In a standard three-core cable, the magnetic fields from the three phases cancel each other out naturally. Ground both ends of the metal shield and you’re done. Simple.
Single-core XLPE cables don’t work that way. They follow different physical rules.
Because the individual phase cables are physically separated, their magnetic fields don’t perfectly cancel. That imbalance induces an electromotive force – voltage – directly into the metal shielding layer. The longer your cable run and the higher your load current, the worse it gets.
Ignore this, and you’re building in a problem that shows up as heat, derating, and eventually insulation failure.
Trefoil vs. Flat Arrangement: How Layout Affects Voltage
The physical geometry of your cable trench directly dictates how severe this induced voltage becomes.
Trefoil (Equilateral Triangle)
This is the most electrically efficient layout. Because the cables are tightly bundled, the induced voltage stays low.
For example: a standard 300mm² single-core cable carrying 200A in a tight trefoil formation generates roughly 10.7V per kilometer of induced voltage. Manageable.
Flat Horizontal
Now lay those same cables flat, side by side. The magnetic cancellation becomes uneven. The induced voltage spikes – especially on the outer phases.
Space them 200mm apart, and the outer cables can see induced voltages exceeding 36V per kilometer. The middle phase sits around 31V. That’s a completely different problem.
Same cable. Same current. Different layout. Triple the induced voltage.
The Hidden Cost of Circulating Currents
Here’s the mistake a lot of system designers make: they ground both ends of a single-core cable because that’s what you do with three-core cable.
But when you ground both ends of a single-core run, the induced voltage has a closed loop to drive current through. That current flows straight through the metal shield as a circulating current.
Field measurements on active 300mm² cable networks show these circulating currents routinely reach 10% to 20% of the main load current.
Think about what that means. If you’re pushing 200A through the conductor, you might have 20A to 40A circulating through the shield. Doing nothing useful.
That current generates heat inside the shield. The heat degrades the XLPE insulation over time. And to compensate, you have to de-rate the power transmission capacity of the entire cable run.
You’re literally paying for capacity you can’t use – because of a grounding decision.
Choosing the Right Grounding Strategy
To eliminate thermal losses and protect your infrastructure, you can’t just ground both ends of a high-voltage single-core run.
Depending on total cable length, engineers need to implement single-point grounding or specialized cross-bonding grounding systems to break the circulating current loop.
Single-point grounding works for shorter runs. Cross-bonding is the standard solution for longer distances – it transposes the shield connections at regular intervals to cancel out induced voltages without creating a closed loop.
Get this wrong, and you’re not just losing efficiency. You’re accelerating insulation aging on cable that costs real money to replace.
Getting the Physical Layer Right
Grounding strategy is only half the equation. The cable assembly itself needs to be built for the application – proper shield construction, consistent insulation thickness, and terminations that don’t compromise the shielding system.
If you’re designing high-voltage distribution networks and need structurally optimized XLPE cable assemblies – or just want a second opinion on shield grounding configurations – we can help.