North American Homes make use of a three wire supply that has many good points when installed and maintained correctly properly. However, there are problems with this system, especially when work is done by "do-it-yourself" individuals. There are two basic problems.
In North America, the supply of electricity comes to the home as three wires. A ground connection is created right at the home by use of driven ground rods, buried plates or a metal water line. The third wire is bonded to this ground connection. The three wires come from the "center tapped" secondary winding of a transformer. Two outer legs have 240 volts across them. The third wire is the center tap so there are 120 volts between this wire and either of the other two wires. The third wire carries only the unbalanced load and not the full load current. The center tap is connected to a grounding conductor at the transformer. The frequency is 60 Hertz.
Since there are two "Live" wires, there are two main fuses or, more usually, one Double Pole circuit breaker. There is no doubt that the North American system attempts to provide more safety than the European system. Portable appliances and light bulbs use 120 volts. Fixed appliances and heavy heating loads use 240 volts. In Europe, light bulbs and portable appliances use 240 volts at 50 Hertz on a two wire system.
What are the problems with this system?
Well, the first problem comes because we need to use double-pole circuit breakers for the fixed load circuits. If this is done properly there is no problem. However, there are cases where two single-pole circuit breakers are used instead of one double-pole breaker. If one of these single pole breakers is tripped or switched off, the appliance stops working. The appliance is still "Live" because the second circuit breaker is still on. Trained people know how to test for this and avoid a shock. The do-it-yourself individual may receive a shock. If the correct circuit breaker is used, both poles will trip and there is no problem. This is one of those cases where "making it work" does not mean "making it safe"
A typical situation may involve a person who has decided to move an electrical baseboard heater (perhaps to install new carpet or tile). This person may lower the setting of the thermostat to the lowest setting. This will stop heat from being produced but there is still one live wire at the heater. Most untrained people would assume that there are no live wires present just as would be the case at a light fixture that is switched off. If you look at a 240-volt thermostat you will notice that it probably does not have an OFF position marked. This is because it must completely disconnect the heater if it says OFF. In other words, it must be a 2 pole thermostat. Most thermostats are only a single pole.
The second problem is caused by a poor neutral (third wire) connection. The severity of this problem depends on where the poor connection is located. For example, if it happens at a receptacle (plug) in the kitchen you may destroy a small appliance. This is how it happens. Remember that we have 240 volts available. In a kitchen plug, each half of the duplex receptacle is on a different "live" leg and they have a common neutral (3 wires).
If the neutral wire is disconnected or has a poor connection, we have two 120 volt appliances plugged in to form a series circuit. Suppose that we have a clock radio plugged into the bottom outlet and a kettle plugged into the top outlet. The clock-radio has a resistance that is about 30 times (and maybe more) that of the kettle. When the neutral is removed from the circuit, we have a series circuit and the current is common to both appliances. The voltage drop across each appliance will be the common current multiplied by the resistance. If the clock-radio has 30 times the resistance, it will have most of the 240 volts across it. We will get something like 8 volts across the kettle and 232 volts across the clock-radio. Guess what happens to the clock-radio.
The U.S.A. has dealt with the kitchen receptacle problem by using 20 Amp 120 Volt two wire circuits, whereas Canada still allows 3 wire circuits for the kitchen with the option of using 20 Amp 120 Volt circuits.
If the poor connection is back at the main panel and on the incoming supply, many devices that draw low current can be destroyed. This includes a lot of expensive computer equipment, sound equipment, telephone equipment, etc. If you have a large load on one "live" leg and a relatively small load on the other "live" leg there will be damage. If your light bulbs seem to burn out too quickly, this could be the cause of the problem. The problem is more common in homes that use Aluminum wire. More on aluminum wiring later.
Many untrained individuals do not fully understand how important the grounding connection is to the system. People have been known to remove a main grounding wire while gardening because they thought it was not important (only a ground).