Neutral rise in potential

Discussion in 'Electricians' Talk' started by spen123, Jan 12, 2018.

  1. spen123

    spen123 Active Member

    Okay so I understand that when a neutral is disconnected in a live circuit it will rise in potential, however reading around its said it' because it' connection to the live so voltage has nowhere to go until connected earth.
    But there' no physical connection between the two and also voltage doesnt all flow from positive to negative. Can' rememer what the law is called but in recent times it was found that it' the negative electrons that are drawn to the positive charge and some positive electrons to the negative charge. In this case with no physical connection most Being induced voltages the only way it can be is just that an induced voltage. Or am I completely wrong
  2. spen123

    spen123 Active Member

    Electron flow vs conventional current. Not really a law
  3. Bazza

    Bazza Well-Known Member

    There is no concept of positive and negative in AC circuits.
    leesparkykent likes this.
  4. leesparkykent

    leesparkykent Well-Known Member

    AC in the UK alternates 50 times a second, hence 50Hz. It doesn't flow it moves back and forth.
  5. MGW

    MGW Well-Known Member

    Potential can apply to AC as well as DC. So with the neutral bonded to earth at the supply transformer as that point there will be no potential difference between the two, with a balanced three phase supply there will never be a PD, but where it is not balanced then the neutral will carry current, so simple ohms law, if the resistance of the neutral is one ohm, and one amp is flowing then there will be 1 volt difference between where the neutral is bonded to earth and where the resistance has been measured.

    If the neutral and earth are bonded at many points, then there can still be a PD between transformer and where the last bond is done, but this will also include the mother earth, we have what is called a gradient.

    So if the neutral is bonded to earth at the transformer, and not down its length, and the line is connected to earth at the place of use, then we will have a massive gradient of 230 volt. However most of this gradient will be near to the users earth rod, which is why the ground around an earth rod is protected so animals including the human animal can't touch the ground where there is a high gradient, even 25 volt over 2 meters can kill a cow, our heart is not between our legs so not such a big problem with humans.

    This gradient also is seen where RCD protection is used, where the neutral is bonded to earth there is no PD so a neutral wire going to earth will carry no current so will not trip a RCD. However if we take the supply away from that point, if no current being used then again same potential so no trip, but draw one amp with a one ohm cable then at it's end you will have one volt difference, if the ground is also one ohm then connecting neutral to earth they will share the current, so 1/2 amp through neutral and half amp through ground, result the RCD will trip.

    So a toaster with some damp bread between neutral and earth but switched off will not trip the RCD, but switch on the kettle so 10 amp flows in the neutral wire and the potential rises so current flows and the RCD will trip.

    We are it seems having a rethink about true ground potential where the gas pipe and water pipes are isolated from ground, it seems in the 18th they will not require bonding, There is a balance, if metalwork is bonded, then if it becomes connected to line, high current will flow and cause it to disconnect automatic, however if you touch earth bonded metal work and line high current can also flow through you, if the metal work is not bonded to earth then no current will flow, so there is a risk either way, so size matters, if a bit of pipe is one meter long from plastic pipe under floor to tap on plastic sink, then not being bonded to earth is likely the best option, but have a series of pipes going around the house not bonded to earth, a standard lamp could fall on a radiator in one room and make the radiator in another room live, so likely with this better to bond. However the RCD today is rather sensitive, so even without bonding, there is likely enough current that will flow through bricks and mortar to trip the RCD anyway.

    As it stands we feel where a single item is supplied it is safer not to have an earth bond, what is called an IT supply, as found in bathrooms with shaver sockets, there is some debate with larger supplies, a generator supplying a single drill likely same as shaver safer not to bond, but supplying 100 drills, then to bond is safer, the problem is doing a risk assessment and deciding at what point to bond to earth. So the safe method is if not sure bond to earth.

    Although I show earth to be like a wire, it's not quite that simple, metal will have a reaction with damp earth, so if you put a copper coated rod into the earth and connect it to a steel narrow boat it will cause the narrow boat to be eaten away, even worst if Ali, so some times we want to allow a small potential difference without current flowing, we do this by putting diodes in the earth wire, four diodes two one way around and two other way around will allow 1.2 volt PD without current flowing, so boat is not damaged, we see the problem with steel or iron drain pipes, often they break where they enter the ground due to current flowing and eating them away.
  6. Risteard

    Risteard Well-Known Member

    Well there are positive and negative half cycles.
  7. Bob Rathbone

    Bob Rathbone Well-Known Member

    Any unconnected (floating) conductor parallel to or part of a single or multicore cable that is connected to phase with no intermediate screening, will rise in potential due to capacitive coupling. The 'plates' of the capacitor are the conductors and the dielectric is the cable insulation. The voltage and current supplying capability will depend upon length of run parallel, the distance between the conductors and the supply frequency. With digital voltmeters with a high input impedance, voltages near mains supply can be measured, but it is unlikely that more than a few micro amps will be available. Moral, don't leave a core floating, use it as a CPC.

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