{"id":23591,"date":"2026-10-29T06:05:26","date_gmt":"2026-10-29T06:05:26","guid":{"rendered":"https:\/\/www.tronkiele.com\/?p=23591"},"modified":"2026-10-29T06:07:10","modified_gmt":"2026-10-29T06:07:10","slug":"broken-neutral-three-phase-system","status":"publish","type":"post","link":"https:\/\/www.tronkiele.com\/fa\/broken-neutral-three-phase-system\/","title":{"rendered":"What Happens When the Neutral Wire Fails in a Three-Phase System?"},"content":{"rendered":"\t\t\t\n\t\t\t\t\t
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It is possible for a neutral to fault without affecting all three of a three-phase system. In a three-phase four-wire system, there is a chance that the line voltage will still be stable while the voltages supplying the single phase devices are not.<\/p>

The biggest risk arises where the loads have been connected from different phases using the same neutral. Once that neutral becomes open and there are imbalances in loads, the point of neutral load may move causing some equipment to receive excess voltage while others receive inadequate voltage.<\/p>

This may not be easily detectable in a factory environment where one of the three-phase motors is still working while the PLC power supplies, lighting, and contactor coils become erratic.<\/p>

This article explains why that happens, how to recognize a possible neutral fault, and what to check before relying on a three-phase automatic voltage regulator<\/a> (AVR) for protection.<\/p>

The Role of the Neutral Wire<\/h2>

In a three-phase, four-wire system, the neutral conductor connects line-to-neutral loads to the source neutral point and carries their return current.<\/p>

A typical 400\/230 V supply has about 400 V between phases and 230 V between each phase and neutral. Three-phase motors often connect across the phases, while lighting, control circuits, and other single-phase equipment may use the phase-to-neutral supply.<\/p>

With balanced sinusoidal loads, the fundamental currents from the three phases cancel in the neutral. Real installations are rarely perfectly balanced, though: loads start and stop, and demand on each phase changes over time.<\/p>

Nonlinear equipment can add to neutral current as well. Third-order and other triplen harmonic currents can combine in the neutral instead of cancelling. This is one reason neutral conductor sizing should reflect the actual load and its harmonic content. <\/p>

The neutral is not the same as protective earth (PE). PE provides protective bonding and helps ensure fault protection. It must not be used as a substitute for a failed neutral.<\/p>

What Changes When the Neutral Opens?<\/h2>

If the neutral opens upstream of a group of single-phase loads, the load-side neutral point may no longer be held at the source neutral potential. It can become a floating neutral<\/strong>.<\/p>

The voltage across each load then depends on the connected loads and their impedances. If the loads are unequal, their phase-to-neutral voltages may no longer remain close to the nominal 230 V. One load may see an undervoltage while another sees an overvoltage. Switching equipment on or off can shift the voltages again.<\/p>

There is no single voltage that appears whenever a neutral fails. The result depends on the circuit arrangement and the loads connected to it. The fault does not automatically place 400 V across every single-phase device, but a voltage well above a device\u2019s rating can still cause damage.<\/p>

A Simplified 400\/230 V Example<\/h2>

Consider two resistive loads connected from L1 and L2 to the same neutral point. For this simplified example, assume the third phase has no connected load.<\/p>

With the neutral intact, each load receives about 230 V. If the upstream neutral opens, the two loads are effectively connected in series across the 400 V line-to-line supply.<\/p>

Suppose Load 1 is 100 \u03a9 and Load 2 is 200 \u03a9:<\/p>

<\/th>Load 1<\/th>Load 2<\/th><\/tr><\/thead>
Resistance<\/td>100 \u03a9<\/td>200 \u03a9<\/td><\/tr>
Voltage with neutral intact<\/td>230 V<\/td>230 V<\/td><\/tr>
Voltage after neutral opens<\/td>133.3 V<\/td>266.7 V<\/td><\/tr><\/tbody><\/table><\/figure>

The total resistance is 300 \u03a9, so the series current is:<\/p>

I = 400 V \u00f7 300 \u03a9 \u2248 1.33 A<\/strong><\/p>

The voltage across each load is then:<\/p>