Most residential homes in the US are fed from a single phase 120/240 volt transformer, which has two lines and a neutral. The neutral is grounded at the transformer and at the electrical service entrance location.
The neutral and ground bus are bonded together at the electrical service entrance panel only, but not any other downstream panelboards (which would create a dangerous situation).
For most residential use, line-to-line voltage is 240 volts, single phase, and line-to-neutral or line-to-ground voltage is 120 volts., single phase.
It is generally a good idea to use separate junction boxes, or junction boxes with dividers when two separate circuits each fed from a different line (L1 and L2) are fed into the same box, such as when you have two receptacles side-by-side fed from differnent 120 volt circuit breakers, since the voltage between the adjacent bare screws on the sides of these two receptacles is 240 volts.
Many people confuse the neutral wire, which is a current carrying conductor with the ground wire, which only carries current during fault conditions. Ground wires are often not insulated in residential wiring after the outer insulation jacket is stripped back, and using a ground wire as a neutral creates a dangerous situation, since anyone or anything that contacts the bare ground wire that is being used as a neutral can receive the full voltage and current of that circuit.
A 120 volt ground interrupting receptacle can be installed at the beginning of a circuit, and if connected properly, it can protect all downstream 120 volt receptacles on the same circuit. I prefer the 20 ampere GFCI receptacles since many devices in the shop need that much capacity.
I have used GFCI type circuit breakers, but they are not very reliable (in my experience).
The European systems save a lot of copper by using the higher voltages, since doubling the voltage reduces the current by 1/2 in a single phase circuit.
Steel mills and refineries in this area generally bring in 161 KV circuits, and then break those down into 35 KV distribution circuits, which are further reduced to 480 volts, etc. using unit substations with oil or dry type transformers.
Large shipping facilities use 23 KV distribution here, and many large plants use 12.47 KV distribution systems with outdoor metal enclosed switchgear.
In the commercial/industrial world in the US, most are standardizing on the 480Y/277 volt systems, with 480 volts feeding motor loads, and 277 volts feeding lighting circuits, and then 208Y/120 volt dry-type step down transformers for small loads, with the dry-type being a delta-wye with the secondary neutral being connected to the building steel.
Rural utility cooperatives are beginning to standardize on the modern transformer voltages and connections, but many still use obsolete and dangerous connections such as the corner grounded delta (either 240 volt, 3-phase, or 480 volt, 3-phase). Three phase motors used in remote rural areas used for intermitant loads such as well pumps are often fed from an open-delta transformer connection, which derives 3-phases from only two single-phase transformers.
Some obsolete rural systems use three 240 volt single-phase transformers, with one transformer being oversized and having a centertap to provide 120 volts. Mixing up the conductors in this type system is very dangerous since you can get a whole lot more than 120 volts with the wrong connection.
There are a large number of transformer connection types (delta-wye, wye-wye, grounded, non-grounded, high resistance grounded) and each has its own particular characteristics. You cannot safely work on an electrical system without a complete knowledge of transformer connections.
Another common mistake I see throughout the industry is to use the wrong nomenclature to describe a circuit voltage.
A 120/240 system is a single phase residential-type connection. A 208Y/120 volt system is a 3-phase system, and totally different from a 120/240 volt system, and yet in this region, the nomenclature 120/240 is commonly used to describe both systems.
The Code in the US allows the use of metallic conduit instead of a grounding conductor. This practice is dangerous, since you will almost always have either a discontinuity in a part of the conduit system, or the resistance of the conduit will be so high that during a fault condition, the circuit breaker will never trip, and the equipment frame can be energized and extremely hazardous. A separate equipment grounding conductor should be used with every circuit.
Aluminum wiring should be avoided unless you are a utility company, and use it in an outdoors application, and terminate it with the correct compression lugs and de-oxidizing compound. I have seen high quality indoor aluminum wiring overheat just because the terminations would not stay tight, or the terminations were not cleaned and deoxidized, or the terminations built up high resistance (causing high heat) due to oxidation of the aluminum.
That is the story on the most commonly used US systems.
Pat J
I showed him how to install a dual contact relay to fix it. That was a serious accident just looking for an opportunity.
