- Machinery and automation panels (resistant to vibration loosening)
- High-volume wiring where installation speed matters
- Control panels in buildings subject to movement or temperature cycling
- Any application where you cannot re-tighten screws periodically
ONKA Spring Clamp Terminal
ONKA Spring Clamp Terminal Block — Push-In
ONKA spring terminals use a high-tension stainless steel spring that maintains constant, vibration-proof clamping force throughout the life of the installation. Connection time is typically 50–70% faster than an equivalent screw terminal.
Best used with solid or ferrule-terminated stranded wire. The spring adjusts automatically to the conductor cross-section — no torque setting to remember and no risk of over-tightening.
Shop Spring Terminals →Amateur tip: If you're wiring a DIN rail consumer unit at home for the first time, screw terminals are more forgiving. Spring terminals are fantastic once you're familiar with the lever action, but wire ends must be prepared neatly — stripped to the correct length, and fitted with a bootlace ferrule for stranded wire.
Screw vs Spring Clamp — Head to Head
| Feature | Screw Terminal | Spring Clamp Terminal |
|---|---|---|
| Connection method | Tighten screw with screwdriver | Depress lever, push wire in, release |
| Installation speed | Standard | Up to 70% faster |
| Tool required | Flat-blade screwdriver | Small screwdriver (to open lever) |
| Wire types accepted | Solid, stranded, with or without ferrule | Solid or ferrule-fitted stranded (best) |
| Vibration resistance | Can loosen over time | Excellent — constant spring pressure |
| Beginner-friendly | Yes — intuitive | Learning curve on lever technique |
| Re-termination | Very easy — loosen screw | Easy — depress lever |
| Maintenance | Periodic re-torque recommended | Maintenance-free |
| Best for | DIY, general wiring, field work | Industrial panels, automation, OEM |
| Cost | Slightly lower | Slightly higher |
Bottom line: For a home consumer unit or a small project with occasional re-wiring, go screw. For any machinery enclosure, industrial panel, or application where vibration or temperature swings are present, spring clamp terminals repay their slightly higher cost many times over in reliability and wiring time saved.
STUD Terminal Blocks — For Heavy Currents
A STUD terminal block uses a large central bolt (stud) around which ring-eye cable lugs are clamped with a nut. Unlike standard terminals, the STUD design is engineered for large-cross-section cables — typically 25 mm² (M6) to 240 mm² (M12) and beyond — where the currents involved (100 A to 415 A+) demand very low contact resistance and maximum mechanical strength.
When do you need a STUD terminal?
- Main supply feeds into large distribution boards (63 A, 100 A, 200 A , 400 A fuses / MCBs , MCCBs)
- Motor feeds for large 3-phase motors (pumps, compressors, fans)
- Battery connections in UPS systems, solar inverter banks, and EV charger panels
- Connecting earth busbars to the main earthing point
- Any application where multiple large cables share one connection point
ONKA STUD Terminal 240 mm² / M12
ONKA STUD Terminal Block — Up to 240 mm² / M12
Rated to 415 A and 1000 V, the contact is CK45 steel / copper alloy for minimal resistance at high current. The polyamide (PA6.6) housing withstands temperatures from −40 °C to +140 °C. DIN 35 mm rail mounting as standard.
A protective transparent cover is included on larger models to prevent accidental contact with live conductors — an important safety feature at these voltages and currents.
Shop STUD Terminals →Important: STUD terminals must always be used with correctly crimped ring-eye (or fork) cable lugs. Never insert a bare conductor under a stud nut — the clamping area is insufficient for a secure connection at high current, and loose connections at these currents cause fires.
Porcelain (Ceramic / Steatite) Terminal Blocks
Where standard polyamide housings begin to degrade above 130–140 °C, steatite (the engineered artificial porcelain used in ONKA ceramic terminals) remains stable and safe at temperatures up to 300 °C and beyond. ONKA uses steatite — a sintered magnesium silicate ceramic — rather than common porcelain, giving higher mechanical strength and better electrical insulation.
When do you need porcelain terminals?
- Wiring inside or near kilns, furnaces, and industrial or domestic ovens
- Boiler control panels where flue or case temperatures can be extreme
- Sauna heater wiring and connection to heating elements
- Ceramic hob and range cooker element connections
- Infrared heater and radiant panel installations
- Any location where sustained temperatures exceed 120 °C
ONKA Steatite Porcelain Terminal
High-temperature rate
ONKA Steatite Porcelain Terminal — 0.75 to 25 mm²
Available in 1-pole, 2-pole, and 3-pole versions across three cable size ranges: 0.75–1.5 mm², 2.5–4 mm², 6–10 mm². and 10-25 mm² All versions are white — the standard colour for ceramic terminals worldwide — with a stainless steel clamping screw for corrosion resistance at elevated temperatures.
Designed to be panel- or surface-mounted, typically screwed directly to a refractory or metal backing plate inside heated equipment.
Shop Porcelain Terminals →Choosing the right size: Match the pole count to your circuit. A single-phase heating element needs a 2-pole connector (live + neutral). A 3-phase delta-connected element needs a 3-pole connector. Always select the version whose cable range matches your conductor cross-section.
Terminal Block Colours — What They Mean
Terminal block colour is a recognised identification system under IEC 60445 / BS EN 60445. Using the right colour makes panel inspection and fault-finding faster, and in professional installations it is expected by inspectors.
Practical rule for beginners: If in doubt, use grey for all phase/signal conductors, blue for neutral, and green-yellow for earth. This combination is always correct, always inspectable, and will never confuse a qualified electrician who services your panel later.
Quick-Pick Guide — Find Your Terminal in 30 Seconds
Grey screw terminal, 2.5 mm² for lighting & sockets. 4–6 mm² for cooker / shower circuits. Blue for neutral, green-yellow for earth.
Spring clamp terminals, 1.5–2.5 mm². Grey for signal/24 V phase, blue for 0 V, green-yellow for earth. Spring type for vibration resistance.
Grey screw terminals 4–16 mm² for motor feeds. STUD terminals for the main supply incomer (35–70 mm²). STUD with cover on 63 A+ circuits.
White steatite porcelain terminals. Choose 2-pole for single-phase elements, 3-pole for 3-phase. Match the cable range to your wire size.
Red terminals for positive, blue for negative. STUD type for battery cables 35 mm² and above.
Spring clamp throughout — fastest wiring, zero re-torque maintenance, vibration-proof. Saves 40–60% of wiring time on large panels.
Always green or green-yellow terminals. Never use grey for earth conductors. PE STUD terminals for main earthing conductors above 16 mm².
1.5 mm² for 13 A, 2.5 mm² for 20 A, 4 mm² for 32 A, 6 mm² for 40 A, 10 mm² for 63 A. Call us on 020 45 770 115 for advice.
Shop All ONKA Terminal Blocks at pelek.uk
All terminal blocks in this guide are available to order online with UK stock, VAT included in the price, and next-day dispatch on most items. Mix and match 10 or more terminals for a 10% quantity discount.
Have a question? Call us on 020 45 770 115 (Mon–Fri, 9:00–17:00) or email info@principalelektrik.co.uk.