Wow, there is a lot more variation in Europe than I imagined.
Luckily, there is some consistency across the US, especially in residential wiring.
Most US residences are fed from single phase transformers, with a 240 volt secondary coil and a center tap. Line-to-line is 240 volts, and either line to the centertap (neutral) is 120 volts. The centertap of the transformer is grounded at the transformer, and another ground rod is installed where the electricity enters the house.
The two lines and the neutral are brought into the home (3-wires), and the ground and neutral busses are bonded at one point only, which is in the service entrance panelboard. The neutral and ground wires are always completely isolated from each other except at the since bonding point in the service entrance panel, since it would be dangerous to do otherwise.
We don't do relays in residential work, but use electromagnetic breakers or fuses only. Generally fuses are only seen in the older homes.
The problem with fuses is that you can blow only one of two fuses that feed a 240 volt load, whereas a two-pole breaker will open both lines.
Land in the US is so far flung and rural that we generate at 3-phase, but split the circuits for residential/rural areas with small loads, so that only one phase feeds a given area. That allows the power company to bring only one phase wire through an area.
Three phase distribution is generally via a grounded "Y" system, and the neutral is brought along the pole below the crossarm for the three phases. The neutral is grounded at every third pole or so (multi-grounded neutral) to help stabilize the phase voltages.
Our thermomagnetic breakers have two elements. One element is the instantaneous function coil, which trips on sudden and large faults, and the other is an overload element, which is a bimetalic thermal element which trips the breaker on long-time overloads that may be just over the rated current of the breaker, and which would not otherwise trip the instantaneous element.
Industrial breakers are generally thermalmag for the smaller sizes, and electronic for the larger sizes, with ground fault required for breakers 1,200 ampere and larger on Y systems. Industrial breakers generally have dials to adjust the settings for long-time, short-time, instantaneuous, and ground fault.
The only place we use DPDT (other than as a relay in control systems) is for transfer switches for switching loads to generator sets.
I have not seen a DPDT breaker used in the US, and I am not sure why you would use this to feed a load under normal conditions.
Most appliances in the US are either 120 volts, which use Line-Neutral-Ground, or 240 volts, which use Line1-Line2-Ground.
240 volts referred to in residential work is generally assumed to be 240 volts, 1-phase.
Appliances line clothes dryers use 240 volts and 120 volts, and a feeder to this appliance is L1, L2, N, G, which allows the heating element to run off 240 volts, and the motor to run off of 120 volts. The motor is generally has separate overcurrent protection or overload protection in the dryer since the breaker feeding a dryer is generally 30 amperes, 2-pole, which is too large to protect the motor.
The home machinist folks in the US have to purchase phase converters to get 3-phase in the home, since the power companies generally do not provide 3-phase for residential services.
Transfer switches used with generator sets are generally two switches built into a common cabinet and tied mechanically, to allow the load to be transferred between one of two sources. Transfer switches are normally break-before-make to avoid paralleling the genset with the utility company power.
Large industrial installations sometimes use break-before-make transfer switches if they want an uninterrupted load transfer, and can afford the additional paralleling relay system for synchronization with the power company. You also need permission from the power company with this arrangement.
Most residential panelboards are in the 200 ampere size, and the better panels have a 2-pole, 200 ampere main circuit breaker (thermal-mag).
Breakers feeding 240 volt loads are suppost to have their handles tied (from the factory), and the inductrial breakers have internal ties for multi-pole breakers.
Using two separate single-pole breakers to feed a 240 volt load is a bad idea since one may trip while leaving the other energized. Our residential panelboards have staggered bussing in them to give 240 volts between any two adjacent poles, otherwise the 2-pole breaker would not give 240 volts.
GFCI is being used more and more in residential work, since it limits stray fault currents to below 10 mA (the level at which the heart is affected).
Arc-fault breakers are also now being required in some areas of residences, but I do not believe these types of breakers are ready for main stream application, and can give false trips to any load that generates noise such as electronic lighting ballasts, VFD's, computer power supplies, noisy motors, etc. (noisy being electrical noise, not acoustical).
In the industrial world in the US, generally the 408Y/277 volt and 208Y/120 volt systems are used in new installations, with a dry-type transformer used to derive the 208 from the 480.
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