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Hypothetical: I have two circuits that are being powered from the same electrical service. They both have a hot and a neutral wire from the same transformer.

  • The first circuit and the transformer are earth grounded locally with a ground rod.
  • The second circuit is earth grounded on the other side of the country with a second ground rod.

Assuming the voltage drop is acceptable for circuit #2:

  1. Are these equivalent circuits?
  2. What problems could occur when a circuit is grounded elsewhere than where its electrical service is grounded?

Hypothetical Schematic

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Just to clarify, you are talking specifically about earth ground. In general, "ground" is any arbitrarily chosen reference point in a circuit. \$\endgroup\$ Commented Aug 2, 2019 at 16:45
  • \$\begingroup\$ @ElliotAlderson, yes that is correct. Specifically earth ground. Thanks for the clarification edit. \$\endgroup\$
    – user199402
    Commented Aug 2, 2019 at 17:10
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    \$\begingroup\$ not same ... circuit #2 does not have neutral tied to ground locally \$\endgroup\$
    – jsotola
    Commented Aug 2, 2019 at 18:26
  • \$\begingroup\$ Is this a homework question? It reads exactly like one. \$\endgroup\$
    – TonyM
    Commented Aug 3, 2019 at 1:27
  • \$\begingroup\$ @TonyM, not a homework question -- uni's been done for a few years but it has obviously had an effect. Just a thought I had at work when discussing with clients. \$\endgroup\$
    – user199402
    Commented Aug 3, 2019 at 2:06

2 Answers 2

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Are these equivalent circuits?

Not really. The resistance between G and N on CCT#1 is likely to be very low. It may be less than an ohm.

The resistance between G and N on CCT#2 is likely to be much higher. With a simple rod hammered into the ground, it could be anything from a few tens of ohms to a few kilohms. Dirt isn't a good conductor, especially if it's dry.

The first supply would be classified as "TN-S", the second is "TT".

What problems could occur when a circuit is grounded elsewhere than where its electrical service is grounded?

You haven't said what circuit protection devices CCT#1 and CCT#2 have. A short between L and G on CCT#1 will cause a large current to flow. This should trip a circuit breaker or blow a fuse.

A short between L and G on CCT#2 is unlikely to pass enough current to trip a breaker or blow a fuse, unless you have an unusually good earth rod. Such circuits really need to be protected by an RCD/GFCI to be safe (RCD and GFCI are the same thing under different names).

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  • \$\begingroup\$ Surely the first is not really TN-S if there is a local ground rod? It could be either TN-S or TN-C-S depending on where the earth and neutral are connected (away from the property or as the supply enters the property). However, with the local ground rod it’s not like either of those conventional earthing systems (as used in the UK). \$\endgroup\$
    – David
    Commented Aug 2, 2019 at 23:14
  • \$\begingroup\$ @Simon B, I did forget to add explicit circuit protection in my diagram. CCT#1 and CCT#2 would each have standard circuit breakers. I'm looking at this from an industrial mindset. So for CCT#2 are you saying the breaker wouldn't trip? Isn't the current to ground what causes the trip though? \$\endgroup\$
    – user199402
    Commented Aug 3, 2019 at 2:13
  • \$\begingroup\$ Ground rods can have different resistances to ground. A special meter is used to measure that. Sometime a longer ground rod can provide the low resistance spec'ed for a system. Sometimes a spread-finger system is used when a ground rod cannot be pounded in. There are other methods as well. google.com/… \$\endgroup\$
    – CrossRoads
    Commented Aug 3, 2019 at 13:11
  • \$\begingroup\$ The problem with earth rods is that the resistance is often too high to trip a circuit breaker. For example, if the supply is 120V, and the earth resistance is 24 ohms, then only 5A will flow. That isn't going to be enough to trip most breakers. That's where an RCD/GFCI comes in. That will trip at a few milliamps. \$\endgroup\$
    – Simon B
    Commented Aug 3, 2019 at 22:40
  • \$\begingroup\$ @SimonB, that makes sense. If the ground were tied back to neutral then you'd have a low resistance current pathway and pop the breaker, correct? \$\endgroup\$
    – user199402
    Commented Aug 6, 2019 at 14:16
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Are these equivalent circuits?

Yes, with small stipulations.

As far as the circuits go? If they are both going to earth, it will accomplish the same thing. Any fault current will go back to earth as intended. Ground normally doesn't carry current, and is generally used in a fault, while there will be slight variations of voltage depending on where the ground rods are place (even the earths potential isn't always at zero volts, but can be considered to be at zero volts)

What problems could occur when a circuit is grounded elsewhere than where its electrical service is grounded?

If there are other circuits that use ground for fault detection then the configuration above may not work. It may also not satisfy local electrical codes so check those. It may also cause ground loops with sensitive equipment if cables were ran between the two devices on separate grounds

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    \$\begingroup\$ For a major fault to Earth ground, this happens. No, the Earth is not designed to carry current. \$\endgroup\$
    – rdtsc
    Commented Aug 2, 2019 at 19:05
  • \$\begingroup\$ @rdtsc That's probably the worst place for a fault! There is a power plant a few miles away from one of the places I lived, about 1600MW. I read an article in the paper that a very small current could be measured in the ground from the transmission lines. \$\endgroup\$
    – Voltage Spike
    Commented Aug 2, 2019 at 19:10
  • \$\begingroup\$ @Voltage Spike, is dissipating fault current to earth all that is required to make it safe? This scenario doesn't satisfy local codes, I'm aware of that. One code that stood out is that multiple ground rods for a building need to be connected by a ground conductor. If all grounds were equal I didn't think that'd be required. \$\endgroup\$
    – user199402
    Commented Aug 3, 2019 at 2:09
  • \$\begingroup\$ @rdtsc, I'm surprised that lasted so long! \$\endgroup\$
    – user199402
    Commented Aug 3, 2019 at 2:10

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