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Losing neutral in the utility system

  1. Sep 26, 2016 #1

    Averagesupernova

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    The consequences of this have been discussed many times here on PF. Here is an example of what can happen.
    http://forums.mikeholt.com/showthread.php?t=178779
    If you live in a rural community with your own water system or your water supply comes in on plastic this sort of thing seems impossible. But the truth is that you could likely cut the neutral conductor completely off in the service panel and it is unlikely you would notice it if you live in a section of town that everything is well bonded with metal water supply pipe. The current that should be in your own neutral conductor is finding its way back to the transformer through your neighbors neutral. Scary to think that some hack electrician my neighbor may hire could cause problems in my own house. Messing with the neutral can certainly be more dangerous in the long run than being a little careless with one of the hot wires.
     
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  3. Sep 26, 2016 #2

    nsaspook

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    I came close to losing the house power neutral a few weeks ago. The local cable guys were repairing a underground coax when their pusher machine cut one of my and the next house on the block 240vac split-phase feeder wires underground. Only lost half of the lights and outlets to the house but a full neutral loss with unbalanced house loads could have been an electrical disaster. The original house ground was bonded to the water supply but that was replaced years ago with plastic from the street meter so I installed two of my own rods and bonded the entire system outside before feeding it to the main panel.

    28612245233_beedcbf316_z_d.jpg
     
  4. Sep 27, 2016 #3
    In the UK it is usual not to have an Earth electrode at the house but to use the Neutral at the entry point to the premises to provide the Earth connection for the three-pin sockets. All metalwork and pipes at the premises are bonded together and to the "Earth". The Neutral point is truly Earthed at the sub station. An Earth electrode is, however, used in cases of overhead cabling in order to just operate an Earth Leakage trip. I am sure real electrical experts will pick me up on this post but it is clear there are differences between countries.
     
  5. Sep 27, 2016 #4

    Averagesupernova

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    While there are some differences there are enough similarities (from what I can tell from your post) that something similar could happen in the UK. I have heard of electrical fires getting starting because of a poor neutral connection somewhere and the neutral current finding it's way back through cable TV coax connections. Here in the USA now we have intersystem grounding buses. This is a busbar where the cable company, phone company, etc. can all ground their 'stuff'. This busbar is usually bonded to the service panel or meter socket with a pretty large wire.
     
  6. Sep 27, 2016 #5

    nsaspook

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    I've had problems with UK type non-local earthed systems in Asia. It usually involved the proper safety grounding of US style equipment with EMI line filter capacitors from hot and neutral to a metal case grounding point near the power receptacle inside the equipment. Leakage currents would result in sometime 80 or more volts AC from a floating metal case to local earth ground. We didn't like to run a solid earth ground connection that could cause large current during a fault so we would at times use a ~1000 ohm grounding resistor from ground connection to limit fault currents while reducing leakage volts to below a fraction of a volt. The proper thing was to use a isolation transformer to break the current path but that was heavy to move around.
    comp_emifilter_large.jpg
     
  7. Oct 16, 2016 #6

    Svein

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    The classic power distribution in Norway does not use a "Neutral" conductor. Instead, it is the responsibility of the house owner to supply a local "protective ground" (usually a thick copper wire either inside the concrete in the house fundaments or buried alongside the drainage all the way around the house).
     
  8. Oct 16, 2016 #7

    davenn

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    interesting comment and difficult to fully understand as a "neutral" and "protective ground", tho they may be ( and usually are) connected at the power meter/fuse box, are two totally different things
    A protective ground isnt designed to do the same thing as a neutral lead in the house or appliance supply cabling with which there may or may not be a protective ground wire included. The neutral is the return path for the phase ( hot) lead

    Would you like to clarify please


    Dave
     
  9. Oct 17, 2016 #8

    Svein

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    A fuller explanation is in http://electrical-engineering-portal.com/phases-and-wires-in-distribution-of-ac-power, but I will just copy one figure:
    three-phase-3-wire-system.png
    The delta connection in (a) has no ground reference and there is no "neutral". If the ground connection is poor (as it usually is in Norway) the voltage between "ground" and the phases may literally be anything. The concept "protective ground" relates to the fact that water distribution used to be buried copper piping and as such had a fairly good connection to the "ground". Thus, any leakage in the power system could give you a nasty shock if you touched a faucet or something else connected to the water distribution in the house. Thus the "protective ground" - introduced in order to ensure that leakage current was directed to the same "ground" as the water system.
     
  10. Oct 17, 2016 #9

    nsaspook

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    The US uses 'split-phase' center tap 240vac to residential 'lighting' circuits. (an historical holdover from Edison three wire DC lighting circuits)
    300px-02-Edison_Central_Station_3_wire_dc_system-17.GIF
    http://ethw.org/Early_Electrification_of_Buffalo

    A few typical types of US power feeds with grounds and neutrals.
    resdt3p3.gif resdt3p4.gif resdtmcn.gif
    In all cases the standard 'split-phase' connection to the house remains the same.

    It's my understanding that UK residential 'lighting' circuits do provide a neutral by bonding to ground at the substation transformer.
    72.png house wiring diagram.GIF
     
    Last edited: Oct 17, 2016
  11. Oct 17, 2016 #10

    Averagesupernova

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    Having something that is called neutral or not, most places have a tie to one of the current carrying conductors that connects to a buried electrode whether it is in concrete, ground rods, a bare conductor that circles the house, etc. Also, this grounded conductor will usually connect to a water pipe in one form or another. Unless your system is completely floating, the danger described in the first post of this thread is a real threat.
     
  12. Oct 17, 2016 #11
    It's my understanding that UK residential 'lighting' circuits do provide a neutral by bonding to ground at the substation transformer.
    View attachment 107595 View attachment 107593 [/QUOTE]
    UK has only one feed for power and lighting, 230V. At a socket there are three wires: Line. Neutral and Earth. Usually, the Earth wire connects to Neutral (the underground cable armouring) at the house entry point. There is no ground electrode required at the house, but water pipes and metal work etc must be bonded to local "Earth". At the sub station, neutral is grounded. It is complicated - I await the comments!!
     
  13. Oct 17, 2016 #12

    Averagesupernova

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    So in the UK there is not a connection between the metal chassis of the service panel and the neutral conductor? So in a fault condition when a hot wire (L in the diagram) touches the chassis of an appliance there will be current in the Earth wire (green in the diagram) which is not guaranteed to be enough current to trip a 30 amp breaker since the only path is through the soil in the earth and not a metal conductor. Do I have this right? Now I believe the UK uses the equivalent of what we call GFCI here in the USA. Ground Fault Circuit Interrupter. If I recall correctly this device is placed in the main leads after the meter. It would sense a difference in current between the hot and neutral which would indicate something leaking off to where it should not be. So if I have this right, there is some protection but to have a faulty blender shutting off my whole house would be something of an inconvenience. Please elaborate.
     
  14. Oct 17, 2016 #13

    nsaspook

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  15. Oct 18, 2016 #14
    The incoming Neutral is Earthed at the sub station, at the centre point of the transformer. This then provides the E connection for everything at the house in addition to being the N conductor. An E wire (third wire) is provided to every socket. There is no actual ground connection at the house, apart from bonding of pipes and metalwork. So the difference between US and UK on the diagram that N is connected to the service panel metalwork. If L touches the metalwork of an appliance, current flows via the E wire back to the service panel where it then flows via the N (armouring) of the incoming cable. So there is no requirement for a Ground Fault Interrupter (Residual Current Device), although such devices are very commonly used for each ring main.
    I think there is a difference between UK and US in that we have less overhead cables, and use mostly underground cables. The UK arrangement I have described is not used for overhead cables, where a local ground electrode is provided for the purpose of operating an Earth Current trip of some sort. (I am not sure of up to date practice about this trip, whether it is now RCD type or Earth leakage type). Can someone in UK help me on this please?
    Regarding the inconvenience of RCD devices, yes it will trip if there is a leaky appliance. But many appliances these days are double insulated with no Earth connection. It is usual to divide the house into two or more zones, so that all power is not lost if the device trips. For instance, upstairs power will not trip the lights etc.
     
  16. Oct 18, 2016 #15

    Averagesupernova

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    My bold. Ok so this helps a bit and this implies that there IS in fact a pathway back to the transformer center tap with a conductor other than the soil. This armouring you refer to I had not heard of. Is the armouring a 3rd conductor? I would think it would have to be to behave in the manner you describe. This is interesting.
     
  17. Oct 18, 2016 #16
    The cable is of "coaxial" construction, with two conductors, a central L conductor and a steel armouring as the outer. The outer provides the N and E path. At the house, E and N house wiring is connected together at the entry point.
     
  18. Oct 18, 2016 #17

    Averagesupernova

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    Ok so there is no difference between the USA and the UK other than it is not split phase in the UK. This is what I would have expected in the first place.
     
  19. Oct 30, 2016 #18
    I do see one glaring difference in the handling of neutrals in US/NA versus UK systems. Specifically the branch circuit devices for premesis wiring are designed to break both grounded, or "neutral", and ungrounded, or "hot" conductors. In USA common practice this is forbidden except under certain permissions (seperately derived prime, unit equipment factory wiring ect). Even GFCIs which are equivalent to RCDs, I am nearly certain do not break the neutral on the load side.

    This has a major implication for troubleshooting since one way to quickly diagnose an open neutral conductor is to shut off the branch circuits connected to it and find continuity from the neutral (or "grounded conductor") to ground since at some point on the premesis wiring system they become the same wire. OL=120v neutral to ground in US...but in UK it just means the breaker's off?!
     
  20. Nov 10, 2016 #19

    jim hardy

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    Wow is this synchronicity ?

    A friend came by a few hours ago to ask for help. He has worked as an apprentice electrician but never finished the training..
    He'd put in a new circuit breaker panel for a neighbor . He said "Some of the lights worked for just a moment. I heard an arc when i plugged in the meter and when i thought i smelled smoke i pulled the meter back out."

    When i got there i saw a wire not connected in the meter box. It was one of the "hot" ones going inside to the panel.
    He said " We had trouble hooking that one up it's almost too short. I guess when we pulled on the other end we pulled this end loose. Maybe that was the arc i heard... "

    Uh oh..

    I had a longer scrap the of same gage with better 90C insulation, gave it to him and he installed it .

    We re-installed meter and observed somebody before had got a screwdriver in the wrong place evidenced by a big spot of arc damage on one of the upper meter stabs.
    Very difficult to push meter into the damaged stab but it went.
    I stayed outside to watch for arcing at back of meter on that damaged stab while he went inside to try the breakers again.
    Fridge and kitchen light came on , light was very bright and fridge was noisy, Breaker tripped after about 30 seconds. No arcing at meter stabs though.

    I went inside opened all the breakers and measured voltages coming in from meter. Found between neutral and one of the 'hots' 120 volts as expected, but neutral to other one read 240. Whoah ! That should be impossible.

    "Hey Scotty did you mark neutral when you reconnected these? " answer "Yeah but my tape mighta fell off when we pulled so hard... "
    Okay we both knew what was wrong but i wanted him to recover face .

    So i strung an ohm meter wire from inside to outside . ( I have ten foot leads on my meter for just such situations.. . )
    I handed him the meter and I went out to meter box, pulled meter .
    I touched my meter probe to a main terminal on downstream meter stab and hollered to Scott "I'm on my left hand hot here , on your right facing me, find me in there. " He replied "Yep , it's on the right hand hot here."
    When i moved my probe to neutral , after maybe ten seconds he said "You sure you're on Neutral? It's the other 'hot' in here

    As you've guessed by now he'd swapped that other hot wire with neutral and applied 230 volts to kitchen lights and fridge.
    He put the wires back where they belong and everything returned to normal.
    By some miracle nothing in the house burned up with neutrals and ground wires elevated to 120 volts.. Had he touched any appliance he'd have got a shock.
    And the fridge seems to have survived the overvoltage but three of five CFL bulbs in the kitchen fixture did not.

    Clearly there's another problem though, the new panel is not bonded to earth ground . Something should have tripped with that connection . Or maybe it melted that first time when Scotty plugged the meter back in and heard an arc.
    I'll get Scotty to add that connection when he comes back in the morning to finish up.

    Moral of story -
    Get a licensed electrician. It's too easy to get confused.
    That's not the place to scrimp. See also this thread . https://www.physicsforums.com/threads/question-on-oversized-wire.892564/

    old jim
     
    Last edited: Nov 10, 2016
  21. Nov 10, 2016 #20

    berkeman

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    Thread closed for Moderation...

    Thread re-opened for now...
     
    Last edited: Nov 11, 2016
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