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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DC Power Supply

Posted by PITHOCRATES - February 13th, 2013

Technology 101

Every DC Power Supply has a Transformer, a Rectifier Circuit and a Voltage Regulation Circuit

Alternating current (AC) power is one of the greatest technological developments of mankind.  It gives us the modern world we live in.  We can transmit it over very long distances.  Allowing a few power plants to power large geographic areas.  Something Thomas Edison’s direct current (DC) power just couldn’t do.  Which is a big reason why he lost the War of Currents to George Westinghouse and Nikola Tesla.  AC power also allows the use of transformers.  Allowing us to take the one voltage produced by a power plant and convert it to any voltage we need.

AC power can power our home lighting.  Our air conditioning.  Our electric stove.  Our refrigerator.  Our doorbell.  Pretty much all of the non-fun things in our house.  Things with electric motors in them.  Heating elements.  Or solenoids.  But one thing AC power can’t do is power the fun things in our homes.  Televisions.  Our audio equipment.  Our cable/satellite boxes.  Pretty much anything that doesn’t have an electric motor, heating element or solenoid in it.  These things that process information or audio and video signals.  Or all of the above.  Things that have circuit boards.  With electronic components.  The kind of things that only work with DC power.

Of course all of these things in our homes plug into AC wall receptacles.  Even though they are DC devices.  So what gives?  How can we use AC power to operate DC devices?  With a little something we call a DC power supply.  And every one of those fun things has one.  Either one built-in.  Or an external power pack at the end of a cord.  Every DC power supply has three parts.  There is a transformer to step down the AC voltage.  A rectifier circuit.  And a voltage regulation circuit.

A Diode is a Semiconductor Device that allows a Current to pass through when there is a Forward Bias

The typical electrical receptacle in a house is 120 volt AC.  An AC power cord brings that into our electronic devices.  And the first thing it connects to is a transformer.  Such as a 120:24 volt transformer.  Which steps the 120 volts down to 24 volts AC.  Where the waveform looks like this.

DC Power Supply AC Input

The voltage of AC power rises and falls.  It starts at zero.  Rises to a maximum positive voltage.  Then falls through zero to a maximum negative voltage.  Then rises back to zero.  This represents one cycle.  It does this 60 times a second.  (In North America, at least.  In Europe it’s 50 times a second.)  As most electronic devices are made from semiconductors this is a problem.  For semiconductor devices use low DC voltages to cause current to flow through PN junctions.  A voltage that swings between positive and negative values would only make those semiconductor devices work half of the time.  Sort of like a fluorescent light flickering in the cold.  Only these circuits wouldn’t work that well.  No, to use these semiconductors we need to first get rid of those negative voltages.  By rectifying them to positive voltages.  When we do we get a waveform that looks like this.

DC Power Supply Rectified

A diode is a semiconductor device that allows a current to pass through when there is a forward bias.  And it blocks current from passing through when there is a reverse bias.  An alternating voltage across a diode alternates the bias back and forth between forward bias and reverse bias. Using one diode would produce a waveform like in the first graph above only without the negative parts.  If we use 4 diodes to make a bridge rectifier we can take those negative voltages and make them positive voltages.  Basically flipping the negative portion of the AC waveform to the positive side of the graph.  So it looks like the above waveform.

All Electronic Devices have a Section built Inside of them called a Power Supply

The rectified waveform is all positive.  There are no negative voltages.  But the voltage is more of a series of pulses than a constant voltage.  Varying between 0 and 24 volts.  But our electronic devices need a constant voltage.  So the next step is to smooth this waveform out a little.  And we can do this by adding a capacitor to the output of the bridge rectifier.  Which sort of acts like a reservoir.  It stores charge at higher voltages.   And releases charge at lower voltages.  As it does it smooths out the waveform of our rectified voltage.  Making it less of a series of pulses and more of a fluctuating voltage above and below our desired output voltage.  And looks sort of like this.

DC Power Supply Capacitor

This graph is exaggerated a little to show clearly the sinusoidal waveform.  In reality it may not fluctuate quite so much.  And the lowest voltage would not fall below the rated DC output of the DC power supply.  Please note that now we have a voltage that is always positive.  And never zero.  As well as fluctuating in a sinusoidal waveform at twice the frequency of the original voltage.  The last step in this process is voltage regulation.  Another semiconductor device.  Typically some transistors forming a linear amplifier.  Or an integrated circuit with three terminals.  An input, an output and a ground.  We apply the above waveform between the input and ground.  And these semiconductor devices will change voltage and current through the device to get the following output voltage (for a 12 volt DC power supply).

DC Power Supply DC Output

All electronic devices that plug into a wall outlet with a standard AC power cord have a section built inside of them called a power supply.  (Or there is an external power supply.  Small ones that plug into wall outlets.  Or bigger ones that are located in series with the power cord.)  And this is what happens inside the power supply.  It takes the 120 volt AC and converts it to 12 volts DC (or whatever DC voltage the device needs).  Wires from this power supply go to other circuit boards inside these electronic devices.  Giving the electronic components on these circuit boards the 12 volt DC power they need to operate.  Allowing us to watch television, listen to music or surf the web.

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Electric Power, Alternating Current, Transformers, Magnetic Flux, Turns Ratio, Electric Panel and Circuit Breakers

Posted by PITHOCRATES - February 6th, 2013

Technology 101

AC Power is Superior to DC Power because it can Travel Farther and it Works with Transformers

Thanks to Nikola Tesla and his alternating current electric power we live in the world we have today.  The first electric power was direct current.  The stuff that Thomas Edison gave us.  But it had some serious drawbacks.  You needed a generator for each voltage you used.  The low-voltage of telephone systems would need a generator.  The voltage we used in our homes would need another generator.  And the higher voltages we used in our factories and businesses would need another generator.  Requiring a lot of power cables to hang from power poles along our streets.  Almost enough to block out the sun.

Another drawback is that direct currents travel a long way.  And spend a lot of time moving through wires.  Generating heat.  And dropping some power along the way due to the resistance in the wires.  Greatly minimizing the area a power plant can provide power to.  Requiring many power plants in our cities and suburbs.  Just imagine having three coal-fired power plants around your neighborhood.  The logistics and costs were just prohibitive for a modern electric world.  Which is why Thomas Edison lost the War of Currents to Nikola Tesla.

So why is alternating current (AC) superior to direct current (DC) for electric power?  AC is more like a reciprocating motion in an internal combustion engine or a steam locomotive.  Where short up & down and back & forth motion is converted into rotation motion.  Alternating current travels short distances back and forth in the power cables.  Because they travel shorter distances in the wires they lose less power in power transmission.  In fact, AC power lines can travel great distances.  Allowing power plants tucked away in the middle of nowhere power large geographic areas.  But there is another thing that makes AC power superior to DC power.  Transformers.

The Voltage induced onto the Secondary Windings is the Primary Voltage multiplied by the Turns Ratio

When an alternating current flows through a coiled wire it produces an alternating magnetic flux.  Magnetic flux is a measure of the strength and concentration of the magnetic field created by that current.  When this flux passes through another coiled wire it induces a voltage on that coil.  This is a transformer.  A primary and secondary winding where an alternating current applied on the primary winding induces a voltage on the secondary winding.  Allowing you to step up or step down a voltage.  Allowing one generator to produce one voltage.  While transformers throughout the power distribution network can produce the many voltages needed for doorbells, electrical outlets in our homes and the equipment in our factories and businesses.  And any other voltage for any other need.

We accomplish this remarkable feat by varying the number of turns in the windings.  If the number of turns is equal in the primary and the secondary windings then so is the voltage.  If the number of turns in the primary windings is greater than the number of turns in the secondary windings the transformer steps down the voltage.  If the number of turns in the secondary windings is greater than the number of turns in the primary windings the transformer steps up the voltage.  To determine the voltage induced onto the secondary windings we divide the secondary turns by the primary turns.  Giving us the turns ratio.  Multiplying the turns ratio by the voltage applied to the primary windings gives us the voltage on the secondary windings.  (Approximately.  There are some losses.  But for the sake of discussion assume ideal conditions.)

If the turns ratio is 20:1 it means the number of turns on the primary windings is twenty times the turns on the secondary windings.  Which means the voltage on the primary windings will be twenty times the voltage on the secondary windings.  Making this a step-down transformer.  So if you connected 4800 volts to the primary windings the voltage across the secondary windings will be 240 volts (4800/20).  If you attached a wire to the center of the secondary coil you can get both a 20:1 turns ratio and a 40:1 turns ratio.  If you measure a voltage across the entire secondary windings you will get 240 volts.  If you measure from the center of the secondary and either end of the secondary windings you will get 120 volts.

The Power Lines running to your House are Two Insulated Phase Conductors and a Bare Neutral Conductor

This is a common transformer you’ll see atop a pole in your backyard.  Where it is common to have 4800-volt power lines running at the top of poles running between houses.  On some of these poles you will see a transformer mounted below these 4800-volt lines.  The primary windings of these transformers connect to the 4800-volt lines.  And three wires from the secondary windings connect to wires running across these poles below the transformers.  Two of these wires (phase conductors) connect to either end of the secondary windings.  Providing 240 volts.  The third wire attaches to the center of the secondary windings (the neutral conductor).  We get 120 volts between a phase conductor and the neutral conductor.

The power lines running to your house are three conductors twisted together in a triplex cable.  Two insulated phase conductors.  And a bare neutral conductor.  These enter your house and terminate in an electric panel.  The two phase conductors connect to two bus bars inside the panel.  The neutral conductor connects to a neutral bus inside the panel.  Each bus feeds circuit breaker positions on both sides of the panel.  The circuit breaker positions going down the left side of the panel alternate between the two buss bars.  Ditto for the circuit breaker positions on the right side.

A single-pole circuit breaker attaches to one of the bus bars.  Then a wire from the circuit breaker and a wire from the neutral bus leave the panel and terminate at an electrical load.  Providing 120 volts to things like wall receptacles where you plug things into.  And your lighting.  A 2-pole circuit breaker attaches to both bus bars.  Then two wires from the circuit breaker leave the panel and attach to an electrical load.  Providing 240 volts to things like an electric stove or an air conditioner.  Then a reciprocating (push-pull) alternating current runs through these electric loads.  Driven by the push-pull between the two bus bars.  And between a bus bar and the neutral bus.  Which is driven by the push-pull between the conductors of the triplex cable.  Driven by the push pull of secondary windings in the transformer.  Driven by the push-pull of the primary windings.  Driven by the push-pull in the primary cables connected to the primary windings.  And all the way back to the push-pull of the electric generator.  All made possible thanks to Nikola Tesla.  And his alternating current electric power.

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Electric Grid, Voltage, Current, Power, Phase Conductor, Neutral Conductor, 3-Phase Power, Transmission Towers and Corona Discharge

Posted by PITHOCRATES - August 15th, 2012

Technology 101

The Electric Grid is the Highways and Byways for Electric Power from the Power Plant to our Homes

Even our gasoline-powered cars operate on electricity.  The very thing that ignites the air-fuel mixture is an electric spark.  Pushed across an air-gap by a high voltage.  Because that’s something that high voltages do.  Push electrons with such great force that they can actually leave a conductor and travel through the air to another conductor.  Something we don’t want to happen most of the time.  Unless it’s in a spark plug in our gasoline engine.  Or in some movie prop in a cheap science fiction movie.

No.  When we use high voltage to push electrons through a conductor the last thing we want to happen is for the electrons to leave that conductor.  Because we spend a pretty penny to push those electrons out of a power plant.  And if we push the electrons out of the conductor they won’t do much work for us.  Which is the whole point of putting electricity into the electric grid.  To do work for us.

The electric grid.  What exactly is it?  The highways and byways for electric power.  Power plants produce electric power.  And send it to our homes.  As well as our businesses.  Power is the product of voltage and current.  In our homes something we plug into a 120V outlet that draws 8 amps of current consumes 960 watts.  Which is pretty big for a house.  But negligible for a power plant generator producing current at 20,000 volts.  For at 20,000 volts a generator only has to produce 0.48 amps (20,000 X 0.48 = 960).  Or about 6% of the current at 120V.

Between our Homes and the Power Plant we can Change that Current by Changing the Voltage

Current is money.  Just as time is money.  In fact current used over time helps to determine your electric bill.  Where the utility charges you for kilowatt hours (voltage X current X time).  (This would actually give you watt-hours.  You need to divide by 1000 to get kilowatt hours.)  The electric service to your house is a constant voltage.  So it’s the amount of current you use that determines your electric bill.  The more current you use the greater the power you use.  Because in the power equation (voltage X current) voltage is constant while current increases.

Current travels in conductors.  The size of the conductor determines a lot of costs.  Think of automobile traffic.  Areas that have high traffic volumes between them may have a very expensive 8-lane Interstate expressway interconnecting them.  Whereas a lone farmer living in the ‘middle of nowhere’ may only have a much less expensive dirt road leading to his or her home.  And so it is with the electric grid.  Large consumers of electric power need an Interstate expressway.  To move a lot of current.  Which is what actually spins our electrical meters.  Current.  However, between our homes and the power plant we can change that current.  By changing the voltage.  Thereby reducing the cost of that electric power Interstate expressway.

The current flowing through our electric grid is an alternating current.  It leaves the power plant.  Travels in the conductors for about 1/120 of a second.  Then reverses direction and heads back to the power plant.  And reverses again in another 1/120 of a second.  One complete cycle (travel in both directions) takes 1/60 of a second.  And there are 60 of these complete cycles per second.  Hence the alternating current.  If you’re wondering how this back and forth motion in a wire can do any work just think of a steam locomotive.  Or a gasoline engine.  Where a reciprocating (back and forth) motion is converted into rotational motion that can drive a steam locomotive.  Or an automobile.

The Voltages of our Electric Grid balance the Cost Savings (Smaller Wires) with the Higher Costs (Larger Towers)

An electric circuit needs two conductors.  When current is flowing away from the power plant in one it is flowing back to the power plant in the other.  As the current changes direction is has to stop first.  And when it stops flowing the current is zero.  Using the power formula this means there are zero watts twice a cycle.  Or 120 times a second.  Which isn’t very efficient.  However, if you bring two other sets of conductors to the work load and time the current in them properly you can remove these zero-power moments.  You send the first current out in one set of conductors and wait 1/3 of a cycle.  Then you send the second current out in the second set of conductors and wait another 1/3 cycle.  Then you send the third current out in the third set of conductors.  Which guarantees that when a current is slowing to stop to reverse direction there are other currents moving faster towards their peak currents in the other conductors.  Making 3-phase power more efficient than single-phase power.  And the choice for all large consumers of electric power.

Anyone who has ever done any electrical wiring in their home knows you can share neutral conductors.  Meaning more than one circuit coming from your electrical panel can share the return path back to the panel.  If you’ve ever been shocked while working on a circuit you switched off in your panel you have a shared neutral conductor.  Even though you switched off the circuit you were working on another circuit sharing that neutral was still switched on and placing a current on that shared neutral.  Which is what shocked you.  So if we can share neutral conductors we don’t need a total of 6 conductors as noted above.  We only need 4.  Because each circuit leaving the power plant (i.e., phase conductor) can share a common neutral conductor on its way back to the power plant.  But the interesting thing about 3-phase power is that you don’t even need this neutral conductor.  Because in a balanced 3-phase circuit (equal current per phase) there is no current in this neutral conductor.  So it’s not needed as all the back and forth current movement happens in the phase conductors.

Electric power travels in feeders that include three conductors per feeder.  If you look at overhead power lines you will notice they all come in sets of threes when they get upstream of the final transformer that feeds your house.  The lines running along your backyard will have three conductors across the top of the poles.  As they move back to the power plant they pass through additional transformers that increase their voltage (and reduce their current).  And the electric transmission towers get bigger.  With some having two sets of 3-conductor feeders.  The higher the voltage the higher off the ground they have to be.  And the farther apart the phase conductors have to be so the high voltage doesn’t cause an arc to jump the ‘air gap’ between phase conductors.  As you move further away from your home back towards the power plant the voltage will step up to values like 2.4kV (or 2,400 volts), 4.8kV and13.2kV that will typically take you back to a substation.  And then from these substations the big power lines head back towards the power plant.  On even bigger towers.  At voltages of 115kV, 138kV, 230kV, 345kv, 500kV and as high as 765kV.  When they approach the power plant they step down the voltage to match the voltage produced by its generators.

They select the voltages of our electric grid to balance the cost savings (smaller wires) with the higher costs (larger towers taking up more land).  If they increase the voltage so high that they can use very thin and inexpensive conductors the towers required to transmit that voltage safely may be so costly that they exceed the cost savings of the thinner conductors.  So there is an economic limit on voltage levels  As well as other considerations of very high voltages (such as corona discharge where high voltages create such a power magnetic field around the conductors that it may ionize the air around it causing a sizzling sound and a fuzzy blue glow around the cable.  Not to mention causing radio interference.  As well as creating some smog-causing pollutants like ozone and nitrogen oxides.)

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