Why doesnt the neutral carry current to ground?

Answer 1

Littlewatts:

If your question is "Why doesn't the neutral carry current to ground, instead of back to the transformer?" then:

It's important to remember current flows in loops. "Ground" is a reference. If you have a single phase transformer, power will flow out of the transformer, through the hot wire to your wall outlet, and back out the neutral wire, then back to the transformer (likely the center tap of the transformer so you have 120 and 240 volts to your house). It is likely the transformer is grounded, so the center tap is at the same voltage as ground, but since there's a perfectly good wire connecting the return wire to the transformer, the current doesn't have to try to go through the ground back to the transformer.

If your question is "Why isn't current flowing in the neutral wire?", then:

In three phase power, with a three phase 4 wire system (the fourth wire is the wye-neutral) there should be little to no current flowing in the neutral. This is because the three phases are balanced - they are of the same magnitude, 120 degrees apart. If you took an instantaneous snapshot of this and referenced to one of the three currents, you would see something like this: Phase 1 = 10Amps, 0 degrees, Phase 2 = 10Amps, -120 degrees, Phase 3 = 10Amps, 120 degrees. If you summed these together, you will see they sum to zero. This summation is what will flow in the neutral. If there is a slight unbalance between the three (say one is 10.1 Amps), you will see some current in the neutral.

If you're referring to your home, where you likely have single phase, and have three prong outlets, the neutral should be carrying the return current from the hot wire. This is not to be confused with the ground plug (the round one), which should not be carrying any current. The purpose of this plug is to provide a low resistance path to ground in the event something bad happens (so the current chooses this wire to flow in instead of you).

There is the chance that you're connected to an ungrounded system. This would likely have three wires instead of four. In this case, there is no neutral. Without knowing exactly what you're measuring it's difficult to provide a clear, short and concise answer.

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Alex156:

Other answer (explains how a circuit works in detail): Because it is not designed to. The neutral line carries the 'return' current; whereas the ground line is reserved as an unobstructed bypass for emergency current drain.

Neutral does not "carry current to ground". Neutral carries current to some return point, which may or may not be grounded. Neutral, however, is always grounded at some point which, conversely, may or may not be the return point.

Confusing? Seemingly reverse defined? Then you better pay attention, because understanding is critical.

Take a light bulb, a switch, a battery, and three pieces of wire, and series connect them. You can turn the lamp on and off by closing and opening the switch.

This is a standalone circuit, and it is not grounded anywhere. It works just fine.

What is the voltage on the battery anode?

Wait! Before you say 1.5VDC you better be asking "with respect to what?"

Well, lets connect battery cathode to ground. Stop and think about this little circuit. You can still turn the bulb off and on by opening and closing the switch.

What did the ground do?

Absolutely nothing - or absolutely something - depending on your point of view...

There is no current on the grounding wire. None. Nada. Zilch. Try it with a micro-ammeter; except for parasitic leakage, there will be nothing.

Now what about that wire going from the cathode to the near terminal of the switch or bulb (depending on how you wired it)? That carries current, don't you say. More on this later.

Back to the "absolutely something" part... That grounding wire provided a connection from some arbitrarily chosen point in the circuit (it could have been the anode, for all I care) to some well defined ground, ground being defined as the common connection between multiple connections that all wind up connected together to the dirt, i.e. ground.

If you measure the voltage on the anode with respect to ground then, lo and behold, you have 1.5VDC. Why? Because you created a circuit with the meter.

Now, go back twelve paragraphs, to my (Alex146) second paragraph... I said "at some point". I'm going to get picky and say "at one and only one point". This point CANNOT be overemphasized, because, with only one point, there can be no current; and with more than one point, there can be no safety

Hence, the neutral does not carry current to ground - it carries current back to the power return point. Conversely, the grounding conductor never carries current except in a fault condition.

Oh - a new word - fault...

Lets modify the circuit and put a 1 ampere fuse between the battery anode and the rest of the circuit. That fuse is effectively a piece of wire because its resistance is very much lower than the bulb, but not as low as the wires. Things still turn on and off. OK?

Now, lets create a fault by connecting a wire between the load side of the switch and ground (across the light bulb). This is a short circuit.

With the switch open, nothing happens. With the switch closed, two things happen. The battery attempt to deliver infinite current, which it cannot, and the extra piece of wire, be a machine frame or whatever, becomes electrified, potentially creating an electrocution/fire hazard, i.e. extra voltage/current where it should not be.

If the original circuit was still ungrounded, the lamp would still light, but the battery cathode would become -1.5VDC with respect to ground. This offsetting of voltage isn't a big deal in single phase power, but can raise three phase power voltage levels with reference to ground to 1.732 times what they should be, which can initiate fault conditions in other phases. Computer like equipment (called protective relays) are used to detect these overvoltage conditions and hopefully warn operators and take care of any potential problems before a full fault develops.

If, on the other hand, the circuit were grounded, then the short circuit would cause the fuse to blow before any wiring could be damaged and, hopefully, before anyone could get electrocuted.

This all sounds trite doesn't it? Well, it turns out that a lot of engineers spend time designing adequate grounding systems, as well as designing the National Electrical Code (US); and I will assure you that they do not do so just because they "want to".