illiad wrote:240V in USA?? or is this just for 'industrial' devices, not domestic??
The US uses a dual-120 arrangement. The "pole pig" distribution transformer has a 240-volt secondary winding, with a "tap" in the center. A three-wire drop cable is run from the transformer to the building's service entrance, at which point the power goes through a meter and a double-pole master circuit breaker.
The two wires from the two "hot" ends of the secondary each provide 120 volts (180 degrees out of phase with one another), with respect to the center tap (known as the "neutral"). Typically, about half of the circuits in the house use one 120-volt side, and the other half use the other 120-volt side. The neutral goes out to all outlets. Also, the neutral is connected to the power system ground (which is also wired out to all of the outlets, and connected to a ground rod hammered into the soil, or to a Ufer ground in the foundation)... and this ground-to-neutral connection is supposed to occur right at the main circuit-breaker panel, and nowhere else.
High-power appliances (electric stoves, dryers, heaters, etc.) usually use 240 volts - they connect to the two "120-volt" feeds and not to the neutral.
So, this isn't a three-phase industrial system... it's a two-phase (180-degrees-out-of-phase) system.
Buildings which need a true three-phase power (commercial/industrial) are wired up differently - they need a different sort of distribution transformer.
This setup makes life very interesting indeed, if something happens to the neutral connection from the transformer. If the neutral wire breaks or comes loose, you end up with the two "120-volt" halves of the house being effectively connected "in series" across a 240-volt supply in an unbalanced fashion... and this means that the voltage seen by each circuit varies all over the place, depending on which devices are drawing current. If you turn on a heavy load on one circuit (say, you start the microwave, or the refrigerator motor powers up), the voltage on that side of the house will sag well below 120, and the voltage on the other circuit increases!
Very bad - this can and does cause damage: it can burn out light bulbs and motors and electronics due to either under-voltage or over-voltage, and it's not unknown for electrical fires to occur due to overheating. The power companies here consider an "open neutral" to be an emergency situation. It's happened at my house twice over the last 20 years (in both cases, a squirrel had chewed through the drop-wire insulation and cut the neutral wire!), and in both cases they had a truck out at our house within 30 minutes to start the repair.
no fuse in plug?? could that be why it is so easy to blow the whole house??? in UK, just the plug fuse would go, no problem!
Nope, the U.S. has never gone for fuse-in-plug as far as I know. Individual electronic components often have fuses, and each circuit from the distribution panel has a circuit breaker (sometimes with ground-fault or arc-fault protection) but most lights and appliances do not have their own fuses.
The U.S. plug body is a lot smaller than the U.K version, and doesn't really have room for a fuse.