Knowledgebase

Introduction

The integration of additional earth conductors in parallel with the steel wire armour (SWA) of armoured cables is a topic of considerable importance within electrical installation practices. This configuration aims to enhance fault current pathways and ensure compliance with electrical safety standards. PD CLC/TR 50480:2011 provides a technical framework for such scenarios, further tailored in the UK through the National Annex (NA).

This article examines the considerations, benefits, and practical implications of running additional earth conductors separately yet in parallel with the steel armour of SWA cables, drawing on the guidance of PD CLC/TR 50480:2011 and the UK National Annex.

PD CLC/TR 50480:2011 addresses the principles of fault current paths, impedance management, and protective measures. It highlights the importance of low-impedance fault current paths to ensure effective disconnection of protective devices during faults.

The UK National Annex Perspective

The UK National Annex refines the principles of PD CLC/TR 50480:2011 to align with BS 7671 and regional practices. The Annex acknowledges the practicalities of using SWA as an earthing path while outlining scenarios where supplementary earth conductors are beneficial or necessary, such as:

• High fault current scenarios where the steel armour alone cannot provide sufficient fault current capacity.
• Long cable runs where impedance in the armour could impede effective fault clearance.
• Specific installations requiring redundancy or enhanced reliability in fault current pathways.

Running Additional Separate Earth Conductors in Parallel with Steel Wire Armoured Cables 

Running additional earth conductors in parallel with SWA steel armour, often designers believe it will offer two key advantages, such as:

1. Enhanced Fault Current Capacity:
   • The steel armour in SWA cables provides a fault current path, but its impedance may be too high in certain installations. Adding a parallel earth conductor reduces overall impedance, facilitating faster and more reliable disconnection of protective devices.
2. Reduced Impedance for Extended Runs:
   • Over long distances, the impedance of the steel armour increases, reducing its effectiveness as a fault current path. Running an additional low-impedance earth conductor in parallel mitigates this issue.

Mathematically for considering the fault currents this is expressed:

R_s and X_s are the resistance and reactance of the upstream system supplying the circuit

Unfortunately, UK National Annex of PD CLC/TR 50480:2011 can in fact negate both of those advantages.

Challenges UK National Annex of PD CLC/TR 50480:2011 creates 

Fixed Reactance value:

When calculating cable impedances the UK National Annex of PD CLC/TR 50480:2011 requires a fixed reactive Value be used in the calculations

The calculation above is adjusted by:

On conductors 95mm² and below where resistive element of the cable is higher than this value this often is not an issue.

Where however the cable is over 95mm² or is run over very long distances, the reactive value required in the calculation has a negative effect on the design.

No matter how meany conductors are placed in parallel the overall impedance calculated from the resistive and reactive elements of the cable can never be below a fixed value. This is because as the reactive will be higher than the parallel sum of the resistive parts. This has a knock-on effect for the calculated Earth Fault loop impedance, causing higher than expected values. Effectively placing a greatly shortened limit on the distance, a Steel Wire Armour with a separate earthing conductor can be run.

Often the only solution to this, is to increase the size of the Steel Wire Armour itself-meaning increasing the overall size of all the armoured cable phase conductors.

Adiabatic Checks:

In most cases, when we parallel a number of conductors together, we sum CSA, parallel earth conductors are treated as a single fault path for adiabatic checks. This to prove that the overall CSA is adequate to carry the fault current that could potentially pass through it.

However, in UK National Annex of PD CLC/TR 50480:2011 it does not stop there. Primarily the separate earthing conductor CSA should not be less than a quarter of the line conductor. This may however not always be feasible, such as where multiple large phase conductors in parallel are used or due to limited space in cable ducts or other containment systems.

Where needed the document gives specific calculations to divide how much fault current is expected to pass through the Steel Wire Armour and the separate earthing conductor respectively.

To calculate:

Fault current carried by the armour:

Fault current carried by the separate earthing conductor:

NOTE: The sum of the calculated currents from these equations individually, will not equal the sum of I_f as may be expected. This is because no account of the relative phase angles in the calculation is considered.

The adiabatic checks need to be done for both the Steel Wire Armour and the separate earthing conductor independently to ensure the proportion of fault current carried individually in the two different materials can be withstood.

Webinar Insights

In my webinar with the ECA (accessible here) (https://youtu.be/UaIdvZjHfzg?si=-TB5-Swn9EyY4TsC), we explored applications of this practice, including case studies and practical examples. The discussion emphasised how UK National Annex of PD CLC/TR 50480:2011 affects design practices and how to ensure compliance and functionality when designing using additional separate earthing conductors and steel armour in parallel. This is showcased in Modecsoft’s ElectricalOM software functionality, developed specifically to address this issue.