Will an appliance metal case get alive if touched by the neutral wire?

Question

My phyiscs textbook only discusses what will happen if a live wire touches a metal case and how it turns alive, however, I was wondering, will the same effect happen to the metal case if touched by the neutral wire instead?

Answer 1

As pointed out in the comment by @rghome, electrical power systems vary around the world. With that in mind, I can only comment with respect to the power systems common in the U.S.

The neutral wire, or more properly called the grounded circuit conductor in the US, is theoretically at ground potential since it is connected to the grounding bus in the service panel along with the protective grounding conductors (third wire in a 120 vac branch circuit). However, the grounded circuit conductor can be a few volts above ground simply due to the voltage drop across the impedance of the conductor between the appliance and the service panel where return current flows. So, under normal conditions, if the neutral wire touches the appliance metal housing it would not constitute a shock hazard.

The problem occurs if there is an open circuit in the grounded circuit conductor between the appliance and service panel. Such an open circuit does not necessarily require a break in a conductor, but could also occur at the various wire connections in the branch circuit, such as wiring terminals of receptacles. If such an open circuit occurs, it could raise the voltage on the housing to line voltage levels and involve a risk of electric shock.

It is because of the possibility of an open circuit or high impedance in the grounded circuit conductor between the appliance and service panel that the conductor is not intentionally used to "ground" the appliance enclosure for safety. The separate equipment grounding conductor (third wire) is used for that purpose. An exception, at one time in the NEC, was the grounding of housings of appliances that are connected to a dedicated circuit (circuit only serving that appliance), such as electric ranges and clothes dryers. But if I'm not mistaken, that exception no longer exists.

UPDATE:

To show what can happen if the neutral (grounded circuit conductor) is used to provide a safety ground for an appliance on a branch circuit instead of the equipment grounding conductor (the proper safety ground) refer to the figures below;

Figure 1 shows three appliances on a branch circuit. The metal housings of each appliance is connected to the equipment grounding conductor (safety ground), or the third wire, of the branch circuit serving the appliances. In the event of a fault between the hot (live) conductor and the housing, the fault current is carried to ground on the equipment grounding conductor limiting the voltage the person might experience if simultaneously touching the housing and some other grounded part (the earth, another grounded appliance, etc..). Note that if there is an open circuit somewhere in the equipment grounding conductor between the last appliance and the service panel, all that happens is a loss of a safety ground in the event of the fault shown. The open circuit does not in and of itself cause a hazardous condition.

In Figure 2 the third appliance is grounded by means of the grounded circuit conductor (neutral conductor). Note that if an open circuit occurs in the branch circuit neutral conductor (as shown in the connections to the second appliance) the load current in the third appliance cannot return to the service panel through the neutral conductor. It has to return through the person's body. The only impedance limiting that current is the load impedance of the third appliance, which may be several amperes. Since for 60 hz sinusoidal current it only takes between 20 mA and 100 mA of current through the body to possibly cause a lethal electric shock and since the body impedance can be as low as 500 Ohms, a lethal electric shock is possible.

Hope this helps.