Why the neutral in USA wiring?

Power is supplied to residences in the US as 240 volts center tapped. If an appliance needs a high power level (range, water heater, heat pump) is gets the full 240 volts and both lines are switched to turn it off. The two lines swing in opposite directions around the center tap (picture an electrical see saw, with the seats being the lines and the center tap being the central pivot. The center tap voltage is grounded at the pole, and at the service entrance (fuse or breaker box) to minimize the peak voltage on any line with respect to ground. The grounded center tap is called the neutraled conductor or the neutral.

Receptacles that supply lower power appliances and lighting circuits use only half of the supply (one line and the neutral) to feed half of the 240 volts or 120. Since the neutral conductor is at or near ground potential (except for the voltage drop along the neutral wiring, back to the ground point) there is little safety or functional reason to break it with a power switch. Breaking the single hot line turns the circuit off, while keeping the load near ground potential.

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John Popelish

That is only one scenario that allows a trap to be created with an ungrounded system. There is actually no such thing as a perfectly isolated AC voltage supply. So if no point in the system is referenced to ground, then the voltage at any part of the system is almost indeterminate with respect to ground. A very high resistance leakage to the high voltage primary side of the distribution system may allow thousands of volts to appear on all the low voltage side terminals, with respect to ground. This would be especially problematical during lightning strikes on parts f the high voltage side that cause faster than normal rates of change of voltage that drive capacitive currents or even flash over from primary to secondary. Without the grounded neutral or some substitute, there would be many more dangerous over voltage situations in residences.

But for traps like you describe, the safety ground wiring in all neutraled systems must be heavy enough to decisively trip the over current protective devices in the system, immediately on a line to ground short. And for that same reason, loads that have the neutraled conductor carrying their current have all their control devices in the hot side, so that while the fuse is blowing, the load is shorted to ground on both sides and thus, turned off.

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John Popelish

Roger Johansson wrote: (snip)

Pedantic quibble for the beginners:

You would die immediately from the high current driven through your body by the 2000 volts this situation would place across two points on it.

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John Popelish

John, while everything you wrote is true, it still doesn't answer the question of why a neutral is needed. You say "The center tap voltage is grounded at the pole, and at the service entrance (fuse or breaker box) to minimize the peak voltage on any line with respect to ground." But of course if none of the lines were grounded, none would be any closer to ground potential than another. The real question is why is a neutral necessary? If none of the lines were grounded, all our appliances would still work the same. Let me explain it the same way it was explained to me.

Suppose you have a steam boiler and a pressure activated switch to prevent the steam pressure from getting too high. The circuit would work perfectly well if none of the lines was grounded. But suppose decades go by and the insulation deteriorates and an unintentional ground develops at point A. Still there's no problem and the circuit continues operating normally and nobody even notices. More decades pass and another unintentional ground develops at point B. Now what happens when the pressure switch opens? The pressure switch is bypassed so the boiler keeps getting heated until it blows up. If however there is a ground at point C, then when the first ground develops at point A, the fuse blows and an electrician investigates and finds the unintentional ground and fixes it.

----o~o--------x--------\--------x--------| fuse A pressure B / switch \ heater / element \

-----------------------x------------------| C

(if the circuit doesn't line up, try changing the font to courier new.)

Hypothetical situation...

You are standing on wet ground and randomly grab wires. Some kill you and some don't. Switches are typically SPST. Where should you put it? By defining one line as ground by tying it TO ground you get a reference which will tell you the ones more likely to kill you and (coincidentally) where to best place the circuit breakers and switches.

We humans live on the earth surface, so we are very often in contact with the real ground potential.

If we had free-floating voltages in our mains sockets: Suppose the wires carrying the voltages for a common light bulb would be at 2000Volt and 2110Volt. The lamp would work just like today, because it would get

110Volt, the voltage difference between the two wires.

But the risk for us humans would be very high. If you happened to touch the metal on the lamp while putting in a new lamp, and your feet would be on the ground, you would die immediately, from the 2000Volt through your body.

That is the main reason why a ground is needed in the mains wiring. It is a reference point for the other voltages in the mains wiring, and it should be very close to the voltage in the earth surface and concrete floors, water pipes, kitchen sinks, etc..

It makes sure you never come into contact with mains voltages which are further from ground potential than 120Volt in USA, or 240 Volt in other countries.

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Roger J.

Others have described why the neutral wire is grounded, but have not really answered your question.

You need two wires to complete an electric circuit - the neutral wire is that second wire in our 120V wiring. If you just connect the hot wire to a lamp, with no neutral, no current will flow, and the lamp will not light.

--
Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html 
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I wasn't clear in my post. I wanted to know why one wire was grounded and the other not. ERS

I understand why the neutral is used for safety reasons now. It makes tons of sense. Especially having controls all on the hot side so failures to ground can't circumvent the controls. Thanks to all who responded to my question. This is what makes usenet great. Eric R Snow

Because if you grounded them both, it would short out the power plant.

;-)

One is grounded for safety. You can't ground bother of them or you would short out the power transformer. The center tap of the power transformer is grounded and connected to the neutral at the main breaker or fuse box. This is done to keep the maximum voltage to ground at 120 volts instead of 240 volts, again for safety reasons.

Neutral = little or no voltage different from safety ground.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

Simply; the neutral is required to carry the current back to the centre tap of the 230/115 volt supply. All circuits must be complete for current to flow. The neutral conductor is close to ground voltage (except for the slight voltage drop due resistance of the wire and the flow of current through it). The neutral is not switched as are the live 115 volt wires. In North American practice the centre tap of the distribution transformer is grounded. AIUI in UK practice one side of the 230 volt supply is the neutral and is basically grounded.

the CT here in the USA is also grounded(Neutral). the advantage we have his is that we have 2 separate

120Vac lines out of phase and non load sharing or, 1 Single Phase of 240Vac using only the outer taps.. so we have a choice. we have a real ground also on site to insure safety.

i am sure you understand this, this is for every one else that is interested.

In Europe we usually have 3-phase 240Volt. I have that in my summer house.

It means I can use any one of the 240V lines and the ground wire for most household uses. And I try to divide my use fairly equally between the three phases to keep the system balanced.

Or I can use any 2 phases and get 380-400Volt between them.

For heavier appliances and machinery I use 3-phase power.

--
Roger J.

Here's my understanding of the need for a ground and a neutral wire on the same circuit. No guarantees. . .

The ground (green, uninsulated) and the neutral (white) wires are both connected to the same place in the breaker box, which is connected to the incoming neutral line. I know this because I wired my house when I built it, and the electrical inspector passed it.

110 volt outlet circuits connect to one hot side in the breaker box, or the other, corresponding to the two incoming hot lines, which are 240 volts apart. The white or neutral wires are connected in the breaker box to the neutral bar, which is connected to the incoming neutral line, being center tapped between the two hot lines which are 180 degrees out of phase. The incoming neutral line is also attached to 'earth', with a heavy wire going to a long heavy copper bar buried in the earth.

When you wire an outlet box, you attach the uninsulated green wire to the box if it is metallic, and to the third 'ground' connection of the outlets. A three prong plug (presumably attached to an electrical device) picks up the hot, neutral, and ground connections.

So . . .

When a device is running, there is current in the hot-neutral loop. If there is a long run between the breaker box and the outlet (or device), there will be some small voltage drop across the neutral wire due to resistance or impedence in the line. It is no longer necessarily at 'earth' level. If someone were attached to the earth and grabbed the neutral line, and were a Very Good Conductor, (s)he could get a shock. But the green ground wire never carries current, so it should be at 'earth' level. It's nice to have that reference around. Especially when you're in a puddle, or a shower.

Well, that's my story and I'm stickin' to it! :^)

-- Barry

For crissakes don't any of you realize that the neutral is the return path for the current and completes the circuit!! The ground (green wire) makes sure the hardware/enclosures the current carrying circuits are housed in are grounded for safety reasons. . . .DUH!!

Fer crissakes, I thought that's exactly what I said. And I described the system for those not familiar with US wiring.

And fer crissakes, learn to bottom post.

--
Barry

That's an Australian term, right? Multiple Earthed Neutral? How is that implemented?

I've seen earth potentials range over a few volts within a few tens of meters. What does that do to the neutral line?

--
Barry
****

**

path

..DUH!!

Also the MEN system helps out adjoining premises if they have a high resistance earth and seems a good way to have a complete earth 'mat'.

-- Regards ..... Rheilly Phoull