How Screw Type Terminal Blocks Hold Wire Tight
- Type:
- Protective conductor terminal block, When aligning with a feed-through terminal block with the same shape, an end cover must be interposed with insulation voltages of > 690 V
- Number Of Connections:
- 2
- Connection Method:
- Screw connection
- Rated Cross Section:
- 4 mm²
- Cross Section:
- 0.2 mm² - 6 mm²
- Mounting Method:
- PE foot with mounting screw, M2
- Color:
- green-yellow
Screw type terminal blocks convert screw torque into high contact force through a clamping yoke. Tighten to rated torque, and the joint becomes gas-tight and vibration-resistant. Three steps matter: strip to the correct length, tighten to specification, then pull-test the wire. A 0.8 Nm torque can generate about 750 N of contact force. This force keeps the conductor pressed against the copper current bar without damage.
The Force Path: Torque to Contact Pressure
Thread amplification
Thread inclined planes multiply turning effort. The screw pushes the yoke downward, so small torque creates large clamping force.
Yoke flex
The yoke tongue flexes upward. This elastic reaction presses the conductor against the current bar.
Gas-tight contact
High pressure closes surface gaps. Oxygen entry stops, so contact resistance stays stable.
Inside the Clamping Yoke
-
Screw: applies torque.
-
Threaded tongue: flexes and stores elastic energy.
-
Clamping yoke: presses the conductor.
-
Current bar: carries current.
-
Spring washer: maintains force under vibration.
| Part | Function | Common Failure |
|---|---|---|
| Screw | Transfers torque | Over-tightening |
| Yoke | Presses wire | Fatigue crack |
| Current bar | Conducts current | Oxidation |
| Spring washer | Holds force | Loss of tension |
What Test Values Show
| Conductor | Torque | Pull-out Force |
|---|---|---|
| 0.5 mm² | 0.4 Nm | 20 N |
| 1.5 mm² | 0.5 Nm | 40 N |
| 2.5 mm² | 0.6 Nm | 50 N |
| 6.0 mm² | 0.8 Nm | 80 N |
IEC 60947-7-1 and UL 1059 define test conditions. Values change with wire class and yoke design.
Three Mechanisms That Keep It Tight
Elastic compensation
Temperature shifts change conductor diameter. The yoke flexes to absorb movement.
Vibration resistance
A din rail screw terminal block survives 168 hours at 130°C plus vibration. Pull-out force stays above standard limits.
Self-locking thread
The thread angle creates counter-pressure. The screw resists loosening under dynamic load.
Selection and Installation Steps
-
Match terminal size to wire gauge.
-
Strip insulation to the marked length.
-
Insert the conductor fully.
-
Tighten to rated torque.
-
Pull-test the wire.
For a din rail screw terminal block, snap the unit onto the rail and check the end stop.
Special Connections
A thermocouple screw terminal uses matched alloys for contacts. This avoids extra EMF in temperature circuits.
A clamp terminal block suits high-vibration panels. The clamp holds force without periodic re-tightening.
Engineers choose clamp type terminal blocks for fine-stranded wire with ferrules, where screw pressure can cut strands.
Failure Modes and Fixes
| Symptom | Cause | Fix |
|---|---|---|
| Loose wire | Low torque | Re-tighten to spec |
| Heat rise | Oxidation | Replace terminal |
| Broken strands | Over-tightening | Use ferrule |
| Signal drift | Wrong alloy | Use thermocouple type |
FAQ
Q: What torque should be applied?
A: Small terminals take 0.4 to 0.8 Nm. Larger sizes reach 1.6 Nm. Follow the maker's rating.
Q: Why is re-tightening rarely needed?
A: Elastic yoke action compensates for settling and thermal growth.
Q: Can fine-stranded wire be used?
A: Yes, with ferrules. The ferrule protects strands from yoke pressure.
| Model | Function | Wire IEC cross-section | Wire UL |
| JHD5-4JD |  |
4 mm² 2.5 mm² 4 mm² |
12 AWG 14 AWG 12 AWG |
| Thickness | Number of packages | Thickness/Height( )/Width mm |
Rated voltage/rated current/rated cross-sectior |
| 6.2 mm | 100 | 6.2/47/42.5 | -/34 A/4 mm² |
Specfication:
| Marking tags | ZB5 |
| Screw/Tightening torque(Nm) | M3/0.6 |
| Material | PA |
| Flame retardant rating UL-94 | V0 |
| Dimensions |  |
With universal mounting feet for standard top-hat(Ω profile) and G-profile DIN rail mounting.
