Electrical Wiring

First, consider single-phase 120/240 service like you would find in a house. the two hot wires are said to be 'in phase'.

Think of it this way. Imagine a ruler12 inches long. Put your finger on the six inch mark, the center. How far to each end? Six inches. Both ends are on the same line, just in opposite directions. So if you go six inches one way, and six inches the other way, the total distance is 12 inches.

120/240 volt systems are like that voltage-wise. The middle of the ruler is the neutral, and each end is a hot wire. When one hot leg is going positive, at the exact same time (that's the 'in phase' part) the other leg is going negative. Same line only opposite directions, like our ruler. So the two 120 volt legs just add up to 240.

Now, here's the important part. Since one leg goes positive while the other leg goes negative, they are said to be 180 degrees apart, 360 degrees making up one complete AC cycle (2 X 180 = 360). In three phase power, there are, not two, but THREE hot wires. Since there are three, they can no longer be 180 degrees apart. They are 120 degrees apart (3 X 120 = 360). So, when phase A peaks, phase B has not yet peaked. it is at some intermediate voltage between zero and maximum. The two phases cannot just be added up because of this fact.

Back to our ruler. Break the ruler in half. bend the two halves so the ruler is no longer straight (180 degrees) but bent at an angle of 120 degrees. Draw a line from end to end. See how we have formed a triangle? See also that the line we drew is not 12 inches long, but is shorter (the two ends are closer together)? That's why the voltages in a three phase system do not appear to add up.

Lets make our ruler 240 inches long. If we bend it into the 120 degree angle and measure end-to-end, we will find the length to be about 208 inches, not 240! In a 120/208 three-phase system, each hot leg (phase) measures 120 volts to ground, BUT phase-to-phase measures 208 volts. This is true whether you measure A to B, B to C, or C to A.

The math guys call this a vector sum. The vector is the angle at which the phases are in relation to each other (120 degrees) and the sum is the distance between the two bent ends (phases).

To figure the phase-to-phase voltage, multiply the phase-to-neutral voltage by 1.73 (the square root of 3). Thus: 120 X 1.73 = 208, 277 X 1.73 = 480, etc.

To figure the reverse, divide the phase-to-phase voltage by 1.73. Thus: 480 / 1.73 = 277, 208 / 1.73 = 120, etc.

You trig people see how this relates to our triangle example using the cosine rule.

It should be noted that the use of the term phase above, as in "phase to phase" is not strictly correct, although a common usage. Each hot conductor in a 3-phase system is correctly called a "line", so the correct terminology would be "line to line", "line to neutral", etc.

There are two types of connections in three phase systems. One is a delta connection where there is no connection to ground, so you should not get any voltage to ground. This is classed as a three phase three wire system. In this type of system any one of the phase wires could become grounded and no one be the wiser. Code requires that grounding lights be added to a delta system to visually show what the phase condition is in relationship to ground. The other type of system is a wye or star point connection. This is classed as a three phase four wire system. In this type of connection the coil ends are all joined together and grounded. You would have voltage between the phases and a lower voltage to ground. The voltage to ground would be the phase voltage divided by 1.73.

Yes. If there is no voltage between a hot and a ground, either the hot isn't really a hot or the ground isn't really a ground.

There is always a potential difference between a line conductor and ground, regardless of whether it is a three-wire system or a four-wire system. This is due to the capacitance between line and ground.

No, a 230V supply is normally single phase, 50 Hz. It is the most common supply used in homes and offices in Europe and many other areas of the world.

For more information see the answers to the Related Questions shown below.

Another View

In Europe, low voltage is distributed as three-phase, four-wire, supply system having a nominal line voltage of 400 V (line-to-line) and a nominal phase voltage of 230 V (line-to-neutral). So, yes, you could describe 230 V as being a 'three-phase' voltage.

Most residences are provided with a single-phase supply, which means that each home is connected between one of the three line conductors and the neutral conductor of a three-phase system. In some European countries, such as Cyprus, it is common for residences to have three-phase supplies, with the various circuits within the home being roughly-evenly distributed between each of the three lines and the neutral.

single phase means that there is only 1 live core in the cable (+) . so dual phase means there are 2 live cores in the cable. this is used to supply different parts of a device where needed. although three phase cable sometimes used to boost power to 400 v

Further answer

Actually it's not correct to say "boost power to 400 v". Power is measured in watts, not volts, although I agree the voltage of three-phase, at least in the UK, is 400v (I don't know about the US - they have lower domestic volatges than the UK anyway)

If, by 'dual phase', you actually mean 'two phase', then this electrical system is quite rare, and consists of two phase voltages displaced by 90 degrees, as opposed to a three-phase system in which the phase voltages are displaced by 120 degrees.

No matter whether we're describing a three-phase service or a a single phase service, the bare copper "earth" or "ground" wire normally carries no current. Its purpose is to provide an emergency path for current if ever there is any accidental contact between a hot wire and the external (or internal) metal parts of any electrical device which a user may be able to touch. The electrical device can be a motor, a water heater, an air conditioner unit or any other kind of appliance.

By carrying away the excess current in a fault condition - which should cause the protecting fuse to blow or the circuit breaker to trip - the "ground" or "earth" wire protects the building and its occupants because the power should be cut off before anyone gets electrocuted or any overloaded circuit wiring or appliances catch on fire.

The neutral is the normal "return" wire. In systems where the load is supplied from only one hot (or "live") wire, the neutral completes the circuit and carries current back from the load to the power station. In "Y-" or "star-connected" three-phase circuits the neutral doesn't normally carry any current if all three phases are properly balanced.

If the three phases actually have unbalanced loads - which can easily happen if each phase is being used to provide power to different single-phase circuits, each with their different loads - then some current will flow in the neutral wire and will result in unbalanced 3-phase currents flowing back to the power station.

All the neutral and ground (or "earth") wires in a building are tied or linked together at the incoming service main breaker panel. This is the only place they should ever be tied together because it is "upstream" of all the fuses and/or circuit breakers protecting the hot (or "live") wires for the various circuits installed in the building.

Warning: we must never assume that a neutral is safe to touch: it has to be checked with a voltmeter or a voltage indicator to be sure it is not "live". This is because a neutral wire is designed to carry current under normal circumstances.

So, if a neutral wire going back to the incoming main breaker panel has not been properly connected - or suffers a deliberate disconnection or some accidental damage which causes it to break - then it and any neutral wires connected to it further downstream will go live up to the break because of being connected to the downstream loads which still have hot feeds coming into them!

That is why we should never use a neutral as a substitute for a proper, separate, ground or "earth" wire.

<><><>

If some external accidental damage or electrical breakdown of the wiring's insulation occurred anywhere to the house wiring, to a socket outlet or to an appliance, these things could be very dangerous if there was no such protective wire.

For example, if there was no protective ground or earth wire, a fault could happen that is of a kind which did NOT draw enough extra current to blow a fuse or make the main circuit breakers on the incoming supply panel "trip" to cut the current off - but the wiring could still catch on fire and/or someone could be electrocuted!

Neutral wires are the return paths to the power generation station for current it supplies to the house or building via single live or "hot" wires in the branch circuits.

For more information please click on the Related Questions below.

<><><>

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.

An electric meter or energy meter is a device that measures the amount of electrical energy supplied to or produced by a residence, business or machine.

The most common type is a kilowatt hour meter. When used in electricity retailing, the utilities record the values measured by these meters to generate an invoice for the electricity. They may also record other variables including the time when the electricity was used.

Modern electricity meters operate by continuously measuring the instantaneous voltage (volts) and current (amperes) and finding the product of these to give instantaneous electrical power (watts) which is then integrated against time to give energy used (joules, kilowatt-hours etc). The meters fall into two basic categories, electromechanical and electronic.

The electromechanical induction meter operates by counting the revolutions of an aluminum disc which is made to rotate at a speed proportional to the power. The number of revolutions is thus proportional to the energy usage. It consumes a small amount of power, typically around 2 watts.

The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil. [1]This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc - this acts as a brake which causes the disc to stop spinning when power stops being drawn rather than allowing it to spin faster and faster. This causes the disc to rotate at a speed proportional to the power being used.

The type of meter described above is used on a single-phase AC supply. Different phase configurations use additional voltage and current coils.Reading

The aluminum disc is supported by a spindle which has a worm gear which drives the register. The register is a series of dials which record the amount of energy used. The dials may be of the cyclometer type, an odometer-like display that is easy to read where for each dial a single digit is shown through a window in the face of the meter, or of the pointer type where a pointer indicates each digit. It should be noted that with the dial pointer type, adjacent pointers generally rotate in opposite directions due to the gearing mechanism.

The amount of energy represented by one revolution of the disc is denoted by the symbol Kh which is given in units of watt-hours per revolution. The value 7.2 is commonly seen. Using the value of Kh, one can determine their power consumption at any given time by timing the disc with a stopwatch. If the time in seconds taken by the disc to complete one revolution is t, then the power in watts is . For example, if Kh = 7.2, as above, and one revolution took place in 14.4 seconds, the power is 1800 watts. This method can be used to determine the power consumption of household devices by switching them on one by one.

Most domestic electricity meters must be read manually, whether by a representative of the power company or by the customer. Where the customer reads the meter, the reading may be supplied to the power company by telephone, post or over the internet. The electricity company will normally require a visit by a company representative at least annually in order to verify customer-supplied readings and to make a basic safety check of the meter.Accuracy

In an induction type meter, creep is a phenomenon that can adversely affect accuracy, that occurs when the meter disc rotates continuously with potential applied and the load terminals open circuited. A test for error due to creep is called a creep test.