Dumb question from this rookie: if all the electrical in my shop is in metal conduit and boxes, and there’s a faulty ground on a machine, won’t I get shocked when I touch the metal conduit?
Especially if the ground wire inside one of the boxes happens to be touching metal somewhere?
Getting the details right for a wall assembly with the control layers to the exterior and lots of drying potential.
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Replies
Technically yes. You would receive a shock. But this shock will be inversely proportional to the impedance of your body to the current path back through both the ground wire within the conduit and the redundant conduit path.
You body will conduct electricity. If it didn't no one would ever get shocked or electrocuted. But compared to the low impedance of the ground conductor and conduit your body is a very poor conductor so the actual current you receive is measured in milliampere, .001 A, possibly microampere, .000001 A.
At these levels you are at no danger and the current will go completely unnoticed.
To confuse the situation even more you can ponder that there is no such thing as an insulator, at least not a perfect one. Everything conducts electricity. Just some things do this very much better than other things. Given a very sensitive instrument it can be demonstrated that turning on a light switch electrifies an entire room with tiny currents that form gradients in the air between every charged surface and every grounded surface.
Technically yes. You would receive a shock. But this shock will be inversely proportional to the impedance of your body to the current path back through both the ground wire within the conduit and the redundant conduit path.
When I was a kid, we would dare each other to touch a particular armored cable in the basement, and at the same time touch a metal collumn. If you did this, you would get a mild shock. You could feel the pulsing of the current. It wasn't strong enough to hurt or make you jerk your hand back, but it required some willpower (and a lot of stupidity) to hold both similtaneously. We did that all the time for fun. Kids aren't always the brightest.
When I told my dad about it a few years later, he was pretty concerned. I think he fixed it right away. But don't really remember.
That's why the metal conduit is supposed to be grounded to a good, solid ground. And also why current code requires an additional ground wire. So, to get a shock from the conduit you have to have three faults: The original short from hot to the conduit, a failure of continuity of the conduit back to its grounding point, and a failure of the ground wire.
> .... a failure of continuity of the conduit back to its grounding point....
... is an all too common condition. Consider how many fittings there are between the panel and the last box on a typical circuit. Maybe half a dozen to a couple dozen. And it only takes one of them getting whacked loose to leave the rest of the run floating. The extra ground wire is a good idea.
-- J.S.
Like I said, that's why the extra ground is now required (in most cases).
Dan,
I don't know what you consider to be "most cases", but the 2002 NEC requires an "extra" equipment grounding conductor (EGC) for metallic conduit in only a couple of cases that are related to our discussion here. That is, for flexible metallic conduit and liquidtite flexible metallic conduit in lengths over 6 feet.
There is no requirement for an EGC for general use of thinwall conduit (EMT), IMC (meduim wall), or RMC (heavywall). An important exception is for wiring in patient care areas of healthcare facilities, but that's a long ways from what we're talking about here. And to extend the discussion from the original poster's EMT situation, type AC conduit is OK without an extra EGC, because of the integral bonding srtip or wire. Type MC cable has an EGC as part of the cable assembly, so I wouldn't consider it "extra".
Could you please cite the section of the NEC that you indicate requires the "extra ground"?
Cliff
Yeah, now that I think about it, you're probably right. I probably was thinking of the change that outlawed the old bare wire neutral scheme.
Re: "The extra ground wire is a good idea."
You said it. I whole heartedly agree. Conduit grounds, using the ground as the only ground path, are legal last time I checked but I strongly urge anyone who will listen, and not a few who won't, to pull in a ground wire. Too often I have seen conduits fail as a ground path. A couple of loose, these are very easy to forget to tighten during rushed production, fittings and possibly a little corrosion would do it. Physical damage, saw a conduit carrying a 200A service being used a a pullup bar that was literally hanging by the live cables, and conduits completely rusted away are also common.
Conduits IMO should be grounded but their primary purpose should be to protect the conductors. As a redundant ground they are OK. As the only ground path they don't pass the test of time and human fallibility.
I know you understand this but it bears repeating a key piece of information: Wiring faults allow currents to find their own way home. If the ground wire, or other ground path, fails anyone who is unlucky enough to get between the current source and another ground gets to carry the current. Ouch. 1A will stop a heart. Much smaller currents can cause someone to fall off a scaffold, building or jerk back into running machinery. A dependable ground wire is very important.
Next week: Why neutrals are more dangerous than live wires.
I picked some home projects magazine on the plane last weekend, and in it one article was about adding a circuit for a shop. Not a single ground wire was pulled. All grounds were from device to box, and the EMT was the only ground path between the 4 boxes. The first box was connected to he bare ground from some romex, as well as the device. Sure was surprised to see something done so bare bones in a magazine.
If everything is done dead right, and stays right, conduit is a grand ground path. The key is that the cross-section area of the conduit is many times that of any commonly used branch circuit ground wire. Even considering that the wire is copper and the conduit is galvanized steel the conduit has, typically, much less resistance.
This apparent effectiveness is more than offset by the probability that not all conduits, connectors or locknuts will get tightened well enough or that they will stay this way years later. The other point is that concentric knock-outs are not suitable for ground paths. They all too often fail when exposed to normal abuse. This failure often eliminates any electrical connections through the conduit after a year or two.
The other point is that adding a ground wire is neither expensive nor very time consuming. Including a ground wire is cheap insurance. Insurance that does not preclude the conduit ground path from working. Make up all the conduit connections as if the conduit was the only ground path. Then pull in a ground wire with the circuit conductors. Redundancy is, as Martha Stewart says, 'A good thing'.
A good thing that could save your life.
It doesn't really surprise me that a magazine might choose to not include a ground wire in a conduit system. IMHO a lot of electricians are used to new work, where they seldom get to see the abuse typical of a residential setting, or industrial sites where electrical systems usually get regular maintenance. Some electricians just never considered the possibilities and consequences. Some will not acknowledge their own fallibility and assume that every conduit connection will be made up flawlessly, and stay that way.
This subject, along with shared neutrals, will get lively, often heated, debates going among conscientious electricians. I say spend the extra few cents and build in redundancy to cover you when your having a bad day.
4Lorn1 knows a lot more than I, but the way it has been explained to me is that we really have to think of electricty as potential energy - the difference in energy states between an energized wire and ground, not as current flow.
Anything on the neutral or ground side has a very low potential compared to ground, so when you touch the neutral or ground side, there just isn't that much energy transfered to you, and the metal path to ground is an lower resistence path, to boot.
Is that close, 4Lorn1?
"It is as hard for the good to suspect evil, as it is for the bad to suspect good."
-- Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)
the way it has been explained to me is that we really have to think of electricty as potential energy
Yea, you don't get shocked, necessarily, but touching a "high voltage" line. In order ot get a shock, parts of your body need to be touching two things that are at different voltage potentials. It's the difference in voltage that causes current to flow through your body, and hence you feel a shock.
As an example, a couple of months ago I saw a show on the History channel (I think) on utility linemen. 'amazing stuff: Nowadays, alot of them work on live ~500KV distribution lines by accessing them from helicopters or high-isolation buckets. They actually attach themselves to the line first, in order to charge themselves up to the same voltage as the cables. Since they're isolated from anything else, no current flows through their body, and they don't get shocked.
As for the original question, in most cases if there is a short to a grounded conduit, enough fault current flows to trip the breaker in short order. That's one reason the neutrals and grounds are tied at the main panel, to ensure a sufficient flow of fault current in case of a short.
But even the the worst-case "perfect storm" short, where alot of current flows through the conduit but not enough to trip the breaker, you still won't get shocked. The current would much rather travel through the conduit back to the panel, than through you, eventually to the ground, and back to the panel somehow.
For instance, assuming it's properly installed, 100 ft. of 1/2" EMT has a resistance of about 0.1 ohms, including couplings. Minimum resistance betweem two points on your skin is going to be about 100K ohms, or about 1 million times greater. So if ~20 amps of fault current is flowing, only 20 microamperes max. are going to flow through you: far below the threshold of feeling of about 1 milliampere. So you don't even feel anything, assuming that there is low enough impedance equipment grounding path all the way back to the panel. That's why proper installation of grounding components is so important to safety.