Electrical Question

[lowen]

I think we're splitting hairs here. Don't use the earthed lead to carry current.

No, actually, it isn't hair-splitting. The term 'earthed' is not defined by the NEC, but, from NEC Article 100:

Grounded Conductor. A system or circuit conductor that is intentionally grounded.
...
Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.
Grounding Conductor, Equipment (EGC). The conductive path installed to connect normally non–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode, or both.

FPN No. 1: It is recognized that the equipment grounding conductor also performs bonding.

FPN No. 2: See 250.118 for a list of acceptable equipment grounding conductors.

Grounding Electrode. A conducting object through which a direct connection to earth is established.

Grounding Electrode Conductor. A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.

The Grounded conductor is bonded to the grounding conductor which is bonded to the grounding electrode conductor which is bonded to the grounding electrode which has a direct connection to earth. The grounded conductor is thus "earthed" through the same electrode(s) as the grounding conductor.

Not splitting hairs, keeping hairs from getting a good 'curling-and-singeing.'

 

[Chuck(G)]

Does my original statement: "Don't use the earthed lead to carry current" still embody the spirit of what you're trying to say?

 

[blackepyon]

The statements about about voltage are very correct and very true; typically the danger point is considered to be 50VDC and above. But respect all voltage, treating every voltage source as being capable of harm, and you'll be ok. Some voltage sources (such as the 12,470V medium voltage feeders here) are more 'aggressive' in their ability to harm you than others, but nearly any voltage can cause harm if it's in the wrong place (watch the 'Baghdad Battery' section of the 'Cooling a Sixpack' episode of Mythbusters for a demonstration!).

I was under the impression that it was amperage that kills you, not voltage. Working from shielded metal-arc welding, you crank up the voltage to make it easier to strike and maintain the arc (and consequently, get more splatter over your work-piece), but turn up the amperage to penetrate the metal. A static shock is well up in the kilovolt range, but the amperage is pretty low and won't penetrate deep into your skin (unless you're talking about a lightning strike or something). Generally though, one should avoid passing a current through the heart or spine.

As for respecting voltage sources, my dad, in one of his brighter moments, when I was around 12 taught me how to replace electrical outlets while they were still live. Under supervision, and on purpose, but none the less facetiously father of the year material. This taught me to always treat the outlet/switch/light socket as if it is live, even if I've switched off the breaker. But when around the same age he taught me to tune the colour and focus on CRTs, he didn't teach me to properly discharge said CRT before servicing it (tuning needs to be done live anyways). I learned the hard way that flyback voltage at your fingertip bloody hurts!

 

[blackepyon]

But I was talking about a wall oven, which uses 240V elements. AFAIK, only the clock runs from 120VAC.

The oven elements, yes, but the stove-top elements run on 120v - two on each line. At least they do on my stove, last I had it apart (need to get around to replacing that other switch at some point - got it in the junk drawer somewhere).

unless it's one of the hybrids with a single center tapped phase. The center tap of that phase then becomes "neutral" and the setup supplies 120-0-120.

I believe that's the high-leg delta I was talking about?

Looking at what specs I could find on the UPS, there is only a single-phase output, with no taps. It's a purpose-built rackmount unit for the HP1000 system, IIRC.

Could you be specific on which HP1000 you're referring to. I'm trying to Google it, and can't get anything specific. Apparently, it's a range of systems.

 

[Chuck(G)]

I think I pointed to a document earlier in the thread.

 

[lowen]

I was under the impression that it was amperage that kills you, not voltage. ...

Correct, the current is what kills you. Current and voltage are related to each other by Ohm's law:
E=IR
Where:
E is the Electromotive force (aka voltage);
I is, in André-Marie Ampère's words, the Intensité du courant (French for 'current intensity')
R is Resistance.

The electrical resistance of the body is rather variable, from hundreds of ohms to hundreds of thousands of ohms. Thus it takes more voltage to kill you in dry conditions than in wet or if the voltage is applied internally via some sort of electrodes. Higher voltages create ion channels (aka arcs and sparks) that reduce the resistance further, which is why higher voltage kills more surely in most cases.

Does my original statement: "Don't use the earthed lead to carry current" still embody the spirit of what you're trying to say?

No, sorry, it doesn't, even though it is close. The grounded conductor (aka the neutral in most services) is in fact earthed by way of the single bond from the grounded conductor to the grounding conductor at the service disconnecting means. That's why I say the term 'earthed' is ambiguous, and thus why NFPA rather narrowly defines these terms to eliminate ambiguity. As ambiguity goes up, the potential for unsafe conditions goes up. So, yes, I know I'm coming across rather pedantic, but it's for the purpose of reducing the potential for unsafe conditions, that's all; I'm not arguing just to be arguing in this case. And it is safety that is the primary concern of the National Fire Protection Association's National Electrical Code and the reason for very narrowly-defined words.

 

[glitch]

So, yes, I know I'm coming across rather pedantic, but it's for the purpose of reducing the potential for unsafe conditions, that's all; I'm not arguing just to be arguing in this case. And it is safety that is the primary concern of the National Fire Protection Association's National Electrical Code and the reason for very narrowly-defined words.

As someone who has had to correct improper and dangerous bonding and grounding in both residential and industrial situations, yeah, it's important. Folks who know what they're talking about using unclear language often leads to folks who don't understand the theory making incorrect decisions. In a previous job, we had every chassis of every machine in a small machine shop electrified after a fault, due to the (certified for industrial/commercial work) electrician's incomplete understanding of "grounded" and "grounding."

 

[Chuck(G)]

I understand what you're talking about, but language is making me struggle without getting into regulation-ese.

One thing about NEC that I found puzzling is that your "earth" potential needn't necessary be at "earth" levels. IIRC, there's a resistance test involved when driving ground rods--and you are only required to try for a good ground a certain number of times (2?)

 

[Dwight Elvey]

Ground is only important if there is a difference in potential. In the desert you may not find a good ground. Connecting to the house water pipes is about as good as you'll get. Surprisingly it will be sufficient for the safety of the people inside but maybe not safe enough to prevent a fire, should the line transformer short out. It is a good idea to not live in the desert.
Dwight

 

[Chuck(G)]

One can get obsessive about this sort of thing. One fellow tried to tell me that my well casing, galvanized pipe, driven 40 ft into wet clay wasn't a good ground, but that a copper rod driven a few feet into loose aggregate was.

 

[lowen]

At my $dayjob it is part of my job description to get obsessive about grounding.... so maybe I do take it to a fault, but it's a big part of what I do day to day for my living.

I have the equipment to perform ground resistance tests here, a nice little clamp-on meter, and our ground here, in heavy mica soil, is 0.25 ohms, thanks to the multiple 20-feet long copperweld rods, chemically enhanced, in an alternating signal/safety ground grid, with three electrode fields, bonded with multiple 500kcmil copper grounding electrode conductors exothermically welded to a 2x16 inch by 15 feet long solid copper 'single point' bond (home runs of 500kcmil each individually welded) with two 500kcmil copper conductors tying the three electrode fields together and to the buildings on site (they're buried quite deep, and the single-point bond is under a really large concrete pad, so it's relatively safe from copper thieves....plus we have security.).

There are over 100 rods in our ground electrode system here. But, with two 26-meter 300+ ton x-over-y parabolic dish antenna structures rising from the valley floor we kindof need it.

Well casings sound like a good idea until you get fault current from a lightning strike that takes out your pump.

The most effective thing you can do is extend the electrode field along one or more radials that will drain the current away from the structure, and use the heaviest gauge grounding electrode conductor you possibly can. And ALWAYS ALWAYS ALWAYS make sure all ground electrodes with conductors coming into the house are bonded OUTSIDE the house prior to entering. I've seen the results of a lightning strike on two separate electrode fields that weren't bonded properly, and the results are not pretty.

 

[Chuck(G)]

The good news is that a thunderstorm in these parts is a very rare occurrence. I think that the only lightning strike in the area hit the top of a butte about 25 years ago and started a small brush fire. Sometimes we'll hear a bit of thunder in the coastal range mountains or in the Cascades--but almost nothing locally.

 

[ibmapc]

The good news is that a thunderstorm in these parts is a very rare occurrence. ....

I was camping at Alsea Falls a few years ago and experienced a terrifying electrical storm that lasted most of the night. It seemed to be traveling in a circle, coming over and over again. I figure you, being only about 40 miles from there, would have been in it's path as well. Maybe the lightning activity got more intense where I was. I guess I'm having trouble believing that lightning is that rare in your neck of the woods when I've witnessed some dramatic lightning in the mid and southern Willamette valley.

 

[Chuck(G)]

I've been here 30 years and only have seen the occasional thunderstorm--and nothing remotely resembling the stuff I'd see in the in the midwest. And I'm above 1000'. Honestly, I'm more afraid of freezing rain.