Using AC Neutral as a Reference

Can someone explain using AC Neutral as a reference? George Carlson talks about using AC Neutral as a reference on the Grainger Potential Relay he uses to build an RPC.

From the schematic it appears that the potential relay uses the 2 hot

legs and then "AC Neutral". Is AC neutral the same white wire that you see on 230 volt appliances such as driers where you have a black, red, white, and green wire?

If so how does this work with the relay? Doesn't this neutral bond to the neutral bar in the houses circuit breaker box?

Any help greatly appreciated.

Thanks, Steve

Reply to
Sierevello
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Think of the AC Neutral as a current return which it is, and not a reference.

Sometimes the ground and AC Neutral are bonded together, still don't loose sight that the AC Neutral is a return and not a reference or a ground.

I'm not familiar with George Carson's talks, nor what he believes that the neutral is a reference to. Ground is always the reference, since the voltage present on a neutral is a function of the current passing thorough it because of its resistive drop, while ground potential is fixed since normally no current should be passing through your grounding wires.

Harry C.

Reply to
hhc314

There are occasional cases where this is not true. If you have a "floating" (ungrounded) power system, as you often do when you use a generator and in some transformer circuits, then ground is reference to nothing.

Vaughn

Reply to
Vaughn

wrote in message news: snipped-for-privacy@f14g2000cwb.googlegroups.com...

Not sure about other parts of the world, but in the US, from the portions of the residential NEC code I have worked with, the neutral and ground only are tied together back at the main panel. If you go through a sub-panel etc, the neutral and ground are different wires. Back at the main panel, they are all tied together and go to a ground (as in copper clad rod in the earth). So, if there is no load that is using the neutral as a return (e.g. 220 volt load), and there is no current in the neutral, then it is the same voltage as the ground, HOWEVER, if there is any 110 volt load (uses one "hot" leg and "neutral"), the voltage will be different between neutral and ground by some amount. Depending on what levels of precision you are measuring at, that may be insignificant or horrible (hospitals are very touchy about grounds -- only takes a few mv to cause all sorts of problems when hooked up to a patient -- lots of opto isolation used there and BIG ground bus bars run around). I would assume you are correct in the colors for the drier that you mentioned. In our area, it is normal to only use 2 conductor + ground for water heaters (and then you must mark the white wire with tape etc to indicate it is no longer neutral). Can't remember right off hand how my dryer is wired -- seems to me our area only has a 3 pin plug so it would be 2 hot and ground. I know some areas do require a 4 pin plug so it would be 2 hot, neutral and ground. Just remember - if there is any current flowing in the neutral (as in a return), it will not be the same voltage as the ground (which hopefully does NOT have any current flowing in it -- if it does, you have a problem).

mikey

Reply to
Mike Fields

The AC Neutral and the ground returned wires of any power system had /better/ be bonded together at the current source! Either at the Generator frame, or at the main service panel for utility feeds. Otherwise, you'll be out of compliance with NEC and in big trouble with the gods of Murphy.

And if you are hooking up a potential relay, and that device is sending any current through the connection, it has to be connected to the white neutral wire. You are not allowed to send any working current through the safety ground wire! (The only exception being grandfathered electric stove and dryer circuits, and only for small ancillary loads like the light or timer at that.)

The whole idea is that the safety ground wire is NOT a current carrying conductor except in the case of a fault in the device.

Right. But if you're running on a generator derived circuit and you aren't in a moving vehicle during it's use, that's why you carry a ground rod, a ground rod clamp, and a short hunk of cable with you.

It's easy enough to poke the ground rod into a convenient patch of dirt, or clamp the wire to a convenient metallic cold water pipe or other permanent grounded point, and ground the generator frame. Then the frame ground IS reference to earth ground, and things that look for a sneak current path to earth ground (like GFCI outlets) will work as intended.

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Reply to
Bruce L. Bergman

Bruce... quick question. I'm running a 240 volt single phase 50 amp subfeed to my (detached) garage panel from a 50 amp CB in main panel. My plan was to run black / red/ white and green from main panel. Do I also want to sink another ground rod out there or just depend on the ground in the house panel which is to the water meter (jumpered around with wire as required). B.G.

Bruce L. Bergman wrote:

nothing.

Reply to
MachineShop

Reply to
Modat22

So do I still want to run the green equipment ground wire from the house service or forget? I think If I remember something about multiple grounds on a system not being good. Believe it or not I was a licensed electrician in Minn. a long time ago. B.G.

Reply to
MachineShop

My initial impulse is to say yes, sink another ground rod at the shed and have the green wire from the house main tie them together into a larger and more effective ground network. But I didn't look in the code book, it's in the truck.

And you do want to run a separate ground and neutral wires to the shed, and make sure that the bonding screw or jumper is NOT used in the shed neutral bar - the neutral and ground are ONLY bonded at the utility service disconnect. You need to use an insulated neutral distribution bar in the shed sub, and a separate grounding bar.

You can not reuse a Meter-Main panel for the shed sub (using a watthour meter to monitor usage) without installing an insulated neutral bar, those usually have factory-bonded neutral/ground bars bolted straight to the case.

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Reply to
Bruce L. Bergman

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