HGV Trailer Couplings: UK/European Electrical + Air Systems

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Modern HGV trailers require reliable connections for both electrical power and pneumatic (air brake) systems. While the principles are consistent, the specific types of coupling heads used can vary between the UK and continental Europe. This guide explains the main types of coupling heads used for both electrical and air systems across different regions.

1. Electrical Couplings

UK Electrical Couplings

In the UK, most trailers are fitted with two separate 7-pin connectors, along with a third to power the trailer ECU:

• 24N (Primary): ISO 1185 - 7-pin socket providing power for side markers, indicators, brake lights and tail lights.
• 24S (Secondary): ISO 3731 - 7-pin socket powering reverse lights and fog lamps.
• EBS/ABS: ISO 7638 - A dedicated 7-pin connector for the trailer’s ECU which controls EBS/ABS braking and other auxiliary systems such as suspension and roll stability. This is standardised across both the UK and Europe.

European Electrical Couplings

In much of Europe, the separate 24N and 24S connectors are replaced by a single 15-pin socket:

• ISO 12098: A 15-pin connector that combines both lighting and supplementary power functions. It simplifies the electrical setup.
• ISO 7638: As with the UK, this remains the standard for EBS/ABS braking and auxiliary systems.

This combined approach allows for a simpler and more streamlined connection between tractor and trailer in European-spec vehicles.

2. Pneumatic Couplings (Air Lines)

UK Pneumatic Setup

UK tractor/trailer connections typically use two air lines, colour-coded and coiled ("suzies") for flexibility:

• Yellow line: Service line – activates trailer brakes when the driver applies the foot brake.
• Red line: Emergency line – provides air pressure to both braking and suspension systems.

These lines connect to C-type coupling heads, which are the UK standard.

European Pneumatic Setup

In Europe, the same red/yellow colour coding is used, but the coupling heads typically differ. Instead of C-type heads, Europe uses:

• Palm Type Couplings: Flat-faced, disc-shaped connectors that twist and lock together. Each coupling is self-sealing when disconnected, offering good protection against contamination and leaks.

Palm couplings are standard across drawbar trailers and full-size European HGVs.

Adapters are available to convert C to Palm type couplings and vice versa.

Drawbar / Rigid-Trailer Combinations

For rigids towing smaller trailers or drawbar combinations, a different system is sometimes used:

• Duomatic Coupling: Combines both service and emergency air lines into a single plug-in head. This is quicker to connect and avoids the risk of cross-connection, but introduces a single point of failure if the coupling is damaged.

Duomatic systems are rarely found on UK tractor/trailer outfits but are more common in drawbar logistics and some European rigs.

Summary Comparison Table

Conclusion

While electrical and air couplings on HGVs serve the same basic functions across regions, the coupling heads and systems vary between the UK and Europe. Understanding these differences — and keeping them separate from the connecting lines — helps operators ensure compatibility, reduce risk, and maintain safe operations across different trailer types.

If you’re ever unsure which setup your vehicle uses or requires, get in touch with Don-Bur’s expert team for guidance.