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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Northeasters, Convection Heating, Thunderstorms, Electricity, Electric Charge, Capacitors, Lightning and Lightning Rods

Posted by PITHOCRATES - February 8th, 2012

Technology 101

A Couple of Centuries ago when a Winter Storm Approached we Stocked Up on Wood for our Cast-Iron Heating Stoves

A study of prevailing weather conditions can predict tomorrow’s weather.  Once you’ve learned some basic weather phenomenon.  Weather generally moves from west to east.  Where cold fronts meet warm fronts we can get storms, tornados, rain, sleet, snow, etc.  And if a swirling northeaster buries a town under snow people in a town northeast of this town can expect the same.  Even though the winds are blowing in the opposite direction.

Today in the worst of winter’s weather we can stay warm and snug at work.  And at home.  Amazing when you consider some of our work places have a lot of exterior glass walls.  Glass curtain walls.  Which really transmit the cold.  Of course, even these rooms can be toasty rooms.  Ever wonder how?  Take a look at the floor under the window.  What do you see?  Fin-tube radiation heating registers.  Copper pipes with metal fins soldered to them.  We pump heating hot water through the copper pipe.  And when we do these fin-tube radiators heat the air as it moves through those fins.  As air heats it expands and gets thinner.  Becoming lighter than the cold air.  And rises.  As it moves up it pulls the cold air below through those heated fins.  Heating the cold air.  Where it, too, expands and gets thinner.  And rises.  Creating a heating convection current.  Heating the room.  And the window.  By washing it with warm air.  All without using a fan to move the air.  Heating units that do have fans and move the air are more for circulating the air to prevent the build of carbon dioxide (produced as we breathe).  While fin-tube heating does the lion’s share of heating our buildings.

So when they predict a winter storm we really don’t worry much about staying warm inside.  Of course, it wasn’t always like this.  A couple of centuries ago when we saw a winter storm was moving our way we made sure we had enough wood available.  To burn in our cast-iron heating stoves.  Where we burned our heating fuel in the room we heated.  And vented the products of combustion out through the chimney.  A big difference to using heating hot water and fin-tube radiators.  But the same principle nonetheless.  These wood-burners heated the cold air and created a heating convection current.  Just like those fin-tube radiators.

During Thunderstorms Clouds act like Charging and Discharging Capacitors

In the summertime when a cold front runs into a warm front it often generates some big thunderstorms.  And some dangerous lightning.  Which has started many building fires throughout history.  Especially churches with tall spires.  Which seemed to be magnets for lightning.  Which they were.  In a way.  Because thunder storms are electrical storms.  Which is why we have lightning.  But first a little about electricity.

Electricity flows between a positive and a negative charge.  The greater the difference in charges the greater the flow of electricity.  A battery can store a charge.  A battery has both a positive (plus) and a negative (minus) terminal.  You charge a battery by applying a voltage across these terminals.  The higher the voltage and/or the longer the charge the more energy is stored in the battery.  When we connect a light to a battery it completes the circuit between the plus and minus terminals.  And electricity flows through the light and illuminates it.  The light will stay lit until the battery runs out of charge.  Or until we open the circuit.  Depending on the voltage or amount of stored charge you may see sparks at the point where the circuit opens or closes.  The charge being strong enough to jump a small air gap just before the circuit is closed.  Or just after it opens.

A capacitor can also hold a charge.  What we used to call a condenser.  Which is a couple of plates separated by an insulator.  When we apply a voltage across the plus and minus terminals the plates charge.  The insulator keeps them from discharging internally.  The bigger the capacitor (i.e., the bigger the surface area of the plates) the bigger the stored charge.  After you charge a capacitor it will hold that charge.  It will dissipate slowly over time.  Or quickly if you short out the plus and minus terminals.  And if you discharge a capacitor quickly you’re going to see some sparking.  As the charge jumps the air gap just before the circuit is closed.  The bigger the capacitor the bigger the sparking.  Funny story.  I saw a kid cutting out the capacitor from an old television set.  The kind your parents had.  With a big glass cathode ray picture tube that used high voltage to move a scanning electron beam to excite (i.e., make glow) the phosphorous coating on the inside of the picture tube.  High voltage and a capacitor mean only one thing.  A very BIG stored charge.  No one turned on that TV for a long time.  But that capacitor held its charge.  As this kid quickly learned.  The hard way.  As he cut the wire going to the plus terminal his un-insulated side cutters touched the metal of the TV chassis.  Which was, of course, grounded.  So you had the plus terminal of a highly charged capacitor coming into contact with the minus terminal of said capacitor (via the grounded TV chassis).  It was like the Fourth of July in the back of that TV.  Threw that poor kid back on his butt.  Funny.  We all had a good laugh.  He was no worse for wear.  Except, perhaps, needing a new pair of undershorts.

All right, back to those electrical storms.  And lightning.  In a nutshell, those ugly black storm clouds are like capacitors.  As the atmosphere churns up these warm and cold weather fronts as they collide something happens.  They charge.  Like a capacitor.  With one plate being on the top of the cloud.  And the other plate being on the bottom of the cloud.  As the charge grows on the bottom of the cloud it induces an opposite charge in the ground below.  The old ‘opposites attract’.  So if a larger and larger minus charge is building up in the bottom of the cloud it attracts (i.e., induces) a larger and larger plus charge on the surface of the earth beneath the cloud.  Until the charges grow so great that they jump the air gap.  But this is no capacitor discharging.  The amount of energy in a lightning strike is so great it can melt sand into glass.  And anything that can do that can play havoc with trees.  And tall buildings.  Igniting a lot of fires along the way.  And killing a lot of people.  Until, that is, we started using lightning rods on our buildings.  Sharp pointed pieces of metal above the highest surfaces of the building.  We attach these rods to conductors running down the sides of the building to ground rods driven below the surface of the earth.  Providing a ‘path of least resistance’ for that charge to discharge through while causing minimal damage to the building.

Ben Franklin gave us Weather Forecasting, Convection Heating and Lightning Rods as well as the United States

Fascinating information, yes?  What’s even more fascinating is that we can trace these developments back to one point in time.  More fascinating still, we can trace them back to one man.  A curious fellow.  With a fascination for scientific experimentation.  Who went by the name of Benjamin Franklin.  Who pioneered weather predicting when a swirling northeaster hit Philadelphia with winds blowing in from the northeast.  Curiously, though, this storm had not yet ravished Boston.  In direct line with those winds.  But the storm moved on to Boston AFTER Philadelphia.  It was Franklin who observed that the northeaster was a counterclockwise spinning storm that moved northeast.  The winds in Philadelphia and Boston were only the top part of that spinning storm.  And weather forecasting was born.

Convection heat goes back to the Philadelphia stove.  What we later called the Franklin stove.  Franklin didn’t discover convection currents.  Or the stove using convection currents.  But he used the available knowledge to make a practical heating stove.  It wasn’t perfect.  But subsequent improvements made it the standard for indoor heating for about a century or two.

Ben Franklin did not discover electricity.  But electricity fascinated him.  And he discovered that lightning was electricity (yes, he actually flew a kite in a storm).  His experimentation gave us the first battery.  The first capacitor.  The standard of using ‘plus’ and ‘minus’ for electrical charges.  The conservation of charge (you can’t create or destroy an electrical charge.  You can only move it around).  The battery.  The capacitor.  Insulators.  Conductors.  Grounding.  All of the fundamentals of electrical circuits we use to this day.  And let us not forget that one other thing.  The effect of points on electrical charges (pointy metallic things help charges jump air gaps).  Which, of course, led to the lightning rod.  This after he set up the U.S. postal service and printed his newspapers and Poor Richard’s Almanac.  But before his political and diplomatic service.  And role as a key Founding Father.  Being the only one to sign the Declaration of Independence, the Treaty of Paris and the U.S. Constitution.  The document that started the Revolutionary War.  The document that ended it.  And the document that created the United States of America.  A busy man that Franklin was.  And a great man.

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