Conduit in stud walls

Discussion in 'Electricians' Talk' started by kidspud, Nov 14, 2003.

  1. ajohn

    ajohn Screwfix Select

    Wow who ever dug this one up must be incredibly happy. :) Can't say I noticed mention of when 50mm depth is needed but no way I am going to read all of this one.
    Maybe I can help spinlondon. It's an idea that some electronics people need. Volts = pressure, current = flow. No pressure no flow. If a cpc is carrying current there has to be some volts behind it and there is no way it would cause current to flow *up* an inactive cpc unless it had an independent earth connection some where on it - bit unlikely. Current has to have somewhere to flow. All that would happen is that the current carrying cpc would cause the voltage at the cu busbar and cable there after's voltage to increase right back to the substation, That's down to resistance. That restricts flow so needs voltage across it to for any current to flow. The volts provide the pressure. The cpc with the fault also has resistance so there will also be a voltage from from the fault to the end of it. Measure it 1/2 way at it would 1/2 of that. As there is voltage drip due to resistance work is being done I^2R= watts again so in this case the wire heats up.

    This relates to why volts are also referred to as a potential. Volts means that it has the potential to do work. Also potential difference - the potential to do work is the difference between the 2 potentials. Unless current flows it can't do any work. When current flows the work that could be done is V x A = watts

    This usually goes on to compare other pressures and restrictions to explain what capacitors do.

    :) Now some one will mention nano second voltage and current jumps etc when the fault occurs. I'd just say get real. Things in the nano second range aren't of interest in this area and wont be predictable anyway in real situations.

    Bazza-spark likes this.
  2. spinlondon

    spinlondon Screwfix Select

    Thanks ajohn.
    So basically your saying that the current will be present on inactive CPCs but flow anywhere as there’s no independent earth connection.
    Is that similar to my statement:
    Some of the current while present on the other CPC, won’t actually flow anywhere because the circuits dead end?
  3. spinlondon

    spinlondon Screwfix Select

    Yes if there is an earth fault, the current will be present throughout the installation, even though there is no complete circuit.
    This is exactly the reason why lost supply Neutrals on PME systems are so dangerous.
  4. ajohn

    ajohn Screwfix Select

    No I'm not. There wont be any volts on them so can't be any flow. There will be current flow from say any appliance that is connected that has leakage to earth - many have but there is a limit on how large it can be. Faults in the appliance causing the case to go high due to loss of an earth connection wont kill but can give a ;) "decent" tingle. We are talking mili amps but not many of them.

    Lost neutral on pme is another miss understood subject. Take one youtube video where a heated tank of water is mentioned. If some one did an insulation test on the heater to tank they might find some figure like 10M, can't remember but over some figure that is lots of megs. Neutral and earth have gone so what ever resistance is there causes the tank voltage to go up to mains potential. It doesn't matter how low the current is, that will happen. Normally the current from the resistance would flow back down to earth but it can't as it's disconnected. So some one touches the tank. They provide the ground. Lets say the insulation resistance of heater to tank is 2 meg R. The current that will flow forgetting the resistance of the person will be 230/2000000 = 0.000115 amps in other words 115uA. There is no way that would kill anybody but they may feel a tingle which is what people who report the problem say.

    Take a number where some one would get a shock. Say 1ma. The resistance between the heater and the tank would need to be 230/0.001 = 230K so that is the number an insulation test would need to show. Again that is unlikely to kill be much more of a shock of another sort - ouch etc. Actually having handle motors with more leakage than this a "severe" tingle is far more likely. Severe is too strong a word really. If it flows through both arms it might cause problems depending on the person conducting it and how well one of there arms is grounded. ;) If some one ever works on live stuff it pays to bear that in mind. Some people might ground a finger to get a neon screw driven to work that is in the other hand. That may generate a tingle, a rather noticeable one if it's a nice bright one.

    Go higher say 30ma, Resistance needed is 230/0.030 = 7K666. At that sort of resistance level the R of the human body would have an effect but this is a level where contact time needs to limited. So will some one go to the tank and grasp it firmly with both hands? The other point of course is that the circuit would have tripped out when neutral was connected. :) The interesting thing is that the fault needs to be that severe or provide circa 50% of this level which means a 15k fault way way less than when everything is ok with the element. Fine providing earth is connected.

    So real danger crops up when there is a double fault. Neutral gone and a some resistance problem to a chassis of some sort. Where it gets interesting is if there is also some bonding. In this case current from live will flow to earth and what ever it is might continue to work. If complete loss which is what matters things wont work full stop. It needs to be considered when things stop working - in this case everything that uses the earth that has failed. Might be the entire house or building.

    Then comes the rcd which can't see a current difference any more. There is another reason why it wont as well - many have active electronics in them that need neutral to power up. This is why rcbo's also have an earth lead - so that they can power up if neutral is broken and still detect current differences. It seems some makers are omitting these and the IEC agrees. Probably on the basis that rcd's need them as well really and they aren't fitted with them.

    Interestingly a rcd doesn't have to have any active electronics in it. I'd guess it allows smaller cheaper ones. If there was no active electronics they could be rewired to just detect earth currents. Same old problem though - many things plugged in that all have leakage at some level so the trip level would have to be the same as it currently is. Detecting complete loss of neutral could be done with simple contactor but would increase electricity bills. Sort of no volts switch arrangement. It seems the powers that be don't see loss of neutral to be worth considering as it's so rare. ;) Having suppliers needing to go round and fit things might be considered as well. It's their circuits.

  5. Bazza-spark

    Bazza-spark Screwfix Select

    No that is not correct. Current cannot flow without voltage.

    Again, incorrect. The VOLTAGE will be present, not the CURRENT.

    It is simple ohms law.

    I = V

    Where I = current
    V = Voltage
    R = Resistance in ohms.

    So with no volts

    I = 0 = 0

    Now when voltage is applied

    I = 230 = 0.23 amps

    No voltage, no current flow. Voltage HAS to be present to allow current flow.

    Edit: The R should be under the V. For some reason it appears that way until I post.
    Last edited: Aug 10, 2019
  6. ajohn

    ajohn Screwfix Select

    People don't seem to realise that voltage doesn't kill Bazza S. It has the potential to kill as it's presence can cause a current to flow. This applies to anything connected to a voltage. Volts just provide the pressure to allow a current to flow when something is connected to it. Current flows between points and voltages are required at each end to allow it to flow. One of them might be earth = 0v or neutral. The other one that causes current to flow may be positive to earth or negative, that sets the sign of the current. If AC the current level is continuously changing and also changing sign in respect to neutral or earth.

    :) Good job it does work like that or summing currents around a point with several current carrying conductors connected to it wouldn't work. The sum of all currents will be zero. If that wasn't the case it would be completely impossible to design many electronics circuits. There are other rules as well base on similar factors.

    So a cpc is carrying current to an earth. There is no reason why any number of other cpc's will carry any of that current as it would have no where to go and it must have for current to flow. They all have earth potential at the far end - no voltage difference end to end so no current flows. "Earth" could be at any imaginable voltages even and there would still be no voltage difference end to end. Same would be true if earth happened to be a 300kv line. ;) In this particular case anyway. The volts might make some ones hair stand on end so some current would be flowing. Thankfully not a lot.

  7. spinlondon

    spinlondon Screwfix Select

    So you’re both saying now, that if there’s an earth fault on say a kettle, there’ll be no volts and no current on the CPC.
    I think you need to re-think.
  8. ajohn

    ajohn Screwfix Select

    No you do as there will be current on what ever cpc the kettle is connected to and none on any other separate cpc circuits.

  9. Bazza-spark

    Bazza-spark Screwfix Select

  10. Bazza-spark

    Bazza-spark Screwfix Select

    Ok, here are 4 scenario's for you.

    Earth fault on kettle, kettle unplugged. No voltage and no current.

    Kettle element for arguments sake has a resistance of 26 ohms.

    Fault example 1
    Kettle element shorted to earth at 22 ohm point

    230 = 10.45 amps

    Fault example 2
    Kettle element shorted to earth at 13 ohm point

    230 = 17.7 amps

    Fault example 3
    Kettle element shorted to earth at 8 ohm point

    230 = 28.7 amps

    The current in the circuit depends on the voltage and the resistance.

    Does that help?
  11. Mr_sweet

    Mr_sweet Member

    Have you admitted it's ok to reply constantly to old threads now?
  12. Bazza-spark

    Bazza-spark Screwfix Select

    Don't recall saying it wasn't, but I am trying to help a fellow member understand some basic electrics.
  13. Mr_sweet

    Mr_sweet Member

    You have moaned when others were just trying to do the same so just thought I'd ask
  14. Bazza-spark

    Bazza-spark Screwfix Select

    Welcome on board.
  15. Mr_sweet

    Mr_sweet Member

    Kind regards
  16. spinlondon

    spinlondon Screwfix Select

    Nope, the current will be present on all CPCs and anything within the installation that is connected to earth.
    Mr_sweet likes this.
  17. Woloumbo

    Woloumbo Active Member

    with a fault to earth,there will be current flow to earth
    this current may flow to the cu and down the main earth
    but may also flow through any other parallel paths
    these could be a person or other metallic paths or other circuits
    if they they have parallel paths
    resistors in parallel (0R50 etc if you can only reed weird R electronic shLt)
    even if no parallel path exists,the POTENTIAL exists for the current to flow should
    someone touch something on another circuit,or a bonded bit of metal
    London is talking sense.think parallel paths,think parallel resistors
    why do you think we have protective bonding?its for a friggin reason
    spinlondon likes this.
  18. spinlondon

    spinlondon Screwfix Select

    Thank you.
    So now we know how much current will bypass the RCD FCU and be present on the towel rail.
    Can you do some calcs to show how much current will flow through a person who is in contact with the towel rail and bonded pipework?
  19. Mr_sweet

    Mr_sweet Member

    He can't do that because you didn't give him advance notice that a towel rail in a bathroom had some pipework. Truly bizarre
    Woloumbo likes this.
  20. Mr_sweet

    Mr_sweet Member

    London is the one talking common sense and the other 2 are just looking to try and use electrical theory to confuse what should be a quite simple scenario,sometimes that smokescreen works with people who have no knowledge of electricity but a real electrician will know you are blagging

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