Earthing arrangements
The key idea
An earth-fault current must return to the transformer through its neutral-to-earth connection. Whatever sits in that connection — a solid link, a resistor, or nothing — controls the current more than the whole rest of the network does.
The idea
A line-to-earth fault drives current around a loop: out along the faulted phase, into the earth, and back up into the transformer through its star-point earthing. The design of that star-point connection is a genuine choice, and each option buys a different trade:
- Solidly earthed. The star point is bolted straight to earth. The loop is complete and low-impedance, so earth faults are big, obvious, and cleared fast. The price: the network must withstand those big currents, everywhere.
- Resistance earthed. A resistor in the neutral caps the earth-fault current at a chosen value — commonly a few hundred amperes. Damage at the fault point and voltage rise around it shrink dramatically. The price: protection must now detect a deliberately small current, which takes dedicated, sensitive earth-fault elements.
- Isolated (unearthed). No intentional connection at all. The first earth fault has almost no loop — only the network's cable capacitance closes it — so the system can keep running with the fault standing. The price: the healthy phases rise to full line voltage, and a second fault anywhere becomes a serious phase-to-phase event. Operating one of these means being good at finding the first fault.
Try it
Earth-fault current
High — limited only by the network impedance
Detection
Easy: the current is large and unmistakable
The star point is bolted to earth. An earth fault sees a complete low-impedance loop, so the current is large — sometimes larger than a three-phase fault. Equipment must withstand it, and protection clears it fast. Common on LV networks and solidly earthed HV systems.
Why it matters
- Earth-fault protection settings start here. A pickup that suits a solidly earthed network would never see a fault on a resistance-earthed one. The arrangement dictates the scheme.
- It dominates the model. In a fault study, the transformer vector groups and neutral treatment set the zero-sequence path. Get those wrong and no amount of accurate cable data rescues the earth-fault results.
- It is a safety design decision. Touch voltages, arc energy at the fault point, and whether a process trips or rides through a first fault — all trace back to this one connection.
See it in Phasor
In Phasor, each transformer carries its vector group and neutral earthing — including any earthing resistor or reactor — and the earth-fault results follow from them. The docs put it bluntly: get the earthing right before scrutinizing cable data.