Allowable Earthing Resistance for Link Box Earthing

The allowable earthing resistance for a link box depends heavily on the type of bonding system (solid vs. cross-bonded) and the location of the link box (at a termination vs. along the cable route).

There is no single "universal" value, but standard industry specifications and practices typically require the following limits:

1. Typical Allowable Resistance Values

Link Box Type / Location	Typical Resistance Limit	Reason
Solidly Grounded Link Box (e.g., at substations/terminations)	< 1.0 Ω (often < 0.1 Ω)	Must handle full short-circuit current and keep sheath voltage near zero. Typically connected to the main substation earth grid.[1]
Cross-Bonding Link Box (Joint Bays along the route)	< 5.0 Ω to 10.0 Ω	Used to ground the Sheath Voltage Limiters (SVLs). The resistance must be low enough to clamp over-voltages effectively during a fault.
Link Box Internal Contact Resistance	< 20–30 μΩ (Micro-ohms)	This is the resistance of the copper busbars and link connections inside the box itself, not the earth rod resistance.

2. Detailed Requirements by Application

A. Solidly Bonded Systems (Direct Earthing)

• Location: Usually found at cable terminations (substations) or for short cable runs.
• Requirement: The link box earth point is effectively an extension of the substation earthing grid.
• Allowable Resistance: It should be as low as possible, typically < 1 Ω.
• Why: During a phase-to-ground fault, the full short-circuit current flows through this bond. High resistance here would cause a dangerous rise in voltage (Rise of Earth Potential - ROEP) on the cable sheath, potentially harming personnel or damaging cable jacket insulation.

B. Cross-Bonded Systems (SVL Earthing)

• Location: Found at joint bays along long cable routes (e.g., every 500m - 1km).
• Requirement: The earth rod at these link boxes provides a path for the Sheath Voltage Limiter (SVL) to discharge transient surges (lightning/switching) and limits the sheath voltage during a through-fault.
• Allowable Resistance: Standards (like IEEE 575 or CIGRE TB 797) recommend a design-based value, but utilities often specify a maximum of 5 Ω or 10 Ω.
• Why: If the resistance is too high (e.g., > 10 Ω), the SVL may not be able to clamp the voltage effectively, or the "standing voltage" on the sheath during a fault could exceed safe touch limits (typically 65V or 50V depending on local regulations).

3. Key Standards & Factors

If you are designing a system, you should not rely solely on a rule of thumb. The "allowable" resistance is actually a calculated maximum derived from these factors:

1. Permissible Touch Voltage: The resistance must be low enough to ensure that the voltage on the link box enclosure does not exceed safe limits (e.g., IEC 61936 limits touch voltage to < 80V in some fault scenarios, or < 50V for general safety).
2. SVL Performance: The earth resistance must be low enough so that the sum of the residual voltage of the SVL + voltage drop across the earth resistance does not exceed the breakdown voltage of the cable's outer sheath (jacket).
3. Relevant Standards:
• IEEE 575: Guide for Bonding Shields and Sheaths of Single-Conductor Power Cables.
• ENA TS 41-24: Guidelines for the Design, Installation, Testing and Maintenance of Main Earthing Systems in Substations.
• CIGRE TB 797: Sheath Bonding Systems of AC Transmission Cables.

Summary Checklist for Verification

• For Substation End: Ensure connection to the main earth mat (Resistance < 0.5 – 1.0 Ω).
• For Joint Bays: Drive earth rods to achieve < 5 – 10 Ω. If soil resistivity is high and you cannot achieve this, you may need to install a parallel earth continuity conductor (ECC) back to the substation.
• Internal Continuity: Measure the contact resistance across the links; it should be negligible (< 20 μΩ).

Sources help

1. dcode.org.uk

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