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Examples of Problems with Grounding Electrical Systems

Faking out the Ground Plug.

Faking out the Ground Plug.

Tonight’s post is a continuation of the last post “Grounding and Neutral in Power Wiring” and I am going to show a couple of examples of problems with grounding in power and even instrumentation and audio systems. Some of these problems are in my opinion still not resolved and I will discuss several of those I have to fight when working on an industrial electronics systems. Normally in those cases I follow the manufacturer’s instructions unless I know there is a safety code violation or I decide to do a “work around”.

The first thing I will talk about is the electrician friend recommendation to Bryan discussed in his comment on post “Ground, Common, Neural, Reference, and Buses“.  Bryan has an older house that have only two conductors going to the outlets.  The National Electric Code (NEC or simply “the code” in this discussion) did not require the ground wire until sometime in the 1970’s, so houses built before that usually had only two conductors, a hot and a neutral.  Many appliances have three prong cords and this causes a problem for those with old houses.   There is an adapter that will allow a 3 prong cord to be plugged into the two prong outlet, but this adapter has a tab or a short wire that is supposed to be connected to ground but usually is not.   Also, it is very hard to find replacement non-grounded receptacles   There is a code authorized way of doing this, but I will first need to talk about a GFCI (most of the time simply called a GFI).   I will do that in the next post.

The “fix” recommended to Bryan was to replace the non-grounding outlet with a grounded outlet and put a jumper between the ground connection and the neutral as I drew in the first drawing.  The problem with this is the Neutral, or Grounded Conductor is designed to carry current in normal use.  If there is a loose or high resistance connection in that wiring the neutral could be several volts above ground potential.  In a really bad case this could be enough to cause a shock hazard.   For example,  lets say a computer with a 3 prong plug is connected to outlet No. 2,  The case of the computer will be connected that ground plug which now is really a neutral wire.   Now lets say someone has one of those turbo jet 1200 Watt blow dryers plugged into outlet No. 3.  (They are not really called turbo jet, but that is the best description I can think of when trying to sleep and someone is running one of them.)   1200 Watts means it will be drawing 10A at a full 120 V.   Now lets assume the neutral connection on outlet 1 is loose and/or corroded and is approximately 2 ohms.   The neutral wire on outlet 2 would be riding about 20 Volts above true ground potential.   The poor guy operating the computer in the basement (to avoid the turbo hair dryer noise) and wearing wet socks and no shoes because he just mowed the grass will probably feel a tingle when he touches the case.   (Yeah, I know, I was stretching to make the story, and I also know my math was not exactly right because the voltage would be low at outlet 3… but you should be able to get the idea.)

So, why would someone give this advice?  Probably because they never really thought about it, and also because it will “fake out” the inexpensive circuit testers such as this one.  The circuit tester is designed to test circuits that were attempted to be wired correctly for mistakes.    The tester is not designed to test things purposely wired.incorrectly.

Now, to counteract everything I just said, until 1996, the code allowed clothes dryers to have 3 prong 240/120 V plugs and receptacles.   The neutral was used as the ground for the frame of the machine and  the neutral did always carry current because dryers use the 240 V circuit for the heaters, but use one of the hot wires and the neutral for 120 V motor.   The excuse for allowing this was it was a dedicated circuit and there was very little history of anyone getting a shock from the dryer.  However, the glaring inconsistency was too much to continue and since 1996 the code has required a four wire receptacle on all new wiring.  This has 2 hots for the 240 V,   a neutral for the 120 V, and a true ground wire for the machine frame.

2 completely different grounds.

2 completely different grounds.

Before I talk about the problem with the 2nd picture I need to describe what an earth ground really is and how it works.   If a charge is somehow discharged into ground the assumption is the earth is a great big thing, and charges dumped in at one point will quickly disperse throughout the earth,.   Please excuse my gross analogy but it makes the idea stick… It is assumed to be a lot like urinating in the ocean.   The problem with that whole concept is it depends upon the earth’s conductivity at that point and it also depends upon how much charges are being dumped.  More than likely you would not want to swim near the sewer discharge line of a large community and you would not like to be standing near a tower being hit by a lightening strike.  Other things can also  be factors such as the earth at a particular point may have minerals and pH value actually causing things to act like a battery.  This small difference in potential can affect low voltage signals by causing current to flow in a ground circuit.   This is called a ground loop.

The 2nd picture shows two circuits.  One circuit is a newer style which includes a ground wire.   The second circuit is an older 2 wire circuit that does not have a ground wire.   This is a fairly common occurrence on older houses that have had new additions installed.  The owner has a device he would like to have grounded at the older outlet (outlet 2 in the picture) so he decides to drive a new ground rod just for this outlet and now feels confident that he has a true ground.   The problem with this circuit is the ground on outlet 2 may not be at the same potential as the ground on outlet 1.   If the device plugged into outlet 2 is off and drawing no current, there could be a difference in potential between the ground  on outlet 2 and the neutral.   This would especially happen if there was a lightening strike somewhere near by.   Because a very large charge is dumped into the earth at the point where the lightening hit the earth.  For a period of time that point would be at a very high potential and that higher voltage would radiate outward from the strike until it finally dissipates the charge.   If one of the ground rods is nearer to the lightening  strike than the other rod there could be a very big potential difference for a short period of time.

The code requires that all grounds associated with a structure all be connected (the code uses the word “bonded”) together for this very reason.   Also, the ground circuit for all communications lines are also all to be grounded at the main grounding electrode of the structure.  This includes things such as cable TV cables, and telephone lines.   The idea is that if one goes up in potential they will all go up in potential together and any arcing and sparking will happen outside of the structure.

If I was in the situation where I really needed a ground wire for a device and a copper cold water pipe was available, I would probably do it.   I would also try to make sure that cold water pipe was bonded to my main grounding electrode near my electric meter or main power panel.   However, I cannot recommend you do that.  Not without reading the NEC and making sure you are meeting the code requirements for your situation.

Now a couple of  “war stories” related to this before I end this post.    In industrial installations, especially at chemical plants the piping and instrumentation is spread out over a large area and often it is outdoors.   Usually these types of installations have large structures holding the pipes.  These structures, called a pipe rack, have large I beam legs mounted on concrete foundations and form a bridge type structure.   Wiring is placed inside some open things called wire trays that look a lot like a ladder place horizontally.  Inside or alongside the wire-tray there is usually a large bare copper wire to make sure that the whole tray remains at the same potential should there be a lightening strike.   Also in the earth blow the pipe rack another bare wire is placed usually about 30 inches below the surface to form a continuous grounding system and this system is bonded to the steel I beam legs.   In addition Ground Rods are driven along this system and bonded to these two sets of ground wires.   In other words, lots of effort is made to make everything at the same potential even in the event of a lightening strike.
To keep electrical noise out of the instrumentation and control systems, an “Isolated ground” is sometimes requested.  Some people in the past assumed this was a completely different ground system.  I have seen cases where three ground rods were driven and sometimes the near-by earth was treated with a salt solution to increase conductivity of the earth for this ground.  This created exactly the same situation as shown in the 2nd picture and actually is a code violation and a real safety and equipment reliability problem.

What is meant by an isolated ground is the equipment ground for the control system and instruments has its own separate wire connecting to the grounding electrode system..   The idea behind this is that if there is any high current flowing into the ground system due to motors or other high current devices, this current will not cause a voltage fluctuation to the ground connection of the control system.   However, again, both grounding systems are connected together so there is no unsafe potential difference between the two.

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“Examples of Problems with Grounding.” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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