# Short Circuits, Ground Faults and Ground Fault Circuit Interrupter

Posted by PITHOCRATES - April 9th, 2014

# Technology 101

## AC Power uses Reciprocating Currents to produce Rotating Electromagnetic Fields

There is a police crime lab television show that can solve a crime from a single fiber.  Many crime lab shows, actually.  Where they use high-tech science and music montages to solve many a crime.  Which is great if you DVR’d the shows as you can fast forward through them.  And save some time.  In one of these shows the writers goofed, though.  Because they didn’t understand the science behind the technology.

Someone murdered a construction worker by sabotaging a power cord.  By cutting off the grounding (or third) prong.  The fake crime scene person said this disabled the ground fault circuit interrupter (GFCI) device in the GFCI receptacle.  Leaving the user of the cord unprotected from ground faults.  So when said worker gripped the drill motor’s metallic case while standing in water and squeezed the trigger he got electrocuted.  And when the investigator saw that someone had cut off the grounding prong of the cord he said there was no way for the GFCI to work.  Which is, of course, wrong.  For the grounding prong has little to do with tripping the GFCI mechanism in a receptacle.

If you look at an electrical outlet you will see three holes.  Two vertical slots and one sort of round one.  Inside of these holes are pieces of metal that connect to wiring that runs back to the electric panel in your house.  One of the slots is the ‘hot’ circuit.  The other slot is the ‘neutral’ circuit.  And the third slot is the ‘ground’ circuit.  Now alternating current (AC) goes back and forth in the wiring.  It will come out of the hot and go into the neutral.  Then it will reverse course and come out of the neutral and go into the hot.  Think of a reciprocating engine where pistons go up and down to produce rotary motion.  AC current does the same to produce rotating electromagnetic fields in an electric motor.

## The Current in our Electric Panels wants to Run to Ground

If the current can come out of both the hot and the neutral why aren’t both of these slotted holes hots?  Or both neutrals?  Good question.  The secondary winding on the pole-mounted transformer feeding your house has three wires coming from it.  The secondary is a very long wire wrapped many times around a core.  If you measure the voltage at both ends of this coil of wire you will get 240 volts.  They also attach a third wire to this coil of wire.  Right in the center of the coil.  So if you measure the voltage from this ‘center tap’ to one of the other two wires you will be measuring the voltage across half of the windings.  And get half of the voltage.  120 volts.

These are the three wires they bring into your house and terminate to your electric panel.  The center tap and the two wires coming off the ends of the secondary winding.  They attach each of the two ‘end wires’ to a hot bus bar in the panel.  And attach the center tap to the neutral bus.  They also connect the ground bus to the neutral bus.  A 1-pole circuit breaker attaches to one of the two hot bus bars.  Current travels along a wire attached to the breaker, runs through the house wiring, goes through the electrical load and back to the panel to the neutral bus.  So this back and forth current comes from the 120 voltage produced over half of the secondary coil of wire in the transformer.  Where as a 2-pole breaker attaches to both hot bus bars.  Current travels along a wire attached to one pole of the breaker, runs through the house wiring, through the electric load and back to the panel.  But instead of going to the neutral bus bar it goes to the other pole of the 2-pole breaker and to the other hot bus bar.  So this back and forth current comes from the 240 voltage produced across the whole secondary coil in the transformer.

Current wants to run to ground.  It’s why lightning hits trees.  Because trees are grounded.  The current in our electric panels wants to run to ground, too.  Which we only let it do after it does some work for us.  When we plug a cord into an electric outlet we are bringing the hot and neutral closer together.  Like when we plug in our refrigerator.   When the temperature falls a switch closes completing the circuit between hot and neutral through the compressor in the refrigerator.  So the current can run to ground.  Which is actually a back and forth motion through the conductors to create a rotating electromagnetic field in the compressor.  Which runs back and forth between one of the hot bus bars and the neutral bus bar in the panel.

## Ground Faults don’t trip Circuit Breakers when finding an Alternate Path to Ground

When we stand on the ground we are grounded.  We are physically in contact with the ground.  We can lie on the ground and not get an electric shock.  Despite all current wanting to run to ground.  So if all current is running to ground why don’t we get a shock when we contact the ground?  Because we are at the same potential as the ground.  And no current flows between objects at the same potential (i.e., voltage).  This is the reason why we have a ground prong on our cords.  And why we install a bonding jumper between the neutral bus and the ground bus in our panels.  So that everything but the hot bus bars is at the same potential.  So no current flows through anything UNLESS that something is also connected to a wire running back to a hot bus in the panel.

Of course, if there is lightning outside we don’t want to be the tallest object out there.  For that lightning will find us to complete its path to ground.  Just as electricity will inside our house.  This is the purpose of the grounding prong on cords.  And why we ground all metallic components of things we plug into an electric outlet.  So if a hot wire comes loose inside of that thing and comes into contact with the metal case it will create a short circuit to ground for that current.  The current will be so great as it flows with no resistance that it will exceed the trip rating of the circuit breaker.  And open the breaker.  De-energizing everything in contact with that loose hot wire.  Eliminating an electric shock hazard.  For example, you could have a fluorescent light with a metal reflector in your basement.  It could have a loose hot wire that energizes the full metallic exterior of that light.  If you were working in the ceiling and had one hand on a cold water pipe when you came into contract with that light you would get a nasty electric shock.  But if it was grounded properly the breaker would trip before anyone could suffer an electric shock.

Ground faults are a different danger.  Because they don’t trip the circuit breaker in the panel.  Why?  Because it’s not a short circuit to ground.  But current taking a different path to ground.  That doesn’t end inside the electric panel.  For example, if you’re using a hair dryer in the bathroom you may come into contact with water and cold water piping.  Things that can conduct electricity to ground.  And if you are in contact with these alternate paths to ground some of that current in the hot wire will not equal the current in the neutral wire.  Because that back and forth current will be going in and out of the hot bus.  And in and out of a combination of the neutral bus and that alternate path to ground through you.  Electrocuting you.  But because of your body’s resistance the current flow through you will not exceed the breaker rating.  Allowing the current to keep flowing through you.  Perhaps even killing you.  This is why we have GFCI receptacles in our bathrooms, kitchens and anywhere else there may be an alternate path to ground.

So how does a GFCI work?  When current flows through a wire it creates an electromagnetic field around the wire.  If you’re looking into the wire as it runs away from you the field will be clockwise when the current is going away from you.  And counter clockwise when coming towards you.  In an AC circuit there are two conductors with current flow.  And at all times the currents are equal and run in opposite directions.  Cancelling those electromagnetic fields.  Unless there is a ground fault.  And if there is one the current in the neutral will decrease by the amount running to ground.  And the electromagnetic field in the neutral conductor will not cancel out the electromagnetic field in the hot conductor.  The GFCI will sense this and open the circuit.  Stopping all current flow.  Even if the ground prong was cut off.

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# Overhead High Voltage Power Lines, Lightning Rod, Grounding Conductor, Ground Rods, Flashover and Underground Duct Bank

Posted by PITHOCRATES - August 29th, 2012

# Technology 101

## Electricity always wants to Take the Path of Least Resistance to Ground

Have you ever noticed bright-color globes on overhead high voltage power lines?  Do you know why they are there?  Because it’s hard to see those wires.  Which could be a problem for ships with tall masts traveling a river where these wires cross.  Or to low-flying aircraft.  Which is why you see them around airports.  And many hospitals.  Why?  Helicopters.  So when helicopter pilots are bringing critically injured patients to a hospital they will be able to see these bright-color globes and take evasive action to avoid flying into these wires.

Of course, not everything takes evasive action to avoid these lines.  One thing in particular tries its hardest to purposely hit these overhead high voltage power lines.  Lightning.  Why?  For the same reason you get a static electric shock after sliding over your cloth seats to get out of your car.  It creates a potential difference between you and your car.  So as your hand approaches your car handle to close your door a little spark jumps between you and your car.  To rebalance that unbalanced charge.  And send those ‘stripped’ electrons back home.  Which is a how a lightning strike occurs.  Only with the clouds being a much, much larger butt sliding across a car seat.  And anything sticking out of the ground being your finger.

Electricity wants to flow to the ground.  But if it flows straight to ground it can’t do much work for us.  So we try to prevent that from happening.  Which can be a struggle as electricity always wants to take the path of least resistance.  Instead of turning a motor it would much rather flow directly to the ground.  And it sometimes happens.  And when it does it can be dangerous.  For if the same amount of energy that can accelerate a subway train is shorted directly to ground there will be arcs and sparks and smoke and even a little welding as that electric discharge melts metal and ionizes the gas into an explosion of heat and noise.

## So Overhead Cabling is Simpler and More Convenient to work with and Requires Fewer Power interruptions

Now these are the last things you want to happen to our electric grid.  Explosions of ionized gases and molting metal.  Because they tend to interrupt the flow of electricity in the power lines to our homes and businesses.  And thanks to work started by Benjamin Franklin we can do something to try and prevent this.  After Franklin made his wealth he became a scientist.  Because it interested him.  He studied the new field of electricity.  And he proved that lightning was in fact electricity.  So he invented the lightning rod.  To attract that lightning and help it go where it wants to go.  To the ground.  Instead of hitting the structure below the lightning rod.  And starting it on fire.

If you look at our overhead high voltage transmission lines you will notice a set of three wires.  Supported horizontally from a tower.  Or two sets of three wires supported vertically from a tower.  These are the high voltage transmission lines.  Above these lines you will see smaller lines.  At the very top of the transmission tower.  These wires are the lightning rods for the power lines below them.  They either terminate to the metal transmission towers.  Or there is a grounding wire running from these wires down a nonconductive pole to the earth.  At the base of the tower these conductors terminate to ground rods driven into the earth.  In the case of a metallic tower there are conductors connecting the base of the tower to ground rods.  So if lightning strikes at these grounding conductors or towers it will take the path of least resistance to go where it wants to go.  Along these grounding conductors to earth.

Low flying aircraft, tall ships and lightning?  Seems like overhead transmission lines give us a lot of trouble.  Wouldn’t it be smarter to bury these lines?  Yes and no.  While it is true it would be difficult for a plane, ship or lightning to hit a buried power line there are other considerations.  Such as infrastructure cost.  Overhead conductors need towers on small plots of land evenly spaced underneath.  Underground conductors need a trench, conduits, manholes, sand, rebar, concrete, etc., wherever the conductors go.  Also, overhead wires are bare.  Because they are in the open air separated from other conductors.  Conductors underground need insulation to prevent short circuits between phases.  Because the three cables of a 3-phase circuit are pulled into one conduit.  And these cables touch each other.  So the insulation, conduit, concrete and sand make it difficult to ‘tap’ a feeder to feed, say, a new substation.  Requiring power interruptions, excavating, cutting and splicing to tap an underground feeder.  Whereas tapping a bare overhead conductor requires none of that.  They can simply attach the new substation feeders to the live overhead wires.  Then close a switch in the new substation to energize it.  So overhead cabling is simpler and more convenient to work with.  And some voltages simply make overhead lines the only option.

## For a Given Current you can use a Smaller Conductor in the open Air than you can use Underground

Current flows when there is a voltage differential.  The greater the voltage difference is the greater the current flow.  In 3-phase AC power generators push and pull an alternating current through a set of three cables.  Think of the reciprocating gasoline engine.  Where the up and down motion of the piston is converted into useful work.  Turning the wheels of a car.  When the current is equal in each of the three cables the 3-phase circuit is balanced.  Which means when current is moving away from the power plant on one cable it is returning to the power plant on another cable.  In North America a complete cycle of current on one conductor happens 60 times a second.  During that second voltage rises and falls as the current flows.   Think of three pistons going up and down.  The crankshaft turns at the same speed for all three pistons.  But the pistons don’t go up and down at the same time.  As it is in a three-phase feeder.  Current leaves the power plant in one conductor.  When it’s one-third of the way through its cycle current leaves in the second conductor.  When the first current is two-thirds of the way through its cycle, and the second current is one-third of the way through its cycle, current leaves in the third conductor.

Current and voltage are both zero twice in each cycle.  Just like the speed of a piston is zero twice a cycle (at the top and the bottom of its stroke).  But it’s never zero at the same time in more than one conductor.  In fact, the voltage is never the same in any two conductors at the same time.  Which means there is always a voltage differential between any two of the three conductors in a 3-phase circuit.  So a current will always flow between two phase conductors if they come into contact with each other.  And if the voltage is high enough the current will arc across the air gap (or flashover) between two conductors.  If they get too close to each other.  And the higher the voltage of these feeders the greater the distance required between the phase conductors to prevent any flashover.  On some of the highest voltage feeders (765 kilovolt) the conductors are more than 50 feet apart.  With one conductor in the middle and one on either side 50 feet away that’s 100 feet minimum distance required for a three-phase 765 kV feeder.  To put these underground would require a very wide trench.  Or cables with very, very thick insulation.  Requiring large conduits.  Deep and wide trenches.  And great cost.

Cables in open air have another advantage over underground cables.  High currents heat cables.  If a cable gets hot enough it can fail. There are only two ways to prevent this heat buildup.  Use thicker cables.  Or cool the cables.  Which can happen with overhead cabling.  The open air can dissipate heat.  Conductors in an underground duct bank have no air blowing across these cables to cool them.  Which means for a given current load you can use a smaller conductor in the open air than you can use in an underground duct bank.  Bigger cable means bigger costs.  On top of all the other additional costs.  And the inconvenience of excavating, cutting and splicing to make a tap.  So despite the risk of a ship, aircraft or lightning hitting our electric grid going overhead just makes more economic sense that going underground.  Because they are less costly.  And are easier to work on.  For replacing a failed overhead cable is a lot easier than replacing a failed underground cable.  Especially if you can’t pull the old cable out.  And don’t have a spare duct to pull a new cable in.  If that happens then you have to install new duct bank before you pull in new cable.  Which will be more expensive than the cable itself.

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