3 ms·

> A common misconception is that the neutral-ground bond at the service entrance is what makes the system grounded. That is not true: it is the neutral-ground b

> A common misconception is that the neutral-ground bond at the service entrance is what makes the system grounded. That is not true: it is the neutral-ground bond at the transformer that makes the system grounded Er, what? It's any/all of the neutral-ground bonds that make the system grounded. The more ground connections, the more redundancy and lower impedance. ... which is probably why the plumber didn't worry too much about disrupting the building's ground connection. If everything else is wired up right it's not going to be that big of a deal - perhaps a few volts on the building's grounding conductor versus actual earth, due to the current on the service neutral.

jnsaff2 4y ago
Yeah. Here in EU I get 3 phase cables only coming from the transformer. Then at my house a grounding rod is used to give combined Protective Earth and Neutral (TN-C). At my house main panel they are split into PE and N (TN-S). They are never connected house side and I now have combo RCD’s so each automatic fuse is also an RCD. EDIT: I was wrong, there are 4 conductors coming from the transformer, 3 phases and a Neutral. Phase-Neutral 230V and Phase-Phase 400V. Grounding rod connects to the Neutral and 4 conductors come to the main electricity cabinet. There Neutral and PE are split.
- mindslight 4y ago
  It sounds like the branch circuits must be wired delta (phase to phase, rather than phase to neutral). What's the neutral actually do? Or does your earth have low enough impedance that you can get away with low power loads returning unbalanced phase current through the earth?
  - jnsaff2 4y ago
    Earth is used as return path for any current. Someone described it as an endless source/sink for electrons. I don’t have any delta loads in my house, but I could of course if I needed to.
    - mindslight 4y ago
      That's odd, and I can only think you must live in an area where the earth is conductive enough to carry significant current (or are the cables coming from the transformer inside a metal conduit?) The earth is nothing like "an endless source/sink for electrons". Rather, electricity always flows in a complete loop (circuit), and the current flow is somehow getting back to the transformer to complete that circuit.
      - derefr 4y ago
        All you need to conduct long distances through the earth is a big buried plate to spread the current across. See e.g. https://en.m.wikipedia.org/wiki/Earth-return_telegraph https://en.m.wikipedia.org/wiki/Earth-return_telegraph. > The earth is nothing like "an endless source/sink for electrons" It’s a bounded sink for electrons, though — i.e. it has capacitance — insofar as a positive ionization required for a static discharge (lightning) built up in the earth can be “spent” by grounding voltage into it. You can think of the earth as a constantly-recharging (but sometimes rapidly discharged) anti-supercapacitor. Just like the atmosphere is a constantly recharging (but sometimes rapidly discharged) regular supercapacitor. Ignoring that, though, what people usually mean by this is that the combined neutrals of the entire power grid are on the other end of any dumping of current you’re doing into ground. It’s not just your local neighbourhood transformer; if your transformer’s resistivity increased too much due to the added ground-return current, then the rest of the current could find another path to some other, further-afield transformer, or multiple such. Or maybe even the local power plant. Or some unrelated grid-connected power plant. Or—dangerously—a power plant connected to a physically proximate but electrically disconnected grid… which is in part what makes power-factor wobble around so much in black-start grid power stations.
        mindslight 4y ago
        Sure, the earth in bulk is essentially a conductor and it's possible to transmit electricity through the ground (eg single wire earth return). I would just think it impractical for last hop residential distribution (eg the effect in the original post), as getting a good connection is hard and the distance is short. > the combined neutrals of the entire power grid are on the other end of any dumping of current you’re doing into ground This only makes sense if the primary side of the transformers have a grounded conductor ("neutral"), and it's bonded to the grounded conductor on the secondary side. I don't know much about common power distribution topologies, but this would seem less safe as it would increase the possibility of the high distribution voltage trying to find its way back to the plant/grid/ground through end-user services.
- jcalvinowens 4y ago
  Residential US electricity primarily uses center-tapped single phase transformers, with 240V between the two ends of the coil, and 120V from either end to the middle. The center tap is grounded and treated as a neutral for 120V loads from either "hot", and 240V loads run between the two "hots". The advantage is that we get 240V power with 120V shocks (assuming you grab a hot wire while you are grounded). The disadvantage is exactly what the original post was about: if the neutral wire breaks, it can fry everything in your house, because your toaster is now in series with your lightbulbs between the hots.
jcalvinowens 4y ago
> Er, what? It's any/all of the neutral-ground bonds that make the system grounded. The misconception is that cutting the neutral-ground bond at the service entrance makes the system ungrounded, which isn't true. You can cut that, and nothing will change under normal operating conditions. > The more ground connections, the more redundancy and lower impedance That isn't true in the US. The only permitted neutral-ground bond in a house is at the service entrance.
- mindslight 4y ago
  > The misconception is that cutting the neutral-ground bond at the service entrance makes the system ungrounded, which isn't true That's a better way of putting it. I agree with that. > That [more ground connections] isn't true in the US. The only permitted neutral-ground bond in a house is at the service entrance. I was talking about the US. By multiple ground connections I mean one for the transformer plus one at each service panel.
- amluto 4y ago
  In the US, if everything else is wired correctly but you disconnect the neutral-ground bond, you may end up with rather high ground impedance. Those two ground rods, 6 feet apart, that satisfy code could easily have quite a few ohms in dry weather. This, in turn, could allow a live (120V to ground) wire to short to an equipment ground, drawing only a few amps and making every grounded object in your house quite dangerous indefinitely. Since the NEC does not require any form of residual current detection except for some branch circuits unless you have a huge service, nothing really protects against this type of failure.
  - jcalvinowens 4y ago
    I didn't mean to suggest the bond is useless, just that breaking it won't cause problems unless there is a fault.

jnsaff2 4y ago

Yeah. Here in EU I get 3 phase cables only coming from the transformer. Then at my house a grounding rod is used to give combined Protective Earth and Neutral (TN-C). At my house main panel they are split into PE and N (TN-S). They are never connected house side and I now have combo RCD’s so each automatic fuse is also an RCD. EDIT: I was wrong, there are 4 conductors coming from the transformer, 3 phases and a Neutral. Phase-Neutral 230V and Phase-Phase 400V. Grounding rod connects to the Neutral and 4 conductors come to the main electricity cabinet. There Neutral and PE are split.

mindslight 4y ago
It sounds like the branch circuits must be wired delta (phase to phase, rather than phase to neutral). What's the neutral actually do? Or does your earth have low enough impedance that you can get away with low power loads returning unbalanced phase current through the earth?
- jnsaff2 4y ago
  Earth is used as return path for any current. Someone described it as an endless source/sink for electrons. I don’t have any delta loads in my house, but I could of course if I needed to.
  - mindslight 4y ago
    That's odd, and I can only think you must live in an area where the earth is conductive enough to carry significant current (or are the cables coming from the transformer inside a metal conduit?) The earth is nothing like "an endless source/sink for electrons". Rather, electricity always flows in a complete loop (circuit), and the current flow is somehow getting back to the transformer to complete that circuit.
    - derefr 4y ago
      All you need to conduct long distances through the earth is a big buried plate to spread the current across. See e.g. https://en.m.wikipedia.org/wiki/Earth-return_telegraph https://en.m.wikipedia.org/wiki/Earth-return_telegraph. > The earth is nothing like "an endless source/sink for electrons" It’s a bounded sink for electrons, though — i.e. it has capacitance — insofar as a positive ionization required for a static discharge (lightning) built up in the earth can be “spent” by grounding voltage into it. You can think of the earth as a constantly-recharging (but sometimes rapidly discharged) anti-supercapacitor. Just like the atmosphere is a constantly recharging (but sometimes rapidly discharged) regular supercapacitor. Ignoring that, though, what people usually mean by this is that the combined neutrals of the entire power grid are on the other end of any dumping of current you’re doing into ground. It’s not just your local neighbourhood transformer; if your transformer’s resistivity increased too much due to the added ground-return current, then the rest of the current could find another path to some other, further-afield transformer, or multiple such. Or maybe even the local power plant. Or some unrelated grid-connected power plant. Or—dangerously—a power plant connected to a physically proximate but electrically disconnected grid… which is in part what makes power-factor wobble around so much in black-start grid power stations.
      - mindslight 4y ago
        Sure, the earth in bulk is essentially a conductor and it's possible to transmit electricity through the ground (eg single wire earth return). I would just think it impractical for last hop residential distribution (eg the effect in the original post), as getting a good connection is hard and the distance is short. > the combined neutrals of the entire power grid are on the other end of any dumping of current you’re doing into ground This only makes sense if the primary side of the transformers have a grounded conductor ("neutral"), and it's bonded to the grounded conductor on the secondary side. I don't know much about common power distribution topologies, but this would seem less safe as it would increase the possibility of the high distribution voltage trying to find its way back to the plant/grid/ground through end-user services.
jcalvinowens 4y ago
Residential US electricity primarily uses center-tapped single phase transformers, with 240V between the two ends of the coil, and 120V from either end to the middle. The center tap is grounded and treated as a neutral for 120V loads from either "hot", and 240V loads run between the two "hots". The advantage is that we get 240V power with 120V shocks (assuming you grab a hot wire while you are grounded). The disadvantage is exactly what the original post was about: if the neutral wire breaks, it can fry everything in your house, because your toaster is now in series with your lightbulbs between the hots.

jcalvinowens 4y ago

> Er, what? It's any/all of the neutral-ground bonds that make the system grounded. The misconception is that cutting the neutral-ground bond at the service entrance makes the system ungrounded, which isn't true. You can cut that, and nothing will change under normal operating conditions. > The more ground connections, the more redundancy and lower impedance That isn't true in the US. The only permitted neutral-ground bond in a house is at the service entrance.

mindslight 4y ago
> The misconception is that cutting the neutral-ground bond at the service entrance makes the system ungrounded, which isn't true That's a better way of putting it. I agree with that. > That [more ground connections] isn't true in the US. The only permitted neutral-ground bond in a house is at the service entrance. I was talking about the US. By multiple ground connections I mean one for the transformer plus one at each service panel.
amluto 4y ago
In the US, if everything else is wired correctly but you disconnect the neutral-ground bond, you may end up with rather high ground impedance. Those two ground rods, 6 feet apart, that satisfy code could easily have quite a few ohms in dry weather. This, in turn, could allow a live (120V to ground) wire to short to an equipment ground, drawing only a few amps and making every grounded object in your house quite dangerous indefinitely. Since the NEC does not require any form of residual current detection except for some branch circuits unless you have a huge service, nothing really protects against this type of failure.
- jcalvinowens 4y ago
  I didn't mean to suggest the bond is useless, just that breaking it won't cause problems unless there is a fault.