In wiring of a building which is kept common neutral or phase?

Answer 1

In mains power generation terminology power is commonly generated at the power station in three separate "phases". The "Hots" ("Live" wires in European parlance) of each of the phases are timed to "lag" each other in wave/cycle terms as follows: the Hot of the first phase is generated at 0o (Zero degrees, which is equivalent to 360o), the Hot of the 2nd phase is timed to lag the Hot of the the first phase by 120o and the Hot of the third phase is timed to lag the Hot of the first phase by 240o.

[In Europe, where power is generated at 50Hz, the "Hots" (labelled as lines L1, L2 and L3) making up each of the three phases commonly have the colors Brown for L1, Black for L2 and Grey for L3; the Neutral wire is Blue. (These are equivalent to the UK's old wiring regulations for 3-phase supply cable of Red, Yellow and Blue where the Neutral wire was Black.) In the US or other countries which use 3-phase power generated at 60Hz the colors for the hots are generally black, red and yellow and neutral is white but other colors such as orange and blue may also be used to differentiate between separate circuits.]

In electrical wiring terminology each "Phase" of a single phase circuit really comprises a pair of wires. The pair is made up of a "Hot" and a "Neutral". (A "Live" and a "Neutral" in European parlance.)

The number of phases delivered from the street to a building depend on its power usage requirements: an industrial building or site which uses 3-phase power for powerful motors, ovens, etc. must have its electricity delivered as 3-phase power; commercial offices or domestic homes typically only use single-phase lights and appliances, so such homes and offices usually only require single-phase power to be delivered from the street.

Homes in the US typically have a single phase service which is "split" into two "hots" with a common "neutral" wire - that comes from the center tap point of the secondary winding on the nearby service voltage-step-down transformer - in order to support high-power 240 volt appliances, such as electric dryers, ranges and ovens. The voltage across the two "hots" is 240 volts while the voltage from any hot to neutral is 120 volts.

So, to answer the main question asked:

All the Hot wires for each circuit within a phaseare kept common to each other within a given building.

and

All the Neutral wires for each circuit within a phase are kept common to each other within a given building.

Under current wiring regulations and codes the hot and neutral wires for each circuit within a phase that is installed within certain designated areas of a building must be protected by a Ground Fault Current Interruptor (GFCI), also known as a Residual Current Device (RCD). That is a sensitive current-breaking device (also known simply as a "circuit breaker" or a "trip") which continuously monitors for significant current imbalances between the Hot and Neutral wires and/or a significant fault current flowing in the protective Ground wire. If any current imbalance or any fault current is detected the protective device must break the supply of power to the circuit within a maximum allowed time. To protect human life a typical design would be engineered to trip within 200 mS (mS = milliSeconds = thousandths of a Second) on detecting an Earth leakage current of 30 mA ( mA = milliAmps = thousandths of an Amp) and to trip in less than 40 mSec at a higher Earth current of 150 mA.

All Ground wires for the various circuits within a building are bonded together only on the supply side of any protective devices. They are connected to a reliable Ground point (Earth in European parlance) at the location where the mains supply enters the building.

[Older buildings, which were wired up under older wiring regulations and codes where no GFCI or RCD was specified, merely have all white neutral/ground wires bonded together and connected to a reliable Ground point (Earth in European parlance) at the location where the supply enters the building.]

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.