Primary Neutral Returns

Ground current has many different names such as ground current, stray voltage, or contact current.  All of these names mean the same thing:  there is current that is not being contained in a wire.  In a perfect world there would be no ground current.  Ground really is zero.  However, that is not the world that we find ourselves.  A large reason for ground current is the electric utility company’s undersized primary neutral returns from the transformer to the substation.

Our electrical grid has remained much the same despite the advancements of technology and that is where the problem lies.  The introduction of more and more nonlinear loads to the electrical grid (done for energy efficiency) has been a major contributor to this problem.  As time has passed, the use of more and more computers and electronics has become a staple in our lives.  Computers and most electronics use what is called a non-linear load.  Non-linear loads do not use current proportionately to the voltage.  In doing so harmonics are created.  These harmonics then ride on the power cycle which increases the impedance of the electricity returning on the primary neutral.  As the harmonics from electronics add up, the neutral wire becomes overcrowded.  At the transformer the neutral and ground wires are bonded.  This is done for safety reasons, which is now being manipulated on a more frequent basis.  Because the primary neutral wire is overloaded, voltage travels back on the ground, because electricity will take all and any paths available.  By traveling back on the ground, so rises your ground current.

This is all measurable.  If you look at the ground current that is present in my home, you can get the following reading using a Fluke 190-202 Scopemeter:

2-28 ground current my home waveform

With a waveform you are able to then identify problems.  Referencing a book on power quality issues such as the Handbook of Power Signatures 2nd edition, by Dranetz BMI pages 44 & 48, you will find a similar wave.  In the book, the solution is simple, decrease the length of the neutral wire or increase the wire size.  Another solution not listed in the book is to add another neutral wire. However, this is just one way to test this.  Here is another.

To further test to see if the neutral wire is in fact over loaded, you can run a simple experiment.  With a 190-202 Scopemeter hooked up sink to floor you can look at the ground current as you turn on phase loads.  For people that are unfamiliar with this, it means the devices in your house that need a special outlet (240V).  In the figure below you will see a sharp jump when I turned on devices in my house that use phase current at the same time (dryer, electric stove, furnace).

phase test done in home

If the neutral wire was sufficient there would be no jump.  The fact that there was this jump further supports that the neutral wire is not big enough to handle the loads in this area.  This isn’t just in my home.  This is in all the homes that are on the same circuit to the substation.

Another test that you can run for a longer duration is another sink to floor measurement that is done using the same Fluke 190-202 Scopemeter for a 24 hr period.   When I did this for my home this was the data that I collected.

24 hr ground current reading

In a 4 wire Wye system dealing with non-linear loads, the max amount of current that could travel back on the primary neutral wire is 173% of the largest phase current.  This is the theoretical worst case; however, this also assumes you are at absolute zero (-273 C).  With respect to temperature the primary neutral should then be able to carry closer to 225% of the largest phase current.  What does that really mean?  Does it mean that it will need to be able to carry as much as 225% of the largest phase current all the time?  The best way to answer that question is with an example.  Consider a bridge and the construction that goes into it.  When engineers construct a bridge they look at what the largest things are that will pass over it.  They then account for a safety factor, which makes it so it can hold even more weight, and then it is built.  They don’t assume that the heaviest tractor trailer will be passing over it all the time, but they do acknowledge it will pass over it some of the time.  When it does, the bridge will hold.   Our electrical grid is not like that.  How can you tell?  Look at the electric poles in your neighborhood (because most residential areas are using more and more non-linear loads) and when you see 3 wires on top and one running underneath, look to see if the one wire running underneath is larger than any of the others on top.  In many cases you will not find it so, and in fact the wire is smaller in a lot of cases.   To finish off the analogy, if the bridge can’t hold the weight of the passing vehicle the bridge will break and the vehicle will not make it to the other side.  In the case of electricity, it doesn’t travel on the wire provided.  It travels in the ground.  In both cases each result can lead to injury.

So, what can you do about this?  The first thing to do is measure.  Measure what is there and then decide what to do.  In many cases this will involve telling your power company at some point.   What your ground current is today will not necessarily be what it is tomorrow.  The real factor is: what is the demand on the power grid at the time you took the measurement?   Was that demand then for linear or non-linear?  The important thing for people to know is what your levels are now and know those levels change slightly from day to day, hour to hour.  They will be higher when the demand for power is higher.  Example is when the heat index over 100 and everyone is running air conditioning or when it is -40 and everyone is running their furnace.  Knowing what your levels are now is always a good starting point.   The only real thing a home owner can do is limit the amount of ways for the ground current to get into a home.  In this case if you have all copper piping, and high contact current, look at replacing 2-3 feet of the copper pipe after your water meter or where ever it comes into your home.

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