Electrical experts chime in please

[Norton]

I'm getting some lesson plans updated for my recording class that I teach and was revising the chapter on the fundamentals of electrical systems and their application to home audio systems.

Specifically I'm taking a look at the subject of physical grounding of household electrical systems.

The question inevitably comes up as to why grounding provides protection from electrical shock.  My answer/guess has been that a grounded connection provides less resistance to the current's path to ground and, given the tendency for electricity to take the path of least resistance, the current will take that route rather than go through you.

Yes?   No?  Maybe?  :-)

[Leatherneck]

Yes. The idea is that, in case of an accidental short circuit of one of the current-carrying conductors, the hard-wired grounding conductor (the bare copper one, usually) will trip the breaker or other circuit disconnect immediately and not subject the human who touches the loose conductor to be the current path.

BTW, proper grounding of houses and buildings is far more complex than I ever imagined before researching the topic while wiring a workshop. Whole books are written on the subject, and the NEC goes on and on and on about it.

TC

[280plus]

You are correct sir. However, to the best of my knowledge, grounding your audio system also helps prevent the dreaded 60 cycle hum.

[K Frame]

It's all about the path of least resistance.

A ground isn't an infallible protection, though. Under certain circumstances it's still possible that someone who touches the problem device might become the path of least resistance.

[ilbob]

I'm getting some lesson plans updated for my recording class that I teach and was revising the chapter on the fundamentals of electrical systems and their application to home audio systems.

Specifically I'm taking a look at the subject of physical grounding of household electrical systems.

The question inevitably comes up as to why grounding provides protection from electrical shock.  My answer/guess has been that a grounded connection provides less resistance to the current's path to ground and, given the tendency for electricity to take the path of least resistance, the current will take that route rather than go through you.

Yes?   No?  Maybe?  :-)

Boy did you ask a question.

Grounding is really two separate things.

The first is the act of connecting your electrical supply to earth. The familiar way is via a ground rod. You drive a rod into the earth and hook a wire up to it. The ground rod is referred to as a grounding electrode (GE). The connection to your electrical system is done at what is called the service point, thats where your electrical service enters your house (in the simplest terms). Inside your service panel (usually a circuit breaker style load center), the wire from the GE (we call it the grounding electrode conductor or GEC) is connected to the ground bar inside your load center and a bonding jumper is connected between the ground bar and the neutral bar. In a service panel, the neutral and grounding bars can be, and often are the same bar.

The primary purpose of this is not protection of human life. It is almost solely to keep the potential of your electrical system from floating away from the electrical potential of earth. if that were to happen, such as during a lightning strike, you could see an electrical system failure that might damage your electrical system or some of your electrical equipment.

The second part of grounding, sometimes more properly called bonding, refers to the green wires that go around your house with your power wires. They are referred to as equipment grounding conductors (EGC). The EGC connects from the ground/neutral bar at the service point to all the metal parts of your house such as outlet boxes that have some potential to be energized in the event of an electrical fault (such as a loose wire). If a hot wire were to come loose from the outlet for instance, and touch the metal outlet box, you could receive a shock if you touched the outlet box. By bonding the outlet box back to the source, a hot wire touching the metal box will cause a short circuit, tripping the branch circuit breaker. Thus the metal box is no longer a hazard because the breaker tripped and removed the electrical power from that circuit.

The idea that electricity follows the path of least resistance is just plain wrong and dangerous.  It will always follow all paths back to the source. the higher resistance paths will see less current, but it will still see some current. And 10 millamps can kill you.

[Headless Thompson Gunner]

It isn't strictly accurate to say that "electricity follows the path of least resistance."  Taking this statement literally could kill you.

Given a circuit with multiple paths to ground, most current will flow through the path of least resistance.  But some current will flow through ALL of the paths, even the paths with a high resistance.  It only takes a few milliamps of current and a few miliseconds of contact time to shut down your heart.  So in the case of a short circuit, even if the shorted object is grounded it could still be lethally dangerous to touch it.

Leatherneck is right.  The purpose of the ground wire is to make the circuit breaker trip.  If a hot wire comes into contace with an ungrounded metal object (usually the case/chassis of an appliance with a loose wire) then that entire metal object becomes hot.  Until power is disconnected from that metal object, it can electrocute anyone who touches it.  

But when a hot wire comes into contact with a grounded metal object, the ground wire creates a low resitance path to ground.  This results in a very large current flow from that hot wire's branch, which is hopefully enough to trip the breaker.  But that metal object will still be hot, even though grounded, until the breaker has time to trip.  Breakers are pretty quick, but again it only takes a few milliseconds of contact with a voltage source to kill someone.

Grounding is good, but it isn't foolproof.

[ilbob]

It's all about the path of least resistance.

A ground isn't an infallible protection, though. Under certain circumstances it's still possible that someone who touches the problem device might become the path of least resistance.

Forget the nonsense about the path of least resistance. it is NOT true and it is an extremely dangerous assumption. Electricity flows in all the paths it finds back to the source.

Proper grounding (of the bonding type) does not provide any protection at all to ungrounded electrical supply systems, such as may be common in industrial plants (3 wire delta being the most common).

[280plus]

Wow...30 years of working with electricity and I'm still learning stuff. I understand what you guys are saying about path of least resistance and electricity flowing through ALL paths. Never looked at it that way before but I used to teach series parallel circuits for a living and it suddenly became so clear when you all said that. Of COURSE it will follow all paths.

Damn...

Thanks!

Follow up question...

I've been nailed by many a volt. Why ain't I dead yet? Just lucky?

[ilbob]

I've been nailed by many a volt. Why ain't I dead yet? Just lucky?

Normally your skin resistance is high enough that the resultant current is low. It is also possible that no current flowed through your heart. This might happen if you touched two hot wires on the same hand. Current would have no path to your heart.

But if you ouched a hot wire in both hands, current could flow across your body, through your heart.

[Harold Tuttle]

what i think is amazing is how the guys in the highlift buckets jack into the flow while working on the wires live

Incredible as it seems, live high voltage transmission lines can be worked barehanded. The lineman must be isolated from the ground by using an insulated bucket truck or other method. The lineman wears special conductive clothing which is connected to the live power line, at which point the line and the lineman are at the same potential, allowing the lineman to handle the wire safely.

http://en.wikipedia.org/wiki/Lineman_(occupation)

[280plus]

I actually did take 200V across the chest once. I was holding a gas pipe in one hand to steady myself while holding up a piece of galvanized duct with the other and it came in contact with the jack chain holding up the flourescent lights in this place. I damn near blacked out, I was at the top of a 10 ft stepladder to boot. When I regathered my senses I took a meter and read 217V IIRC from the jack chain to the gas pipe. The lights were hung from wooden beams, apparently they weren't grounded either. Another time I crossed two terminals on a 440 V compressor with my sweaty forearm and watched the pretty blue "lightning bolt" travel across my skin. No harm done that time.

I watched a thing once where they check the high tension wires with a chopper and the chopper and guy doing the inspection became "live" while they worked. Hopefully they make LOTS of money to do this.

[K Frame]

"Forget the nonsense about the path of least resistance. it is NOT true and it is an extremely dangerous assumption. Electricity flows in all the paths it finds back to the source."

OK, sounds reasonable.

Just remember, there are few things more effective for probing problematic wiring than a silver fork...

[Norton]

You are correct sir. However, to the best of my knowledge, grounding your audio system also helps prevent the dreaded 60 cycle hum.

Yup....you're right on that account.  Plug everything to the same path to ground and it lessens, if not eliminates, the 60 cycle disease.

[Norton]

Boy did you ask a question.

The second part of grounding, sometimes more properly called bonding, refers to the green wires that go around your house with your power wires. They are referred to as equipment grounding conductors (EGC). The EGC connects from the ground/neutral bar at the service point to all the metal parts of your house such as outlet boxes that have some potential to be energized in the event of an electrical fault (such as a loose wire). If a hot wire were to come loose from the outlet for instance, and touch the metal outlet box, you could receive a shock if you touched the outlet box. By bonding the outlet box back to the source, a hot wire touching the metal box will cause a short circuit, tripping the branch circuit breaker. Thus the metal box is no longer a hazard because the breaker tripped and removed the electrical power from that circuit.

This is mostly the context of our discussion in class.  We're mostly applying the concept of grounding as was noted above.....that proper grounding of audio gear reduces the chance of 60hZ hum being noticeable in a mix.

The kids want to know what "grounding" is so I gave them a diagram of a simple house system showing a grounding rod and further explained the difference in a grounded receptacle and a non-grounded receptacle.

I figured out it was a complicated subject when I looked up some web pages and the discussion got very involved very quickly.

Would it be fair to say that grounding the chassis of a device LESSENS the chance of shock as the current seeks a path to ground?

[ilbob]

Would it be fair to say that grounding the chassis of a device LESSENS the chance of shock as the current seeks a path to ground?

There is a very bad misconception going on here that needs to be dispelled first. Electricity does not usually go to ground (earth). Grounding something is not like a storm sewer. A storm sewer collects whatever stray water comes its way. Ground (earth) is not where stray electrons go. Electrical current always returns to its source through all paths available to it.

The only reason grounding a chassis would offer protection is if the chassis was also connected to the equipment grounding conductor which went back to the service point (the source) and connected to the neutral at that point. Then if a hot wire were to contact the chassis, you would have a very low impedance path back to the source (essentially a short circuit). The high current this produces will open the branch circuit breaker shutting off the power. Shutting off the power is what is protective. If the breaker did not open, there would be no protection from grounding the chassis.

You cannot eliminate 60 cycle hum by grounding (earthing). Most 60 cycle hum is actually produced by either improper grounding or unbalanced circuits. It is not uncommon for some idiot to connect a neutral and a ground together inside an outlet, especially what used to be a 2 wire outlet. This is referred to sometimes as a bootleg ground. It is not only unsafe (because it causes current to flow in the green wire which is never desirable) but also can cause an unbalanced current flow between the hot and neutral wires of the branch circuit because part of the current is returning through the green wire instead of all of it going back through the white wire.

This means the magnetic fields that are more or less canceled out by having the hot and neutral run together, no longer cancel each other out. This can be a source of 60 cycle hum.

This does not mean that bootleg grounds won't sometimes mask a 60 cycle hum problem, but it is highly dangerous to do so, even if it does reduce the hum.

[Harold Tuttle]

the other factor with ground and musical instruments is that it acts as a shield to stray interference & noise

i had a guitar that would pick up radio stations until i made a grounded box around the internal components

[Norton]

the other factor with ground and musical instruments is that it acts as a shield to stray interference & noise

i had a guitar that would pick up radio stations until i made a grounded box around the internal components

I just had a picture of that episode of "Gilligan's Island" where Gilligan bumped his head and realigned his fillings, creating a radio in his head.

[Norton]

ilbob,

Thanks again for the informed reply.  I understand what you're saying about the bootleg ground.  I've seen a few of those when I was doing demo for a construction company and couldn't figure out why the receptacles were wired that way.  Now I get it a little more.

So, on the subject of 60 cycle hum.....in the audio world, we're told to always run our gear to the same source of power (assuming the circuit can handle the load) because it reduces the chances for 60hZ hum.  I always assumed (in error apparently) that it was because that way every device was taking the same path to ground or something like that.  Can you 'splain to me?
 
I hate telling the kids something that's wrong.

[lupinus]

yep, (I'm not an electrician just a do it yourselfer so take my post for what you think it's worth)

Electricity wants a path to the ground and will take the easiest route.  Copper wire is more conductive then you are and therefor it will go through the wire unless you make a big whoopsi....like having a finger on the prong of a 220 outlet while plugging it in....and yes it was a shocking experience.

Also if you don't provide a path to the ground you wont get electrocuted or as badly so.  That is why linemen if in the right equipment can work on live wires, birds don't get electrocuted, and if you work on electrical stuff you are supposed to use a nonconductive ladder.  I'm personally not gonna go hooking myself up to any live wires in one of them buckets just to test the theory, but that's what the history channel said

[280plus]

I thought grounding the shield on shileded cable is what eliminated hum? (and interference). In other words if the shield on the cable is not grounded or there is no shield that's when you get a hum, say from a flourescent light in the area.

[Firethorn]

Electricity wants a path to the ground and will take the easiest route.  Copper wire is more conductive then you are and therefor it will go through the wire unless you make a big whoopsi....like having a finger on the prong of a 220 outlet while plugging it in....and yes it was a shocking experience.

It will not take the 'easiest' route.  It will take _Every_ route.  It's just that the current flow will be less through the higher resistance path.

[Norton]

I thought grounding the shield on shileded cable is what eliminated hum? (and interference). In other words if the shield on the cable is not grounded or there is no shield that's when you get a hum, say from a flourescent light in the area.

A balanced cable has the extra shielded braid that isn't carrying an audio signal.  I've never disected the female connections on the devices to see where that connection (from the 3rd pin on an XLR, or the sleeve of a 1/4" jack) goes.  I always presumed that it was connected to the chassis which is, in turn, connected to the electrical ground.

[Azrael256]

I thought grounding the shield on shileded cable is what eliminated hum?

That's a part of it.  A grounded shield creates a Faraday Cage that will stop RFI from affecting cables.  Outside sources, like a flourescent light, will not cause a hum on a properly shielded cable.  It doesn't actually have to be grounded to work, but it helps.

Problems with incoming power are a different story.

[280plus]

LOL...Faraday cage, I like it, kind of like a shark cage but for electrons and magnetic waves.

In the case of incoming power it's an out of phase thing I'm guessing.

[Harold Tuttle]