You're right about the potential for significant voltage shifts when you lose the neutral on a 480Y/277V system. However, the instantaneous jump to 480V L-G isn't quite the whole story. Here's a more nuanced explanation

What Happens When the Neutral is Lost

1. Floating Neutral

You're absolutely correct. The transformer's neutral point becomes "floating." It's no longer firmly tied to ground potential. This means the voltage between the neutral and ground at the transformer now becomes variable and depends on the load imbalance in the system.2.

Voltage Imbalance

The key factor is the

load imbalance across the three phases downstream. This is what causes the voltage to shift. Let's break it down: Balanced Load

If the loads connected to each of the three phases are perfectly balanced, the neutral will remain close to ground potential, and the line-to-neutral voltages will stay relatively close to 277V. However, perfect balance is almost impossible in a real-world scenario.

Unbalanced Load

This is the common situation. When the loads are unbalanced, the neutral voltage shifts toward the phase with the least load. Think of it like a seesaw. If one side has very little weight (light load), the other side (heavier load) will pull the pivot point (neutral) closer to it.3.

Voltage Distribution

The line-to-neutral voltage of each phase is inversely proportional to its respective impedance (mainly due to load) with reference to the neutral. This means that the phase with the least load will see its line-to-neutral voltage increase, potentially exceeding 277V. The phase with the heaviest load will see its line-to-neutral voltage decrease.4.

Overvoltage and Undervoltage

The most dangerous consequence is the potential for overvoltage on lightly loaded phases. This can easily damage or destroy equipment designed to operate at 277V. Conversely, the heavily loaded phases experience undervoltage, which can also damage equipment and cause it to malfunction (motors drawing excessive current, lights dimming, etc.).

How Fast Does This Happen? Almost Instantly

The voltage shift happens very, very quickly – essentially as fast as the electrical system can react to the change in impedance. It's not an instantaneous jump to 480V for all phases, but the redistribution of voltage occurs in milliseconds. The speed is dictated by the inductive and capacitive characteristics of the wiring and connected equipment.

Delay Factors (Minor Influence) Transformer Inductance

The transformer's inductance does provide a tiny bit of buffering, but it's not significant enough to prevent the voltage imbalance.

Line Impedance

The impedance of the distribution wiring also plays a role in how quickly the voltage shifts, but again, not a substantial delay.

Why Not Always 480V L-G? While it's possible for a lightly loaded phase to approach 480V L-G if the other phases are heavily loaded, it's not a guarantee. The actual voltage depends on the degree of imbalance. The system tries to maintain the line-to-line voltage (480V) between phases. Therefore, the neutral shift distributes the voltage among the phases depending on the load connected to each.

Example Scenario

Let's say you have a 480Y/277V system with the following loads: Phase A: 10 amps Phase B: 2 amps Phase C: 5 ampsIf the neutral is lost, Phase B, with the lightest load, would likely see a significant increase in voltage, potentially well above 277V. Phases A and C would see their voltages decrease.

Consequences and Protection Equipment Damage

The overvoltage is the biggest threat. It can quickly burn out electronics, ballasts, motors, and other sensitive equipment connected to the lightly loaded phases.

Undervoltage Problems

Motors can overheat and fail, and other equipment may not function properly.

Safety Hazard

Although less directly dangerous, the voltage change can create a shock hazard from equipment cases that are nominally at neutral potential.

Solutions



Ground Fault Protection

Properly designed and installed ground fault protection systems can detect and interrupt the fault before serious damage occurs.

Neutral Monitoring Relays

These devices continuously monitor the neutral voltage and can trip the circuit if it exceeds a pre-set threshold.

Load Balancing

Distribute loads as evenly as possible across the three phases.

Redundant Neutral Conductors

Using multiple neutral conductors increases reliability.

In summary: Losing the neutral on a 480Y/277V system is a serious problem. The voltage imbalance happens very quickly and can cause significant damage to equipment. The specific voltages on each phase after the neutral is lost depend on the load imbalance and can be difficult to predict precisely. Therefore, preventative measures and robust protection systems are critical. Flag for review