During a storm, a large branch landed on the drop line to my house. This snapped the bare neutral cable cleanly in two but the two hot legs remained intact. I had no idea this had happened until I saw the wire the next morning. I still had power and nothing seemed amiss. My computer UPS did not even trip. How could losing the neutral to the transformer go unnoticed? Did all the (unbalanced) current return to the transformer through the ground?
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4\$\begingroup\$ Which country are you in? \$\endgroup\$– virCommented Nov 8, 2023 at 20:24
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1\$\begingroup\$ Probably your loads can stand with bigger voltage and difference in phase current not so big. Over current will not trip if we're is not over current. Most modern power supplies can work at big range of voltage. \$\endgroup\$– user263983Commented Nov 8, 2023 at 22:21
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\$\begingroup\$ These effects are fairly comprehensively discussed on home improvement under lost neutral diy.stackexchange.com/search?q=Lost%20neutral \$\endgroup\$– user1937198Commented Nov 9, 2023 at 13:40
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2 Answers
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In the US, where you reside, the earthing scheme applicable is the TN-C system in which the neutral and protective earth are bonded at the consumer.
Should the neutral line break, continuity is still maintained via earth.
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2\$\begingroup\$ What's the typical resistance of the ground connection? Higher or lower than the neutral wire? I'm wondering that for two reasons: 1. If the ground resistance is lower, most of the current would flow through that anyway, and (2) if it is high, there may now be none-negligible potential on the home's neutral (is that possible when it's grounded?). \$\endgroup\$ Commented Nov 9, 2023 at 12:22
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2\$\begingroup\$ What's key which is not mentioned in this answer: in a well designed split phase setup, the current on the neutral supply should be minimal to the difference between the two hot currents, with most of the load being hot to hot, with the internal neutral connecting the phase loads, rather than return to supply. As such any ground load like this may not be large and/or compensated by the neutral swinging to one phase or the other. \$\endgroup\$ Commented Nov 9, 2023 at 13:30
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\$\begingroup\$ Thank you both for your thought-provoking comments \$\endgroup\$– vu2nanCommented Nov 9, 2023 at 14:10
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How could losing the neutral to the transformer go unnoticed?
If you have GFCI breakers they will not trip from this sort of fault.
In some countries, such as the US, the neutral is bonded to ground at the transformer pole, and again at consumer's location. If the neutral wire between the pole and the consumer is broken, current can still flow between the pole neutral and the consumer's neutral through the ground. (And as user28910 points out, also through the ground wire on any neighbors that are on the same transformer.)
Because the consumer's bond between neutral and ground is before the GFCI circuit breakers, current in a consumer load will return through the neutral wire and the circuit breakers. Hence a GFCI will NOT be triggered by a load current in this type of fault.
Did all the (unbalanced) current return to the transformer through the ground?
Yes, the unbalanced current flowed through the ground. But as @Justme points out, if you have split phase power, and the two phases happen to have approximately equal current, the unbalanced current might be small. If the ground is particularly moist, like during or after a storm, the ground resistance might be quite small.
answered Nov 8, 2023 at 21:27
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5\$\begingroup\$ Also if the phases happen to have approximately equal load, the neutral current is approximately zero or small anyway, and the result is that ther is not much voltage imbalance between phases. Sure some amount might return via soil. \$\endgroup\$– JustmeCommented Nov 8, 2023 at 21:41
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1\$\begingroup\$ Re, "consumer's bond...before the circuit breakers." In my panel, the bond is neither before nor after any circuit breaker. The neutral line does not go through a breaker. \$\endgroup\$ Commented Nov 8, 2023 at 22:10
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6\$\begingroup\$ @SolomonSlow: It would if you had GFCI breakers. \$\endgroup\$ Commented Nov 8, 2023 at 22:22
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\$\begingroup\$ Neutral is also grounded at any neighbors that are on the same transformer \$\endgroup\$ Commented Nov 9, 2023 at 10:15