Correct scheme of Neutral,Live and Earth system?

Discussion in 'General Electronics Chat' started by alphacat, Dec 2, 2009.

  1. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Hey.

    I sketched a scheme of the home distribution system including the wires resistances and was wondering if i was right about it.

    Please have your opinion on it.
    Thank you very much.

    [​IMG]
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Earth and neutral should not be connected at the wall outlet

    The earth terminal should be separately connected to earth at the home's end. There is no direct connection between the home's earth and the transformer earth.

    Review the resistance in the neutral line. I doubt you can actually quantify the resistances of the feed lines.

    Not sure what Voff and Vamp represent?

    In theory there should be no rsistance between the neutral and the connection to earth at the transformer. In practice there is some, which is measured, not calculated.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Studiot is correct, Neutral and Ground are only connected once, and are not connected again. Your setup can create ground loop, where current flows between ground and neutral.

    Think about the following scenario, you have a line that has 10A on it. This means the neutral also has 10A (ground should be 0A). If there is 0.1Ω resistance in that line you will have 1V measured at the wall plug (which is a normal situation). If you connect ground there there will be some current flowing through neutral too.

    A ground fault outlet would not hesitate to blow in that situation, which is what it is supposed to do.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, wait a minute. ;)

    In the USA, the power company's transformer has three outputs; L1, L2, and Neutral.
    L1 and L2 are "hot" or "live". They are equal in amplitude (120VAC RMS) but opposite in phase (180° out).

    The electric company connects the Neutral to earth ground at the transformer.

    Once the three lines (L1, L2, and Neutral) get into the home's electrical service/distribution/breaker panel, Neutral is also connected to Earth Ground. The Earth Ground connection is usually via an AWG 4 solid copper wire that is connected to two 8-1/2' copper-clad grounding stakes that are driven into the earth at least 6' apart.

    This is the only place inside the home that Neutral and Earth Ground are connected.

    The earth ground in the electrical service/distribution/breaker panel is for safety; in case the power company's ground fails.

    All wire has some resistance. Here is an online calculator for wire size and resistance:
    http://www.stealth316.com/2-wire-resistance.htm
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Is this true throughout the United States, or do you also use 3 phase and only take one phase into domestic properties, like Europe ?
     
  6. Wendy

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    Mar 24, 2008
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    3 Phase in the USA is mostly industrial, for homes we don't have actual multiple phases.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    In my area (which is somewhat rural) the power company's lines are 3-phase 60Hz with neutral, somewhere between 11kv and 22kv. However, the neighborhood power transformers just use one of the three phases and Neutral on the primary. The secondary winding is split-phase. Without the Neutral wire, we would just have 240VAC 60Hz.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I wasn't suggesting that individual homes had 3 phase, that is very uncommon in Europe too.

    Prior to the 1920s most AC supply was known as the 3 wire sytem (L, N, E, single phase).
    All consumers were on the same phase.

    During the 1920s this was replaced by a 3 phase distribution system (3 phases of 240 volts, L,L,L,N,). All three phases were brought to a locality substation and the (3 phase) transformer neutral earthed there.
    Different properties, street lights etc were supplied from a different phase to balance the load.
    Often the supply cable was metal armoured and also earthed at the substation. This armour was often used by consumers as their domestic earth, but the supplier has never been under an obligation to provide one. If the supplier doesn't supply an underground cable or the earth tests are not good enough it is the consumer's responsibility to provide the earth.
     
  9. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Just for info, the UK domestic scheme is called 'Protective Multiple Earthing'.

    Each house is fed via a two conductor cable, one being the live feed and the other being both neutral and earth.

    The cable (which is typically coaxial style) is connected to a fuse-plus-terminal unit which has separate live, neutral and earth connections to the electricity meter and on to the masterswitch-plus-fuses board (known as a 'consumer unit').

    Generally the earth busbar in the consumer unit will also be bonded with heavy cable to both the incoming water main (where copper - or lead - pipes are in use) and also the gas supply pipe if gas is installed.

    As the neutral is earthed at multiple locations (over many properties), the neutral to earth voltage is very low.

    This was introduced as the older system where each house had it's own independent earthing led to significant neutral to earth voltage and possibly very high currents if a neutral to earth short occurred (as one fault could be taking a good part of the neutral current from a whole street of houses). Fires and severe wiring damage were not uncommon.

    Also, as the house earth often relied on connecting to a metal water pipe, earth connections were often lost when old lead pipes were replaced with plastic, again leading to hazardous conditions.
     
  10. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Wow, thank you so much guys.
    So much great information.

    I fixed up the scheme -
    1. I connected the Earth to Netural at home's end instead of at wall outlet.
    2. I removed the connection between the two Earths (transformer's end and home's end).

    Is it ok now?
    Or is it wrong to say that both Earths are 0V? (as shown in the scheme).

    [​IMG]

    By the way, if too much current flows through RH(EARTH), then a power outage occurs (as a protection mean), right?

    So, if R(Neutral) is too high, such power outage could occur for not so large currets that flow through the Live and Neutral line, right? (Theoretically).
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    More like this:

    [​IMG]

    Fuses are seldom used in homes nowadays. F1 and F2 simply represent circuit breakers.

    There are usually a number of outlets connected to one breaker/fuse.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    One thing to remember is that there is a difference between overhead supply cabling and underground supply cabling.

    Because the underground cable is buried and usually has a metal sheath it can easily provide a good earth connection.

    Overhead supply is inherently without an earth. Supply companies do not run an extra wire for this.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    This could be the case where you live.
    Here in the States, it used to be that each residence had to have a single 8-1/2 foot long 1/2" diameter copper-clad ground rod driven into the ground, with at least AWG 4 diameter solid copper wire connecting it to the Ground and Neutral buses in the service panel. A few years ago, they amended the national electrical code to require two such copper-clad grounding rods driven in at least six feet apart.

    In the States, transformers on poles are generally grounded via the supporting cable, which are usually galvanized steel. I have no idea what resistance they might be.
     
  14. studiot

    AAC Fanatic!

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    Up to about 1950 supply cables were only laid underground in the most modern or most built up areas. Voltage varied between 200 and 250, two wire L and N single phase. So the supply did not arrive at the property with an earth. The regulations provided for two types of circuitry within a property - low current of up to 5 amps with no earth and high current of 16 amps with an earth. It was the propertyholder's responsibility to provide this earth, with a spike as described.

    Subsequent to 1950 it became policy to lay all new supplies underground and the cables until recently were wrapped in metal strip and and bitumen. All power circuits were required to have an earth, although earthing for lighting circuits was not introduced until the 1970s.

    At the end of the 1960s a programme of 'undergrounding' older overhead supplies was begun and has progressed ever since, so the percentage of properties supplied overhead is diminishing all the time.

    Since the 1990s all plastic clad underground cables have been employed, leading to renewed earthing challenges and some arguments.

    Other changes introduced have been plastic piping for other services. One good effect of this is the new colour coding; bright yellow for gas, bright blue for water and bright red or black for electricity. However it has proved difficult to locate plastic pipes with tracers so more recently pipes have been introduced with metal strips sufficient to activate detectors.

    No one has yet introduced standard positions in the highway for services, although a few places have experimented with standard service troughs and concrete cover planks, to avoid all the digging up.
     
    Last edited: Dec 3, 2009
  15. Wendy

    Moderator

    Mar 24, 2008
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    I have underground wiring to my house. Far as I know, it follows the USA standards. If I see a utility tech I'll ask next time.

    Thing I like most about it is relative immunity from lightning.
     
  16. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    In Canada, following the CEC, overhead services do not include a ground. NS duplex or triplex cables are characteristic with a bare supporting neutral (NS designates Neutral Supported). Underground service also do not include a ground. Sheathed cables are not required underground for voltages under 3KV phase to phase.

    The idea is that short currents will return to the source on the low impedance neutral, thereby ensuring operation of over current devices. If left to a high impedance ground route, sufficient current may not flow to activate O/C devices.

    At the supply side, the transformers are grounded via a stake, while at the home, insitu electrodes are common, and include copper water lines. Size of Common grounding conductor is based on ampacity of service conductors.

    All metalic, non conducting equipment is bonded to earth to limit possible potential difference with respect to ground.
     
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