Grounding wire in household circuitry?

[remorris44]

Hi guys,

I know this sounds crazy but I do not understand why household circuits are grounded. It is this grounding that causes more harm than good in my opinion. I understand the need for it, e.g. if your washing machine's hot wire shakes loose and contacts the housing and it weren't grounded then you'd be in for quite a surprise with the next load of laundry... but if the transformer weren't grounded in the first place it wouldn't matter if you touched the housing because neither the hot or neutral wires individually would shock you.

Without grounding you would have to complete the circuit and touch both wires to get shocked. Now I am proposing that the neither the transformer or the circuit breaker panel itself be grounded for this to be the case. Just three wires (the two ends of one side of the transformer and the center tap for 120VAC) from the pole transformer coming into the house and distributed via the circuit breaker.

I am assuming that perhaps this is more of a solution to the needs of the power company in that they need periodic grounding of their transformers such that a lightning strike to a line would rapidly dissipate instead of surging more widely across the grid. In this case I would understand the need to ground the house's circuity as well as the transformers but otherwise this makes no sense to me.

Any thoughts?http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html#c1http://postimg.org/image/f9niinv2h/6f933d44/

 

[sophiecentaur]

but if the transformer weren't grounded in the first place it wouldn't matter if you touched the housing because neither the hot or neutral wires individually would shock you

This is very true - as far as it goes. But if you have a house full of devices, some of them could have been installed years ago. It is very possible that there could be a resistive connection from 'one side' of your supply to Earth. No one would know about that; everything would be working fine. However, the 'other side' of your circuit would now have full mains volts on it so you could no longer go ahead and touch it, secure in the knowledge that you would be safe. The value of leakage resistance wouldn't need to be that low, for the 'hot' side to be able to pass a significant enough current to kill you without blowing a fuse.
What you are suggesting is, in fact, done when connecting individual electrical devices in outdoor applications. An isolating transformer with floating outputs is often used and the protection this gives is well worth having. However, the appliances are built to a higher standard of insulation so there is very little chance of the fatal combination of a leakage to ground on one side and an exposed conductor on the other side.
Of course, nowadays most supplies use a 'residual current' circuit breaker which cuts the supply when there is any unbalanced current in a circuit - so the Earth conductor is not so necessary. But there's no such thing as a free lunch and a circuit breaker that will protect you from serious shock will also trip at the slightest excuse - unusual loads and old equipment - which means that sometimes you can't get the supply to stay on when it would be actually considered as being quite safe. In that case, the Earth system gives you excellent protection

 

[JakeBrodskyPE]

Here's a good reason: lightning strikes. If you have a ground, everything rides up and down with the ground potential. If you don't have a ground, the lightning will couple capacitatively into your wiring and destroy loads of equipment.

In general, if you could maintain isolation in everything, it would be fantastic. But that isolation isn't perfect. Things DO short to ground all the time. It is important to have a reference to that ground so that you can trip a breaker.

 

[the_emi_guy]

Another issue with grounding the neutral wire is the difficulty in detecting ground faults. This is becoming a significant issue with the introduction of large photovoltaic systems.

Some high profile fires have been caused by ground faults that went undetected because the neutral was at ground already. Then, at a later time, the intended path was cut and large current flowed through the ground fault.

This is a tough problem since, as the other posters mentioned, there are often good reasons to ground the neutral.

http://www.solarabcs.org/about/publications/reports/blindspot/

 

[sophiecentaur]

Here's a good reason: lightning strikes. If you have a ground, everything rides up and down with the ground potential. If you don't have a ground, the lightning will couple capacitatively into your wiring and destroy loads of equipment.

In general, if you could maintain isolation in everything, it would be fantastic. But that isolation isn't perfect. Things DO short to ground all the time. It is important to have a reference to that ground so that you can trip a breaker.

My point was that you do not need a 'short' to ground (implying a very low resistance path). A perfectly harmless 100mA leak will give a lethal situation.
Also, you do not need a ground reference to detect current to growing. A modern RCD looks at unbalanced current. This is not the same as an old fashioned Earth Leakage trip, which measured the actual current in the Earth conductor, going to the Earth Spike.
I don't know why there should be a problem detecting unbalanced current in a PV system. Is there a reference to it? [Edit: I read what those links (other post) said and they just make the point that systems should be engineered 'properly'. That really goes without saying, imo. Otherwise, I son't see any difference between PV systems and any other.]

 

[dlgoff]

Food for thought.

image compliments of http://ecmweb.com

 

[JakeBrodskyPE]

My point was that you do not need a 'short' to ground (implying a very low resistance path). A perfectly harmless 100mA leak will give a lethal situation.

Agreed. That was the reason behind the GFI breaker. It is commonly used anywhere household appliances are likely to contact water (example: an electric shaver or a hair dryer).

Also, you do not need a ground reference to detect current to growing. A modern RCD looks at unbalanced current. This is not the same as an old fashioned Earth Leakage trip, which measured the actual current in the Earth conductor, going to the Earth Spike.

These things do exist in larger machinery. For example, three phase motors in a delta configuration are isolated from ground. The devices you are talking about are feasible and they are used for exactly the reasons you state. The problem is that you need a lot more of those protection devices and you need them to be closer to the load to avoid complications from having more than one connection to ground.

The ground based circuitry is a result of the fact that many places actually have to deal with ground potential up close: water pumps, HVAC air handlers and compressors, washing machines, and the like. In the isolated wiring situation, you need advanced detection capabilities EVERYWHERE, or you risk injury. Note that experienced electricians have all sorts of war stories about things that made contact to ground in weird ways where they weren't supposed to. However, with devices based at ground potential, you can detect these faults over a larger number of nodes in wiring.

The faulty assumptions here are

1. Such devices are cheap (they're not, and you'd need more of them to trip reliably in the presence of multiple faults).
2. There will never be more than one connection to ground

The issue here is safety. If we had perfect devices, we wouldn't need the safety. It is usually less expensive and easier to design systems that are inherently safe around grounded wiring than around isolated wiring.

 

[the_emi_guy]

My point was that you do not need a 'short' to ground (implying a very low resistance path). A perfectly harmless 100mA leak will give a lethal situation.
Also, you do not need a ground reference to detect current to growing. A modern RCD looks at unbalanced current. This is not the same as an old fashioned Earth Leakage trip, which measured the actual current in the Earth conductor, going to the Earth Spike.
I don't know why there should be a problem detecting unbalanced current in a PV system. Is there a reference to it? [Edit: I read what those links (other post) said and they just make the point that systems should be engineered 'properly'. That really goes without saying, imo. Otherwise, I son't see any difference between PV systems and any other.]

On the link that I posted, check out the full report pdf link: The Ground-Fault Protection Blind Spot: Safety Concern for Larger PV Systems in the U.S:
Particularly the two paragraphs at the top of page 1, and figure 1 (also on page 1). Then skip down to section 3 (page 4).

The issue is apparently that in PV systems, in practice, there is always some ground leakage currrent, and the larger the PV array the larger this leakage current.
Thus, the specification for the trip threshold for ground fault interrupters increases with the size of the PV system. This leads to a large enough "blind spot" in large PV systems that dangerous ground faults (from a fire perspective) are not detected because they are below this threshold.

Their recommendation for making sure the system is "engineered properly", seems to be to not to do anything in the installation that could cause a ground fault in the first place (insulation techniques, wire management), and to perform specific annual preventive maintenance/inspection.

In other words it sounds like they are essentially saying: since we cannot reliably prevent fires caused by undetectable ground faults in larger PV systems, for heavens sake make sure you don't nick any insulation when you bury that darn cable.

 

[JakeBrodskyPE]

In other words it sounds like they are essentially saying: since we cannot reliably prevent fires caused by undetectable ground faults in larger PV systems, for heavens sake make sure you don't nick any insulation when you bury that darn cable.

There are grades of direct burial cable designed for certain uses. The key is to ensure that this cable is used. A lot of PV system installers are not familiar with industrial wiring standards and so they bury cable with insulation that is not suitable for exposure to rodents, or they put in a conduit bank, not realizing that often these underground conduits get condensation or flood. It's not an issue of nicking cable. It is about water, critters, sunlight, and other enemies of wiring.

Anyone can put a system in that will last for a year or two. But most install such systems with a notion that it will last for a decade or more. That takes real engineering.

 

[the_emi_guy]

There are grades of direct burial cable designed for certain uses. The key is to ensure that this cable is used. A lot of PV system installers are not familiar with industrial wiring standards and so they bury cable with insulation that is not suitable for exposure to rodents, or they put in a conduit bank, not realizing that often these underground conduits get condensation or flood. It's not an issue of nicking cable. It is about water, critters, sunlight, and other enemies of wiring.

Anyone can put a system in that will last for a year or two. But most install such systems with a notion that it will last for a decade or more. That takes real engineering.

Agreed,
But, at the same time, even when all of the correct cable is used and all of the best practices are followed, we still install ground fault protection presumably because it provides the safety net for when something goes wrong with this "perfect" install.

The papers that I cited are essentially saying that, for fire safety purposes, we may not have that safety net in large PV systems.

10 years is not a very long time. We lost power in a section of our neighborhood because a buried cable failed shorted to ground after 27 years (I got to watch them do the repair hot!).

Losing power once in 27 years is no big deal, but having undetected ground faults that can lead to burning down buildings should never be able to happen.

 

[JakeBrodskyPE]

The papers that I cited are essentially saying that, for fire safety perposes, we may not have that safety net in large PV systems.

That safety net is still being written. You are correct, there are very few standards dealing with large PV arrays.

10 years is not a very long time. We lost power in a section of our neighborhood because a buried cable failed shorted to ground after 27 years (I got to watch them do the repair hot!).

Losing power once in 27 years is no big deal, but having undetected ground faults that can lead to burning down buildings should never be able to happen.

Go look up Arc-Flash hazards. This is no joke. You should stand a good distance away from these guys. Hopefully they were wearing face masks, ear protection, and Nomex suits.

That said, you are right, and such systems typically linger for much longer than the designers ever expect them to. I'll bet the people who installed knob and tube wiring in homes back in the 1920s never dreamed that stuff would still be around to cause problems almost a century later.

 

[jim hardy]

I know this sounds crazy but I do not understand why household circuits are grounded. It is this grounding that causes more harm than good in my opinion.

The reason is real simple.

Stuff happens.
A system that was built ungrounded will not remain ungrounded - something will get wet or a mouse will eat the plastic off a wire.
Then you have a system that's not ungrounded like you think it is and that's dangerous.

So we ground the system. Now an accidental ground will trip the breaker which announces that something needs to be fixed.

That's the basic thought.
For an in depth tutorial look up IEEE Green Book. It is well written and starts with the basics.

 

[sophiecentaur]

That safety net is still being written. You are correct, there are very few standards dealing with large PV arrays.

But let's just plough ahead regardless!

Stuff happens

It certainly does

A lot of PV system installers are not familiar with industrial wiring standards

This third quote is the most scary. It seems to reflect the 'fast buck' mentality (plus the political capital to be made) that's associated with the sustainability drive.

 

[JakeBrodskyPE]

I hate to be the bearer of bad news, but...

In any new endeavor like this, there will be accidents until the industry can gain enough experience to set standards for likely safety problems. Some of them are quite predictable. Some of them may already have similar, but not prescriptive notions in the National Electric Code (The NEC is used in the US, I'm sure Europe has similar documents). The problem is that when someone is under contract to provide an installation at the lowest price, they do exactly what is legally required and not one bit more or less. So even though it might be advisable to use a better grade of cable, you can bet that nobody will use it unless they are told to via a consensus document such as the NEC.

This is one of those areas where lawyers make loads of money until people get a clue. I have nothing nice to say about the process except that this is how standards happen. Like many such standards, it exists only because of someone's massive property loss or death.

 

[jim hardy]

Like many such standards, it exists only because of someone's massive property loss or death.

e.g. who publishes the NEC ? Not the academic IEEE but NFPA, National Fire Prevention Association.

Imagine how the boiler and pressure vessel code came about..

http://machinetec.blogspot.com/2010/11/safty.html

 

[dlgoff]

I'll bet the people who installed knob and tube wiring in homes back in the 1920s never dreamed that stuff would still be around to cause problems almost a century later.

And the problems aren't just fire and shock. My parents old house doesn't have a ground wire anywhere and I can't get it sold because it's not cost effective for prospective buyers to bring it up to code.

 

[jim hardy]

Oh no, Don -

is it two wire Romex or conduit?

Plaster or drywall ?

 

[Windadct]

The basic thinking is valid - all original wiring, not just housing had this thinking ( as well as saving cost) -- but also all industrial was underground Delta - it was learned, that is the real world grounding is safer, more relabel, easier to find the inevitable problem - etc. The final nail in the un-grounded vs grounded debate in commercial and industrial settings was probably electronics - computers, industrial controls, more sensitive devices - they suffered greatly at the voltage spikes & noise that exist in the ungrounded set up.

 

[jim hardy]

A system that's completely floating is by definition at unknown potential respect to earth.
So the voltage can be anything.
Experience in industry showed that around big systems with rotating machinery, ungrounded system voltage to Earth can get high enough to pierce insulation.

So systems are "grounded" by something.
In your house wiring that's a solid bolted connection, the "Bonding jumper".
That s known as "Solidly Grounded System".
In industry it common to use some impedance instead of a bonding jumper in order to limit fault current, with detection to announce presence of an unintended 'ground'. That let's equipment keep running if the flaw is non-fatal to it, so they can shut down the process gracefully.
That is known as "Impedance Grounded System".. It is used in industry where it is expected that well trained and attentive people will keep the system in good shape.

IEEE "Green Book", standard 142, is an excellent tutorial. I think EE curricula should include a one hour course on it.

 

[russ_watters]

I just skimmed this, but I'm not seeing the most direct answer:

...but if the transformer weren't grounded in the first place it wouldn't matter if you touched the housing because neither the hot or neutral wires individually would shock you.

Without grounding you would have to complete the circuit and touch both wires to get shocked.

That isn't true unless you are well insulated. If you are in a carpeted house, made of wood, you might have a chance, but what if you are leaning against the faucet while messing with the washer? Sitting on an air conditioning supply register while working on the outlet? Or have a flood? Or using an outdoor receptacle? Or have metal studs?

Yes, there are some situations where it won't shock you to touch a hot wire, but instead of judging each individually it is much safer to not have to do that, but to provide a better path for the electricity to take than through you.

And that also reduces fire danger.

 

[dlgoff]

Oh no, Don -

is it two wire Romex or conduit?

Plaster or drywall ?

Two wire knob & tube in attic, two wire Romex in plaster walls.

 

[jim hardy]

sounds do-able but a big job , for winter when attic isn't so hot.

Extension drills down from attic , pull new romex up from new rework boxes , cover and abandon existing wiring in place , then a lot of plaster repair

call "This old house" crew? Be fun to show Bob Vila and Norm Abrams some Kansas fishing. Crappies in that lake ?

http://www.oldhouseauthority.com/archive/receptacles

 

[rude man]

And the problems aren't just fire and shock. My parents old house doesn't have a ground wire anywhere and I can't get it sold because it's not cost effective for prospective buyers to bring it up to code.

What agency enforces 3-wire electricity in older houses in your area?
When we sold our old apt. bldg. we had to add 3-wire receptacles but the place was 3-wired. So that was no problem.
Then we bought our house (built in 1953) which unfortunately had just 2 wires. The FHA exempted older houses from the 3-wire requirement & the sale went thru without problems.

Later I bought & installed a microwave & immediately felt a shock when touching the case. Fortunately we were having to replumb the house with copper (old place was galvanized iron thruout!) so I took advantage of an opening in the drywall & ran a ground wire from the exposed copper pipe to a 3-wire receptacle before they sealed the place up.

Bottom line is I can't believe how long it took for the code to require 3-wire systems! And you didn't dare run the low side to an appliance case - were the house ground to lift accidentally you'd get the full 115 from the appliance thru you to ground! I'm still in the same house & it still makes me nervous ...

 

[dlgoff]

What agency enforces 3-wire electricity in older houses in your area?

It's not that I can't sell it "as is", it's the city codes that the buyer would have to follow; making it to costly for them.

 

[rude man]

It's not that I can't sell it "as is", it's the city codes that the buyer would have to follow; making it to costly for them.

I understand.

What city? This was not required in Phoenix, AZ. (Not that that's some kind of virtue, to be sure).

 

[dlgoff]

What city?

Let's just say, it's in Kansas.

 

[rude man]

Let's just say, it's in Kansas.

You poor fellow. Almost as bad as Phoenix, AZ ...

 

[gt 5]

The statement that you must touch two wires to complete the circuit is false! The ground wire is there to create a path of least resistance, if in fact you do touch the housing of your washingmachine and a wire is loose inside. You, being of higher resistance are, then, not the path of least resistance. If houwever there is no ground wire, you MAY become the conduit to the Earth and complete the circuit into the Earth's infinite 'sink' or 'supply' of electrons. This is true particularly if you are not insulated. If, for example, there is water on the floor and you are barefoot, and the water also touches a cold water pipe...

 

[the_emi_guy]

The statement that you must touch two wires to complete the circuit is false! ..

No, I believe you are wrong here.

The OP is correct in asserting that if the two wires supplying electricity to your house were completely isolated from ground, you would need to touch both of them to get shocked.

If you touch one side of a battery to the Earth, will it drain the battery?

The topic of this thread is the practicality and tradeoffs of maintaining this isolation.

 

[jim hardy]

The statement that you must touch two wires to complete the circuit is false!

Kirchoff's current law says that current must get back from whence it came.
If, as postulated in post#1, "ground" will not take it there, current will ignore "ground"..

 

[rude man]

No, I believe you are wrong here.

The OP is correct in asserting that if the two wires supplying electricity to your house were completely isolated from ground, you would need to touch both of them to get shocked.

If you touch one side of a battery to the Earth, will it drain the battery?

The topic of this thread is the practicality and tradeoffs of maintaining this isolation.

Appliances are connected one way or another to ground whether intentional or not. It may be by sitting on ground making Ohmic contact, or it may be capacitive - but a floating power system would have enough imbalance to ground to present a potentially lethal hazard to anyone making ground contact to either hot side of the floating source.

The idea of a perfectly floating source is science fiction.

 

[gt 5]

Ahh, yes. I see your point, Jim. You are correct. But keeping the system isolated is the problem. The idea, here, is to assume that the worst could happen. If a tree were to make contact with with the transmission lines, or any other form of ground fault should occurr, the system would not fail safe. It would then be grounded waiting for you to make the circuit complete (again referring to the loose wire touching the housing of the wash machine). If the housing were also grounded, you couldn't complete the circuit.

 

[sophiecentaur]

You, being of higher resistance are, then, not the path of least resistance.

I have to point out that "least resistance" doesn't mean 'all the current'. The current is shared between the Earth path and the person holding the faulty equipment. There will be a finite but small current through the body (often detectable as a mild tingle). The presence of the Earth conductor means that the two parallel paths present a very low resistance so the voltage drop to Earth will be safely low.
Kirchoff 1 always applies and the naive statement that "current takes the easiest path" is not to an appropriate standard for PF discussions.

 

[jim hardy]

Maybe this will help cement the concept:

but a floating power system would have enough imbalance to ground to present a potentially lethal hazard to anyone making ground contact to either hot side of the floating source.

The idea of a perfectly floating source is science fiction.

it's a matter of scale. Large system yes; small system, well, maybe.
When a system gets large enough that its surface area creates capacitance sufficient to pass a good part of an amp, maybe 1/50th, it can deliver a painful shock through that capacitance as rude man stated.

That capacitance has some impedance at line frequency.
In industrial systems it is recommended(IEEE142) that the grounding impedance be a smaller number of resistive ohms, so that Q remains low.
In residential systems that number is zero.

Everything powered by one of these

is powered from a "floating" supply. The system may or may not be earthed. The laptop I'm typing this on is not.

There is minute capacitance between primary and secondary windings, as mentioned.
That's minimized by split-bobbin winding, which also assures that an insulation failure won't let their two wires touch.

Here is a little transformer using a divided (or split) bobbin. This is very practical, because it completely separates the primary from the secondary winding, making it much easier to achieve the degree of insulation required for safety.

http://ludens.cl/Electron/trafos/trafos.html

What's important for beginners to grasp is that "electricity" has no magical affinity for "ground", current only goes there in obedience of Kirchoff's laws.
Ground is simply another wire that happens to go most everyplace. We have this discussion a lot here.

 

[dlgoff]

Ground is simply another wire that happens to go most everyplace. We have this discussion a lot here.

With that in mind, I'll post this image again from http://www.rapidtables.com/electric/electrical_symbols.htm#ground symbols.