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Ground, Common, Neutral, Reference, and Buses

Common Symbols – in Common Use
(No that is no typo… hahaha)

Today I am fussing and cussing while working on my car and while doing that I thought “I sure hope it is not a ground problem”.  At that point I remembered I stumbled over that word in my last post.  So, as much as I hate to, I am going to have to devote a post dedicated to precisely defining some words and then at the same time talk about the imprecise way we normally use them in common language.   The imprecise language is commonly used by electricians and engineers as well as others and that in part is what leads to the confusion.   It gets even worse, the symbols I show in the first picture are not always used consistently either.

A BUS:

In electrical terms a bus is a common connection point.  It is assumed that the voltage at one point along the bus is the same as all other points.  A good example of a bus is the long skinny side connections on the breadboard test board I talked about in the previous post.   I wired these so one is the +Vcc, one is the -Vcc, one is the common/reference/neutral bus.   The final one was wired to +5 V that we are not using at this time.  A bus in electrical terms is very similar to a manifold when talking about piping or a plenum when talking about air or some other gas.

Because schematic diagrams quickly become very busy drawings, often some wires are not shown in the diagram.  Instead a common symbol is used at each point where a connection is made to a bus.  I did that in my diagrams and used the hollow triangle symbol to show the connection to the reference point.  Some would call that the ground connection.  I often would do that except that I am teaching mode and it is not really correct.  So, do as I say…. not as I do!  Seriously, I sometimes get sloppy in my talk, but never when it is important and while you learn it is very important.  Also, you will have to learn how terms are meant from the context of how they are used.  I also showed wires hanging out in the air with the label +Vcc or -Vcc instead of drawing an actual connection to the power supply.

Common vs. Neutral vs. Ground:

In an automobile, one of the goals is to save money while manufacturing it.  One of the ways of doing that is to save copper wiring.  As you look at wiring of lights etc. you will find there is only one wire.  The body and frame of the car is metal and used as the second conductor.  In common terminology people call this ground but that is not precisely correct. The automobile sits on rubber tires and is nowhere connected to the earth, or ground. The correct term would be common or reference point because this is the common return for the electricity.  (Conventional current definition was used there… flowing from + to -.  Being precise can be a pain!)  The shop manual on my Honda refers to this as the ground connection.

While on the subject of automobiles there is a very important safety statement related to all of this.   Should you ever have to disconnect or replace a battery, always remove the connection going to the frame first.   By doing so if your wrench slips and touches the metal frame while touching the terminal you will not be creating a short circuit and possibly destroying the battery and even injuring yourself.

In our power supply for our Op-AMPS we are using both a positive and a negative power source with the center connection tied together to make a common or reference point.  Going back to the dimension lines analogy, if we assume the reference is ground floor, then the negative power supply is the basement level and the positive power supply is the first floor and ground floor is a reference point of 0.  Many would call this ground.

House power in the United States is very similar to our op-AMP power supply, but this is where ground becomes very important.  For now we will assume DC instead of the more normal AC.  There are three wires coming into your house from a transformer.  Think of this exactly as the power supply except one wire is +120V the other wire is -120V and the 3rd wire is zero.  At the corner of your house there is normally a rod driven into actual earth.  This is called the ground rod and is your grounding point.  The zero wire is connected to this rod and splits into two bus connection in your main circuit breaker box.

One of these buses is called “the neutral bus” and one is called “the ground bus”.  Both of these are at exactly the same voltage and both are at zero volts.  However they serve two completely different purposes.  On a normal 120 V circuit in your house this will go to a receptacle with 3 connections.  One connection will have the 120 V wire, commonly called the hot wire.  One connection will have the neutral wire and the round connection will have the ground connection.  Although the neutral wire and ground wire go back to the same point, the neutral has the purpose of normally carrying current. The ground wire will only carry current if something fails.  It may be connected to metal frames of appliances and keeps them at the same potential as the earth.  This provides protection to you if you have wet feet and touch the metal frame. (I would not test that… it could be that you have a faulty ground connection in that circuit and that would be a shocking experience of a way to find out.)

So now after all the story: Is ground a good way to refer to a common connection?  It works, it is commonly done, so it is OK but it is not precisely correct.  In other words, know the difference because sometimes it is important and sometimes it is not.

Now the symbols shown in the picture.  All are used to show a common return connection.   Sometimes, in some circuits there are two return connections; one for a digital circuit and one for the analog circuits.  In that case, sometimes they denote the difference by filling in one triangle and not filling in the other triangle.  On those sorts of drawings somewhere on the drawing will be a legend explaining what each one means.   The third symbol, the one that looks like a garden rake is almost always used to show an earth ground connection.   The key word there is “almost”, I believe I have seen it used otherwise.  The fourth symbol is used either for a true earth ground or just a common reference point.

Now to really confuse things, a floating ground is a common connection that is not connected to earth.  The problem with this is if there are are high voltages near by this voltage could be very high in reference to the earth.  The circuit could be operating like normal and not having any problems, but if you were to touch it you could get a shock because it is at a high potential to you because you are more than likely at earth potential.

Announcements:

On the resources page I have put links to several CAD drawings I have created.  Those are there for your download.  None are very pretty drawings, but they are drawings I use to create the diagrams used on this blog.

As always if you find this blog useful and interesting please consider subscribing by using one of the methods on the upper right column on the home page. If you know anyone who might find this useful and informative please share the site with them.

Gary


 

 

 

 

 

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4 comments to Ground, Common, Neutral, Reference, and Buses

  • […] Electrical Ground, Common, Neutral, Reference, and Buses – Although Ground, Neutral, and Common are used almost interchangeably, there are some slight differences and these in some cases can be very important. […]

  • […] blog to put no time limit for comments and I got a very interesting comment/question in the post: Ground, Common, Neutral, Reference, and Buses.   I am not going to repeat the question and my answer here, but that question and my reply made me […]

  • Bryan

    Hi Gary,

    I have a couple of questions. One being about using the Neutral as a Ground. I live in my grandparents old house in a rural area. It was wired probably back some time after 1953 when the rural electric cooperative got electricity to the area. Naturally, it has the old two wire romex with no ground.

    Over the years things were semi-upgraded. I helped my dad put in a new service entrance and add a couple of 110 circuits. I wired everything with 12/2 with ground, but that’s not the problem. The problem is the old original circuits. I’m concerned about the old romex wire and not having a ground. Granted a lot of things these days are double insulated and have no ground prong, but some of the electonic stuff requires a ground. I took care of some of that with new circuits, but occasionally we run into a situation where we need a three prong recepticle in other parts of the house.

    We moved in about 8 years ago and the first thing I did was to put a GFI recepticle in the bathroom. I know it’s not kosher, but I ran a ground wire to the cold water pipe under the sink. It works so good or bad that a nearby lightening strike trips it during a thunderstorm occasionally. I haven’t put GFI recepticles in the kitchen though, since everything is double insulated and we know not to go probing in a toaster while touching the kitchen sink, faucet, or stick our hand under running water. Actually, we’ve done away with the old toaster and have a toaster oven.

    I hope one day in the future to totally rewire the old house when remodling, but for now an electrician told me just to put in three prong outlets with a jumper wire from the neutral to the ground. I know it will work, but is that sound electrical advice? That’s my first concern.

    My second concern is the age of the old two wire Romex. It was put in back in the day when the electricians twisted the wires together and soldered them in the junction boxes, complete with a little electrical tape. I guess wire nuts came along after that. Should I be concerned about the age of the wire? I worry about an electrical fire all the time.

    Thanks,

    Bryan

    • Gary

      Hello Bryan:

      There are lots of questions hidden within your question. I will not be able to give you a simple answer for two reasons. First, and foremost is I am not there looking at the situation so I cannot answer if it is safe or if it is to code. The second, is liability. No matter how well an installation is done, there is a chance something unforeseen can happen and cause a problem, including a fire. So… since I am giving you a non-answer answer right now, (I am sounding like a politician… shoot me now… put me out of my misery), the best I can do is give you a whole bunch of education about it all so you can make your own decisions on what is acceptable to you and what is not.

      I will explain how a GFI works and why those are very good things. What Grounding does and while I am at it I will look up what the current National Electric Code, NEC, requirements are when replacing 2 wire receptacles. The code is updated every 3 years and Grounding has been an ever evolving issue. Before 1992 I owned a house that was built in 1955 and wired with 2 wire romex so I know what you are facing and the code had a work-around back then, but it may not be acceptable now.

      For now, I will answer that the putting in 3 prong outlets with a jumper between the neutral and ground is very lousy advice. Both eventually do go to the ground rod at your service connection, but the neutral wire is intended to carry current in normal situations, the ground wire is only intended to carry current in fault situation or leakage current in normal situations. I actually like your method of using the cold water pipe much better, but it has some problems also. (Plastic pipe has ruined that fix many times.)

      This whole thing is probably going to take several blog posts to “answer” and then you may not feel like you have an answer, but you will know a lot of stuff.

      As a final comment for right now I will give you two very good resources for these sorts of questions. First there is a magazine than is now on the web called Electrical Construction and Maintenance. They have a couple of sections about code issues. Their home page is http://ecmweb.com/

      The second resource, and pretty much the code Guru is Mike Holt, the forum is a good place to go to get lots of opinions, His home page is http://www.mikeholt.com/

      And finally keep checking out my site I will be creating a few blog posts where I talk about these issues and hopefully lace it with a few “war stories” about actual situations I have ran into in industrial, commercial, and some personal residential electricity.

      Gary

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