Conductor Creep and Thermal Cycling

Important Notice

This article is intended for qualified electricians and engineers working in the field of electrical energy systems. All installation, maintenance, and inspection work must be carried out in accordance with national regulations and company safety procedures. Always observe personal safetySafeguarding Yourself: A Guide to Personal Safety with Electrical Installations rules and ensure that circuits are de‑energised before handling conductor connections.

Introduction

In electrical installations, the long‑term stability of conductor connections is critical for safety and reliability. Engineers often assume that once a terminal is tightened to the correct torque, the joint will remain secure for decades. In practice, both copper and aluminium conductors are subject to mechanical and thermal effects that reduce contact pressure over time. This behaviour, known as creep or relaxation, can lead to higher resistance, overheating, and in severe cases, fire hazards.

This article explains the mechanisms behind conductor creep, the differences between copper and aluminium, and the engineering measures that can mitigate these effects. It also provides recommended tightening torques for common M5 and M6 brass terminals used in metering and distribution equipment.

Material Behaviour of Conductors

Copper and aluminium are both ductile metals. When clamped under pressure in a terminal, they deform slightly. Over time, this deformation continues at a microscopic level, even under constant load.

Creep: Time‑dependent deformation of the conductor under constant pressure.
Relaxation: Stress reduction in the clamping screw or bolt under constant strain.

Both phenomena occur in electrical connections. The conductor creeps, reducing contact area, while the bolt relaxes, reducing clamping force.

Copper: Copper has higher mechanical strength than aluminium and lower thermal expansion. It still exhibits creep under sustained pressure and elevated temperature, but the effect is relatively modest.
Aluminium: Aluminium has about 1.5 times the coefficient of thermal expansion of copper and lower hardness. It is much more prone to creep. In addition, aluminium forms a non‑conductive oxide layer that increases contact resistance if not properly managed.

This difference explains why aluminium connections require special handling, such as oxide‑inhibiting pastes and terminals designed for aluminium conductors.

Thermal Cycling

Every conductor expands when heated and contracts when cooled. The coefficient of thermal expansion for copper is approximately 17 \times 10^{-6} \, \text{K}^{-1}, while for aluminium it is about 24 \times 10^{-6} \, \text{K}^{-1}.

In distribution networks, conductors often experience temperature swings of 40–70°C between no‑load and full‑load conditions. Each cycle of expansion and contraction causes micro‑movements in the joint. Over time, the contact surface settles, and the clamping force reduces.

The result is a self‑reinforcing process:

Reduced contact pressure increases resistance.
Higher resistance increases heating.
More heating accelerates creep and oxidation.

If not corrected, this cycle can lead to severe overheating.

Practical Consequences

Loose connections are among the most common causes of electrical faults. In metering cabinets, distribution boards, and switchgear, the consequences include:

Localized hot spots detectable with thermal imaging.
Oxidation and corrosion at the contact surface.
Arcing under load, leading to carbonization of insulation.
In extreme cases, fire ignition.

For utilities and metering applications, this is a critical issue because installations are expected to operate safely for decades with minimal maintenance.

Engineering Countermeasures

Several measures can reduce the risk of conductor creep and thermal loosening:

Torque‑controlled tightening: Always tighten screws and bolts to the manufacturer's specified torque. Under‑tightening reduces contact pressure; over‑tightening damages threads or deforms conductors.
Spring‑loaded terminals: Where available, use terminals with spring elements that maintain constant pressure despite conductor creep.
Special handling for aluminium: Use oxide‑inhibiting paste and terminals rated for aluminium conductors. Avoid mixing copper and aluminium directly without bimetallic connectors.
Periodic inspection: Thermal imaging during routine maintenance can identify hotspots before they develop into failures.

Recommended Tightening Torques for Brass Terminals

For brass terminals commonly used in metering and distribution equipment, the following tightening torques are recommended:

M5 brass terminal: 2.0 – 2.5 Nm
M6 brass terminal: 3.0 – 3.5 Nm

These values provide sufficient contact pressure without damaging the conductor or the terminal. Always verify with the equipment manufacturer's documentation, as specific designs may require slightly different torque values.

Standards and Guidelines

International standards such as IEC 60947 (Low‑voltage switchgear and controlgear) and IEC 61238 (Compression and mechanical connectors for power cables) provide guidance on conductor connections and performance requirements. Utilities often have their own specifications for torque values and inspection intervals.

Safety Reminder

Work on electrical connections must only be performed by qualified personnel. Always follow the correct lock‑out and tag‑out procedures, verify absence of voltage, and use appropriate personal protective equipment (PPE). Never attempt to tighten or re‑torque live connections. Safety comes first — reliable connections are important, but your life is irreplaceable.

Takeaway

Copper and aluminium conductors do not literally shrink, but they do creep and relax under sustained pressure and thermal cycling. This reduces contact pressure, increases resistance, and can lead to overheating or failure. Proper installation practices, torque‑controlled tightening, and periodic inspection are essential to maintain safe and reliable connections. For M5 and M6 brass terminals, recommended tightening torques are 2.0–2.5 Nm and 3.0–3.5 Nm respectively.