In low-voltage electrical systems, the integrity of the protective earth (PE) and protective earth-neutral (PEN) conductors plays a critical role in ensuring operational safety. Unlike phase or neutral conductors, whose disconnection typically results in immediate and noticeable interruptions in equipment function, the failure of a PE or PEN conductor often goes undetected. Despite the continued operation of connected equipment, the absence of a reliable grounding path eliminates the essential protection against electric shock hazards. When equipment insulation fails and a conductive enclosure becomes live, the lack of a functional PE or PEN conductor can lead to catastrophic consequences. Therefore, from a safety perspective, ensuring continuous and secure connectivity of PE and PEN conductors is even more vital than maintaining the integrity of live conductors.

To guarantee effective fault protection, the cross-sectional area of PE and PEN conductors must meet two primary criteria: mechanical robustness and thermal stability under fault conditions. In the case of PEN conductors, which serve dual functions as both protective earth and neutral, additional requirements for current-carrying capacity and permissible voltage drop must also be satisfied.

I. Based on Mechanical Strength Requirements

1. Single-Conductor PE Lines
When a single conductor is utilized as a PE line, minimum dimensions are mandated to ensure sufficient mechanical durability:

Copper Conductors:

With mechanical protection: not less than 2.5 mm²

Without mechanical protection: not less than 4 mm²

Aluminum Conductors:

Regardless of mechanical protection: not less than 16 mm²

These thresholds are established to prevent accidental breakage or degradation during installation or operation, particularly in industrial or outdoor environments where vibration, corrosion, or physical strain may occur.

2. PE Conductors Within Multi-Core Cables or Enclosed Raceway Systems
When one core within a multi-core cable or a sheathed wire assembly is designated as the PE conductor—especially when routed within a common enclosure or cable tray alongside phase conductors—specific mechanical strength thresholds may be waived. This exemption acknowledges the added physical protection provided by the cable sheath or housing infrastructure.

3. PEN Conductors in Fixed Installations
PEN conductors, due to their hybrid function and higher safety risk upon failure, are subject to more stringent requirements:

Copper: Minimum cross-section of 10 mm²

Aluminum: Minimum cross-section of 16 mm²

These limits are applicable only in fixed installations that are free from frequent bending or movement, ensuring long-term integrity without the risk of fatigue-related breaks.

II. Based on Thermal Stability Requirements

Thermal stability is crucial in preventing conductor degradation or destruction under fault conditions. While PE conductors usually carry negligible leakage currents under normal conditions, fault scenarios—especially earth faults—can impose high current surges comparable to or exceeding those in phase conductors. Without adequate cross-sectional capacity, these fault currents can cause PE or PEN conductors to overheat, compromising their protective function or even initiating secondary hazards such as fire.

To counteract these risks, the following thermal stability rules apply:

1. When Phase Conductor ≤ 16 mm²
The PE and PEN conductors must match the cross-section of the phase conductor. This ensures they can safely handle any potential fault current, even in worst-case scenarios.

2. When Phase Conductor > 16 mm² and ≤ 35 mm²
The minimum cross-section for PE and PEN conductors is standardized at 16 mm², balancing safety with practical conductor sizing.

3. When Phase Conductor > 35 mm²
The PE and PEN conductors must be no less than 50% of the phase conductor’s cross-sectional area. This proportionate sizing reflects the statistically lower—but still critical—risk of sustained high fault currents.

These sizing principles are equally applicable to IT and TT earthing systems.

Additional Considerations for IT and TT Systems

In IT and TT configurations, fault current pathways differ significantly from TN systems. Under standard single-fault conditions, grounding currents are generally modest, meaning that PE conductors are not typically subjected to thermal overload. However, if two separate equipment enclosures develop simultaneous earth faults—especially on different phases—then the PE conductor may carry a phase-to-phase short-circuit current, a highly hazardous condition. In such a scenario, the resultant current could severely exceed the conductor’s rated capacity.

To mitigate such risks, PE conductors in IT and TT systems must also be dimensioned in accordance with the same cross-sectional criteria outlined above. This ensures that the system remains resilient even under rare but highly dangerous double-fault events.

Conclusion

The reliable operation of PE and PEN conductors is foundational to electrical safety in low-voltage systems. Their sizing must not only account for electrical performance but also withstand mechanical and thermal stresses across the full range of foreseeable operating conditions. By adhering strictly to established standards for minimum cross-sectional area, engineers and electricians ensure the enduring reliability of grounding systems, preventing latent faults from escalating into life-threatening accidents. In any installation, the unseen conductors tasked with protection deserve the same—if not more—attention as the conductors delivering power.