Household Circuits, Safety and Static in Practice

A pole or pad transformer steps the local distribution voltage down and sends three conductors into the house: two live conductors and one neutral. Each live conductor sits at $120\text{V}$ relative to neutral, and the two are opposite in phase, so measuring across both gives $240\text{V}$. That single arrangement supplies everything in the building. Those conductors land in the service panel, the grey box with the main breaker, which splits the incoming supply into branch circuits. Each branch leaves through its own breaker to feed one part of the house, and nothing in the building connects to the supply except through one of them.

Live and neutral are the working pair: every electron that leaves through one comes back through the other, so in a healthy circuit they carry identical currents. The ground wire does nothing at all until something goes wrong. It is bonded to the metal case of an appliance and runs back to a rod driven into the earth, so it is a spare path held at zero volts.

• Every outlet gets the full supply voltage — a true $120\text{V}$ at each one. In series the supply would be shared out, and each appliance you added would dim the rest.
• Devices switch independently. One lamp turned off is one open branch; the others are untouched. In series, one failure would darken the house.
• The rules are familiar. Voltage is the same across every branch and the branch currents add to the total: $I_T=I_1+I_2+\cdots$.
• The cost is the current. Every extra appliance lowers the total resistance and raises the total current in the wires feeding that branch — which is exactly the quantity a breaker is installed to limit.

Wire heats as current passes through it, and the power turned into heat grows with the square of the current, so doubling the current makes four times the heat. Push enough current through the wiring buried in a wall and the insulation chars and ignites. A fuse or a circuit breaker is therefore a deliberate weak point: the one place in the circuit designed to fail first, cheaply and safely, before the wiring can fail expensively and dangerously. Both devices are wired in series with the branch they protect, because only a series element can interrupt the whole path. A fuse contains a thin metal strip that melts at its rated current. A breaker uses a bimetallic strip that bends as it warms, for slow overloads, plus an electromagnet that snaps the contacts open almost instantly when a huge fault current appears.

A breaker cannot protect a person. It is watching for the tens of amperes that would cook the wiring, and a current far too small to move a $15\text{A}$ breaker at all is more than enough to stop a heart. Bathrooms, kitchens, garages, basements and outdoor outlets therefore use a ground fault circuit interrupter, the outlet with the test and reset buttons. A GFCI compares the current flowing out along the live wire with the current returning along the neutral. In a healthy circuit they are identical, because every charge that leaves must come back. If some current is leaking away by another route — through a person, through wet concrete, through a puddle — the return is smaller than the outgoing. A difference of about $5\text{mA}$ makes the GFCI open the circuit in roughly $25$ milliseconds, long before that current can do lasting harm.

Voltage on its own does nothing to you. It is the current pushed through your body that interferes with nerves and with the electrical rhythm of the heart. A carpet shock can be twenty thousand volts and is merely startling, because the charge available is tiny; a $120\text{V}$ outlet can be lethal, because it will push current for as long as you are in contact with it. How much current flows is simply Ohm's law, $I=\frac{V}{R}$, with your body as the resistor. Dry, unbroken skin is a fairly good insulator at roughly $100000\Omega$ from hand to hand, while wet skin is around $1000\Omega$, because water carrying dissolved salts conducts and because moisture opens up the skin's surface. The voltage does not change; the resistance falls by a factor of a hundred, and the current rises by the same factor.

Charge separated by friction and left with nowhere to go is static electricity, and once the potential difference is large enough the air itself breaks down and the charge jumps across. That single idea explains the spark from a doorknob, the lightning over Lake Ontario, and the failure of a memory module handled carelessly. It is worse in an Ontario winter because humid air lets charge leak away gradually as fast as it is generated, while heated indoor air in January is bone dry and lets it accumulate instead.

• Lightning — colliding ice crystals in a storm cloud separate charge until the potential difference to the ground reaches tens of millions of volts. The air ionizes and an enormous current flows for a few ten-thousandths of a second.
• Lightning rods — a grounded metal point at the highest part of a building, wired to earth with heavy conductor. It does not attract strikes so much as provide a safe path, and its sharp tip bleeds charge quietly into the air so that the difference often never reaches breakdown at all.
• Electrostatic discharge — a spark you can barely feel is thousands of volts, and the microscopic layers inside a processor or memory chip break down at far less than that. Technicians work on a grounded mat with a wrist strap, so charge drains away continuously rather than arriving all at once.
• Fuel-truck grounding straps — fuel flowing through a hose builds up charge by friction, and a spark near petrol vapour is catastrophic. A bonding cable clipped between truck and tank puts both at the same potential, so no difference can build and no spark can jump.
• Photocopiers and laser printers — here static is used on purpose. A laser writes a hidden pattern of charge onto a drum, oppositely charged toner powder is attracted to exactly that pattern, the powder is transferred to charged paper, and heated rollers fuse it permanently.