How to Size High Voltage Earthing Conductors Correctly

What size conductors should I use? And, is there a simple table I can use for this high voltage earthing design? These are great questions that I'm often asked. This post answers these questions together with the choice of conductor materials and jointing method within a high voltage earthing design project.

Assessing conductor size is entirely dependent on the electrical configuration and the load that the conductor must take. For example, an above-ground bonding conductor serves to transfer current with minimal voltage drop, from A to B. A directly buried conductor has an additional purpose, that of leaking the fault current/voltage into the local geology (as part of an electrode).

When considering Lightning, the same conductor above might also see a high-frequency component and which will impose yet another requirement.

High Voltage Earthing Design Considerations

In IEEE-std80 Guide for Safety in AC Substation Grounding, section 11 - states the basic requirements are:

Each element of the grounding system, including grid conductors, connections, connecting leads, and all primary electrodes, should be so designed that for the expected design life of the installation, the element will

a) Have sufficient conductivity, so that it will not contribute substantially to local voltage differences.

b) Resist fusing and mechanical deterioration under the most adverse combination of a fault magnitude and duration.

c) Be mechanically reliable and rugged to a high degree.

d) Be able to maintain its function even when exposed to corrosion or physical abuse.

Materials

Copper has traditionally been the go-to material for years in high voltage earthing because it is not only highly conductive but also resistant to most sources of in-ground corrosion.

Similarly, aluminium has sufficient conductivity but suffers from in-ground corrosion, and the oxidation that forms around its surface is not conductive, therefore, compromises the conductor's ability to leak current when buried, which is why buried aluminium conductors is a no-no.

Stainless Steel or mild steel (when appropriately coated - galvanised or copper bonded), on the other hand, may not have the conductivity of copper or aluminium but it is sufficiently conductive to be utilised as a buried conductor.

Dealing with the Heat

So now we’ve covered the materials that are widely used for high voltage earthing. Point ‘b’ from IEEE-std 80 above calls for the conductor to retain its mechanical strength when being exposed to a fault.

Conductors get hot when current flows through them. And they will potentially be at their hottest when subjected to a fault from the electrical system. This fault means the conductor needs to maintain its physical integrity as temperature increases and not transform into a shower of hot molten metal!

Up to this point, we’ve discussed conductor conductivities, materials and thermal-mechanical characteristics.

Another consideration for high voltage earthing is the method of the joint between conductors.

Conductor Joints

Joints in conductors are as critical to current-flow as the conductors themselves. The jointing method must not allow the joint to introduce excessive resistance. Therefore, the selection of the jointing method will impact the earthing system’s thermal resilience massively. This thermal resilience is why welded joints are the go-to choice for the high voltage earthing system.

Welded joints achieve the nearest physical match to the native conductor itself and are the gold-standard in conductor jointing because they accomplish a molecular similarity to the native material's conductivity, as well as similar mechanical robustness.

Here’s an example of a safe method of igniting an exothermic weld.

A Word of Caution

Bolted joints and clamps are made up of multiple parts which can creep and loosen over time. Loosening will add resistance across conductor joints, which as previously mentioned... is not a good thing (thermal runaway is a topic for another day).

Bolted joints fall into the category of mechanical joint methods and have a derating factor to keep the conductors operating at lower temperatures. When designing a high voltage earthing system, this means the seamingly unrelated selection of jointing method will also impact the conductor size calculation.

IEEE adiabatic equation.

Figure 1 - IEEE adiabatic equation.

In Summary

So there you have it. Without going into great detail, the following factors influence conductor sizing:

  • The magnitude and duration of fault current (i.e. the heat source),
  • The method used for the joint
  • The conductor material
  • The conductor’s role in the high voltage earthing design

These factors may vary project by project, so, using a particular size of the conductor on a previous project does not mean that the same will apply to the next project, which means calculating conductor sizes  (adiabatic equation) to fit the particular requirements of the project is usually very necessary.

Ian Griffiths SelfieIan Griffiths CEng, MBA, BEng, MIET

Ian Griffiths, writes this post. Principal Engineer at GreyMatters, an Earthing & Lightning Consultant of 27 years, one of the top 1% UKAS accredited CDEGS consultants and professional advisor to international utility companies, data centre and infrastructure developers.

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