Expert: Dave Nyce - 7/10/2006

Question

Thank you so much. Your answer was very helpful. Yet, I do not understand the part about the neutral and ground being connected somewhere in the system and how the polarity is reversed by crossing. Can you please elaborate what crossing means? Thank you.
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Followup To

Question -
What is electric polarity ? How can it cause electrocution? What is crossing wires(i.e. crossing neutral to hot) to reverse polarity(does it pose a hazard)? How does a GFCI work? Why can't it work without the third prong? Thank you.

Answer -
Polarity: Electrical current is the flow of electrons. An electron has a negative charge. In an electrical circuit, a conductor having a higher concentration of electrons has a negative polarity with respect to a conductor with a lower concentration of electrons.

Electrocution: If a current flows through a person, it can cause muscles to contract (such as the heart, and possibly stopping the heart). Higher levels of current also can cause tissue damage and burns.

Crossing wires: If you wire a power outlet with the hot and neutral wires reversed, it is a safety hazard. The neutral wire is connected to ground somewhere in the system. Modern appliances are wired to rely on this for safety.

GFCI: A ground fault current interruptor looks for any current flowing between the neutral and the ground wires. Since they are connected together somewhere in the system, there is normally no current flowing between those two points. If a fault causes a current to flow between the neutral and ground wires, then power to the hot wire is disconnected.

Third prong: The third prong is the ground wire. If it is missing, then the GFCI can not measure the current flowing from neutral to it.

Hope this helps!

Dave

Answer
When an electrician wires the power box, he connects the neutral line to a ground.  The ground is usually a copper rod that is driven several feet into the earth, or it can be a water pipe that goes into the earth.  This is also where the ground wire connects, and then goes to the third wire in an electrical socket.

When you asked about crossing the neutral and hot, I think that you mean to connect the hot wire to an electrical socket in the place that should have been connected to the neutral, and to connect the neutral wire to the place that should have been connected to the hot wire (don't do this!).

This does not change "polarity".  In a AC circuit, the hot wire changes polarity from + to - and back to +, at a rate of 60 times per second (in the USA).  A measurement of the voltage on the hot wire is made with respect to the neutral wire.

Hope this helps!

Dave