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The answer to Is the neutral wire considered safe? states:

When everything is working correctly, it should be at most a few volts from ground. However, and this is the big gotcha, if there is a break in the neutral line between where you are and where it is connected back to ground, it can be driven to the full line voltage. Basically in that case you are connected to the hot line via any appliances that happen to be on in that part of the circuit. Those can easily pass the few mA it takes to kill you.

And something similar is stated in an answer to this question:

A second problem with connecting the ground to the neutral happens if your neutral wire breaks between the outlet and your service entrance. If the neutral breaks, then plugged in devices will cause the neutral to approach the "hot" voltage. Given a ground to neutral connection, this will cause the chassis of your device to be at the "hot" voltage, which is very dangerous.

Can someone shed light on this behavior? Specifically, how a break/disconnect in the neutral wire can suddenly cause it to have full voltage?

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Here is a simplified version of AC power lines:

schematic

simulate this circuit – Schematic created using CircuitLab

The two lines at lower right represent you holding the neutral. Since the neutral is connected to ground elsewhere, as you agree you only feel a small voltage. Note that the load is fairly low resistance, so it can produce a lot of power.

Now let's break the neutral

schematic

simulate this circuit

Current will now flow through the upper hot wire, through the load, through you and then the ground in order to get back to the transformer neutral. And since the load has a low resistance, you are the big resistor in the circuit, and you will take most of the voltage.

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Consider this:

schematic

simulate this circuit – Schematic created using CircuitLab

With the neutral wire broken, the "Neutral" at the socket will be at 120VAC relative to ground.

If another 1500W kettle was plugged in on the other side of the line, the voltage at the neutral would be around 0VAC relative to ground, however if a human were to contact the neutral wire and had a path to ground through their body, the voltage would not drop all that much and the person could be injured or killed.

If a 60W incandescent bulb was connected between Hot2 and Neutral, it would glow very brightly and quickly burn out. That's why changes in brightness of incandescent bulbs can be seen as a canary-in-the-coalmine for a flaky neutral connection.

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Essentially it won't be grounded and since the signal is AC it will be the line voltage (work isn;t being done in the incomplete circuit and therefore remains the same voltage as it was on input.) Once the connection is completed (by either fixing the neutral or your body), the voltage will measure correctly at the same point and power will flow.

Purpose of Neutral

Because electric utilities are not required to install an equipment grounding conductor to service equipment, a grounded (neutral) conductor shall be run from the electric utility transformer to each service disconnecting means and this conductor shall be bonded to the service disconnect enclosure [250.24]. The grounded (neutral) service conductor serves as the required effective ground-fault current path necessary to ensure that dangerous voltage from a ground fault will be quickly removed by opening of the circuit protection device [250.4(A)(3) and 250.4(A)(5)].

Dangers of Open Service Neutral

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  • \$\begingroup\$ This is US centric, so consult your local codes... but they do esentialy the same thing. \$\endgroup\$ – Spoon Jul 23 '14 at 23:42

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