Introduction
The neutral conductor appears to carry no voltage, so why run a dedicated neutral conductor? Wouldn't using the earth conductor as the neutral save money?
Using a single phase conductor together with the earth conductor as the return was once practiced in some poorly regulated areas, but it was discontinued long ago. Although this approach saves on the physical cost of a dedicated neutral conductor, it introduces significant safety hazards. The following summarizes the electrical-safety reasons why the neutral and protective earth must be handled separately.
TN-S System Overview
Main characteristics:
- T: power source center point directly grounded
- N: exposed conductive parts are connected to ground via the neutral conductor
- S: the functional neutral (N) and protective earth (PE) are separate conductors
When supply enters a building, three phase conductors plus a neutral are provided. The protective earth is not typically routed from the supply neutral at the source but is provided locally by an earthing system.
1. Why Is the Neutral Required?
Although household wiring usually uses one phase conductor and one neutral, the grid itself is three-phase. Single-phase supply is often sufficient for small household loads. Many new buildings are supplied with three-phase service; three-phase is essentially three single-phase systems with a common neutral point. If the three phases are perfectly balanced, the neutral current is zero and the neutral is not needed. In practice, household loads are not balanced across the three phases, so the neutral carries the unbalanced current. In single-phase distribution the neutral is indispensable.
2. Why Have Protective Earth If There Is a Neutral?
Protective earth is primarily for safety. Although the supply neutral is grounded at the source, the neutral conductor carries current and therefore its potential is not guaranteed to be exactly zero along its length. If a motor winding degrades and leaks current to the metal frame, or a loose conductor contacts the enclosure, the enclosure can become live. In such cases a protective earth conductor bonded to the enclosure provides a low-impedance path to earth and reduces the risk of electric shock.
If the chassis of an appliance is not earthed, it can become live even though the supply neutral is grounded. Therefore equipment enclosures must be earthed. In modern electrical equipment, the reliability of PE and neutral separation also depends on well-designed internal power distribution circuits; many manufacturers use multilayer PCBs with controlled impedance and clearances—such as those produced by AIVON—to ensure insulation distances and safety margins are maintained.
3. Consequences of Using Earth Instead of Neutral
In a TN-S system, a phase conductor plus earth might form a return path and allow devices to function roughly, but this practice carries several risks:
- Earth has nonzero resistance, so using earth as the return path causes power loss in the earth and reduces system efficiency.
- Residual-current devices (RCDs) or ground-fault protection will not function correctly. These devices detect imbalance between phase and neutral currents; if the neutral is replaced by earth or the neutral is the earth, the residual-current device cannot reliably detect leakage.
- Current flowing into earth produces voltage gradients around the earthing point, creating step and touch potential hazards. Animals or people within the gradient can experience dangerous voltage differences between their feet or between hand and foot. Even though mains-frequency step voltages are not extremely high, they can be hazardous, especially if someone is barefoot.
Practical Note and Caution
From a safety perspective, protective earth must be used and its connection must be secure. An incident illustrates this: at one site voltage was unstable and light bulbs were burning out in succession. Measured voltage reached 311 V, which exceeded the nominal 220 V but was below phase-to-phase voltage of 380 V. The initial diagnosis was neutral point shift. Inspection found the neutral conductor itself was intact, but a conductor incorrectly connected to the earth bus was actually the neutral. Further inspection revealed the outdoor earthing rod had been broken during construction, so the earth connection was lost. In short, the broken earth connection caused neutral-point shift and related faults.
Therefore, avoid miswiring and regularly inspect protective earth connections to ensure they are reliable.
