Beware of double voltage i.e. 220 vac instead of 120vac input ,when connecting separate computer components with USB, Firewire, TV -S connector or sound cables such as a printer, monitor, external hard drive or home theater system. This is especially noteworthy when the remote components are in separate room of the house or across the room and plugged into a separate outlet.

Electrical power is supplied to your house from the utility company with three cables; two power and one common. The power cables are identified as line one and line two. The voltage between line one and common is 120VAC . The same is true for line two at 120VAC. The voltage between line one and line two is 220VAC. The three cables are routed into the circuit breaker box and connected to separate buss bars. The breakers are plugged into the box and make connection to the buss bars for line one and line two. The arraignment of the breakers are alternating between line one and line two.

That is the first breaker is connected to line one, the second is connected to line two the third is connected to line one and so on to the bottom. The same arraignment is on the other row of breakers. If you need 220VAC for an electric dryer, an A/C unit or range, you connect the wires to adjacent breaker, so each breaker is supplying 120VAC and the common goes to the grounding screw on the appliance cabinet.

When connecting wiring for wall plugs, the black wire of the 12-3 romax cable is connected to the breaker, the white wire is connected to the common buss bar and the bare wire is connected to the ground buss bar. Collectively, the power to that wall plug is 120VAC, with a ground. At the wall plug, the black wire in connected to the receptacle opposite side from the (green) grounding screw. The bare wire goes to the green ground screw. The white wire is connected on one of the screws located on the same side as the grounding screw.

This provides 120VAC of the correct polarity to the appliance. The receptacles are keyed to prevent plugging a cord in the wrong way. There are two slots and one hole, the common wire was connected to the wide slot, the power was connected on the narrow side and the ground (bare) wire was connected to the round hole.

Now comes the problem. Some of the wall plugs in the house are on line one while some others are on line two, and each is connected correctly and supplying 120VAC respectively.

If your computer is plugged into a receptacle that is on line one and some interconnected device, such a your home theater system is plugged into a receptacle that is on line two, you could have a real problem.

While each unit is correctly connected to a 120VAC outlet, because of the interconnection wiring, collectively the two units are experiencing 220VAC and the electronics of both devices could be burned out, or happily blow the internal fuse.

A GFCI (ground fault circuit interrupter) receptacle is a good way the protect your electronic equipment, but even with a GFCI in place, when the two units are plugged in to different power lines and the devices are turn on, the GFCI will trip off.

Make sure both receptacles are on the same line.It doesn’t matter which, line one or two, but it must be the same line. If your are electric appliance skilled, you could use a volt meter between the two receptacles and test for a mutual connection. Set the meter on 200 + VAC scale and plug one meter lead into the narrow slot of one receptacle. Get a small wire that is long enough to reach to the other receptacle and plug it into the narrow slot there. When the small wire is connected to the other meter lead, it should regester zero.

If not, you have two options, individaully plug either device into another receptacle and test again. If that fails it is time to call an electrician to come out and move one of the breakers to another place in the breaker box.

My first time to blog at this site.I hope this information is of some use to you.

Best regards. Bob T.

Ed: Great stuff Bob! Thanks for contributing!