IEC 61138 — Live Working: Cables for Earthing and Short-Circuiting Equipment

1. Scope and Core Technical Requirements

IEC 61138 — Live working — Cables for earthing and short-circuiting equipment — is the definitive international standard governing flexible power cables used in temporary earthing and short-circuiting assemblies for live working (also referred to as “live line work” or “hot sticking” in North America). The standard applies to single-core and multi-core flexible cables rated for use in AC systems with nominal voltages not exceeding 25 kV phase-to-phase.

The fundamental engineering purpose of these cables is to create a deliberately low-impedance path to earth that can safely conduct the full prospective fault current until protective devices operate, while maintaining galvanic integrity and preventing dangerous voltage gradients from developing across the work zone. This is fundamentally different from ordinary power cables, which are designed primarily for continuous current-carrying capacity with short-circuit withstand as a secondary consideration.

2. Cable Construction and Material Requirements

2.1 Conductor Stranding and Flexibility

Conductor design is arguably the most critical element differentiating IEC 61138 cables from general-purpose power cables. The standard mandates the use of Class 5 or Class 6 flexible copper conductors as defined by IEC 60228. These conductors consist of very fine individual wires — typically 0.21 mm diameter or finer — assembled in a concentric-lay or bunch-stranded configuration. For larger cross-sectional areas (50 mm² and above), a multiple-strand construction is employed, where several pre-stranded bundles are themselves stranded together in a second operation.

The rationale for such fine stranding is threefold. First, it dramatically reduces the cable’s bending stiffness, allowing it to be handled manually without excessive force. Second, it distributes cyclic bending strain across many individual wire surfaces, greatly improving fatigue life. Third, the increased surface-area-to-cross-section ratio improves heat dissipation under short-circuit conditions. The standard requires cables to pass a bending test at both room temperature and −25 °C without conductor fracture or insulation cracking — a demanding specification that directly validates field performance in winter conditions.

2.2 Insulation and Sheath Materials

IEC 61138 specifies thermosetting elastomeric compounds — principally ethylene-propylene rubber (EPR) or equivalent high-performance rubber blends — for both insulation and sheath. Thermoset materials are chosen over thermoplastics (PVC, polyethylene) for several fundamental technical reasons:

• Superior thermal endurance: EPR maintains its mechanical and electrical properties at continuous conductor temperatures of 90 °C and can withstand short-circuit conductor temperatures up to 250 °C for periods not exceeding 5 seconds
• Exceptional low-temperature flexibility: Unlike PVC, which transitions to a glassy brittle state at approximately −5 to −15 °C, properly formulated EPR remains flexible and impact-resistant at −40 °C
• High dielectric strength: EPR compounds typically exhibit dielectric strengths in the range of 20–30 kV/mm, providing generous insulation margins at the standard’s rated voltages
• Excellent ozone and weather resistance: The saturated polymer backbone of EPR is inherently resistant to ozone cracking, a critical advantage for outdoor storage and use

The outer sheath — typically chloroprene rubber (CR) or chlorinated polyethylene (CPE) — provides mechanical protection against abrasion, cutting, and environmental degradation. The standard specifies minimum tensile strength, elongation at break, and retention of these properties after accelerated thermal ageing.

2.3 Insulation Thickness and Colour Coding

3. Electrical Performance and Type Testing

3.1 Dielectric Integrity and Withstand Voltage

Every length of IEC 61138 cable must pass a routine AC voltage withstand test at the factory. For cables rated 0.6/1 kV, the test voltage is 3.5 kV (AC, 50/60 Hz) applied between conductor and earth (water bath or metallic foil electrode) for 5 minutes without flashover or breakdown. For higher voltage ratings, the test voltage scales accordingly.

Beyond the routine test, type tests include partial discharge measurement for cables rated above 6/10 kV. Although earlier editions of IEC 61138 did not mandate partial discharge testing, modern industry practice — and the current edition — increasingly requires PD measurement at 1.5 to 2 times the rated voltage to ensure the insulation system is free of voids, contaminants, or interfacial defects that could initiate premature electrical treeing and eventual failure under repeated switching or transient overvoltage stresses.

3.2 Short-Circuit Thermal Stability and Conductor Sizing

The most critical performance attribute of earthing and short-circuiting cables is their ability to withstand the immense thermal energy of a fault current without fusion or insulation collapse. The conductor’s short-circuit current capacity is governed by the adiabatic heating relationship:

I²t ≤ K² × S²

Where I is the short-circuit current in amperes, t is the fault duration in seconds, S is the conductor cross-sectional area in mm², and K is a material-dependent constant (approximately 115–135 for copper, depending on the permissible temperature rise of the adjacent insulation).

This relationship has profound practical implications. Consider a 50 mm² cable subjected to a 25 kA fault with a clearing time of 0.5 seconds: the I²t value is 312.5 × 10⁶ A²s, yielding an equivalent thermal stress of 125 A²s/mm² — well within the copper conductor’s capacity but approaching the limit for the insulation system. Increasing the clearing time to 2 seconds (e.g., with a downstream fuse coordination issue) drives the stress to 500 A²s/mm², which may exceed the cable’s rated short-circuit withstand.

3.3 Mechanical Performance and Field Durability

IEC 61138 subjects cables to a battery of mechanical type tests that are either absent or less stringent in general-purpose cable standards:

• Cold bending test: Cable is conditioned at −25 °C for at least 4 hours, then wound around a mandrel of specified diameter. The insulation and sheath must show no cracks visible to normal or corrected vision
• Abrasion resistance: A weighted specimen is dragged across a standard abrasive surface for a defined number of cycles. Sheath wear depth must remain within limits
• Tensile and elongation: Minimum requirements for both insulation and sheath before and after accelerated thermal ageing (typically 7–10 days at 100–120 °C in an air-circulating oven)
• Impact resistance at low temperature: A falling weight strikes the cable specimen at −25 °C; subsequent voltage testing must not show breakdown

These tests directly replicate the mechanical stresses encountered in field use: cables dragged over gravel and concrete, stepped on, pinched between equipment cases, and bent around sharp edges in freezing weather. Passing these tests gives engineers confidence that the cable will perform its safety function even after extended service in harsh environments.

4. Engineering Application and Selection Guide

4.1 Key Selection Parameters

Selecting the appropriate IEC 61138 cable for a specific installation requires a systematic evaluation of several interdependent factors:

• System voltage: The cable’s rated voltage U₀/U must equal or exceed the maximum phase-to-earth and phase-to-phase voltages of the installation. For mixed-voltage installations, the highest voltage governs
• Prospective fault current: Obtain the maximum short-circuit current from system studies. Size the conductor using the I²t adiabatic equation, applying appropriate safety factors (typically 1.2–1.5)
• Ambient temperature range: For installations in cold climates, verify the cable’s low-temperature bending and impact ratings. For hot environments, apply derating factors to both continuous and short-time current ratings
• Mechanical duty cycle: Applications involving daily deployment and retrieval (e.g., mobile earthing trucks, temporary work sites) benefit from cables with reinforced sheaths and extra-fine stranding
• Connector compatibility: The cable’s conductor diameter and stranding configuration must be compatible with the compression lugs and crimping tools specified for the earthing and short-circuiting clamps

4.2 Field Inspection and Maintenance

Even the highest-quality IEC 61138 cable requires diligent field care to maintain its protective function. The following practices are recommended:

• Perform a visual inspection before every use — look for cuts, abrasions, crushed areas, exposed conductors, and signs of thermal damage (discolouration, melting, embrittlement)
• Check the minimum bending radius (typically 5× the cable outer diameter); never force a cable around sharp corners
• Keep earthing cable lengths as short as practicable to minimise loop impedance — lengths exceeding 30 metres should be verified by calculation or measurement for adequate fault current clearing
• Avoid contact with sharp edges, hot surfaces, and corrosive chemicals
• Conductor continuity and insulation resistance tests should be performed at least annually, or after any suspected overcurrent event
• Store cables in a clean, dry environment away from direct sunlight, ozone sources (electric motors, welders), and chemical vapours

5. Frequently Asked Questions