shocking table saw

[Scott Lurndal]

How do you know what a 110 shock should be like? It's the current that governs the 'mildness' of the shock, not the voltage.

Two possiblities:

1) The neutral conductor is wired to the frame of the saw (unlikely, but possible). Reversing the plug will give you a much larger shock if this is so (dangerous!) 2) Somewhere on the saw is an unintentional path to the frame from either the grounded or current-carrying conductor.

Unless you find the problem and resolve it, the saw shouldn't be used.

scott

[Scott Lurndal]

That doesn't mean that it is acceptable for the grounding conductor (that you are calling 'ground') to carry _any_ current at _any_ time other than a fault situation that trips a breaker.

The GROUND _MUST_ _NEVER_ _EVER_ _EVER_ carry current!!!

(because it is tied to the metal frame of most appliances and to the conduit, your water pipes, and so forth und so weiter).

scott

[Scott Lurndal]

If he does what you two suggest, there is a real good chance he'll kill someone.

Note that if he wires the ground to the motor frame on an old enough motor (and 1940's is old enough), he'll quite possibly be connecting the grounded and grounding conductors together at the equipment, which will feed current into the grounding conductor.

The current in the grounding conductor could easily fry his SO while his SO is frying his eggs.

don't do it.

scott

[Scott Lurndal]

So answer me this. If he adds this ground, and the motor frame is also connected to the neutral (not at all uncommon in older motors), what will be the effect of this connection on the stove, dryer, and other metal appliances in the home which are grounded?

scott

[GerryG]

It's always interesting to see all the aggressive opinions that come with many electrical questions. Some have tried to explain the difference between a ground and neutral, while others have ignored or misinterpreted it. But no matter how you explain it, getting shocked is not a nice thing. With older devices, those who survive are usually experiencing leakage current which may be more startling than directly dangerous. Unfortunately, one can never tell when this leakage may increase.

In a few very old items, reversing the plug may stop the problem, and you can then put a polarizing plug on to keep it that way.

There is nothing illegal or dangerous about connecting an earth ground to the frame, as he said below. However, a tool that discharges current through this earth ground is dangerous and against any electrical code. Check with any book or reputable site on this. Now, you could always put this into a ground-fault outlet, but it would probably trip immediately.

In short, you have a potentially dangerous situation which shold be fixed. Adding a ground will at least give you some protection if the leakage increases, so the motor will begin to smoke before you do.

This is also the reason I run my shop with ground-fault detectors.

GerryG

[Nova]

Depending on the age of the saw and where it was made the neutral may be internally connected to the saw's frame through the motor casing (one of the reasons for polarized plugs). With the saw unplugged and the switch off use an ohmmeter to check for continuity from the each of the plugs prongs to the frame of the saw. You should see an infinite ohm reading (open).

-- Jack Novak Buffalo, NY - USA (Remove "SPAM" from email address to reply)

[Mike Marlow]

Oh Bullshit! If the grounding conductor was to never carry any current, it could not in any way protect the user. Those of us who have advocated a grounded system have also advocated repairing the original problem in association with installing a grounded solution. If he had the ground he'd likely not experience the current flow he's experiencing now. The breaker would trip and he'd know to fix the problem with far less risk of injury that with an ungrounded saw. It is quite possible in a grounded scheme for the leakage to be less than what will trip the breaker, so your opening comment is pure bull as is your all caps comment. The ground will carry current under a fault condition which serves to protect the user and that flow may or it may not trip the breaker. The fault condition must still be repaired, but no one is advocating anything different.

[Mike Marlow]

Hey - stop it. Remember, we invented 'lectricity.

[Tom Veatch]

One response to this, in a different thread, brought up the possibility of static discharge causing the "zap". The OP use of the term "frequently" implies it isn't an "every time" occurrence which reinforces the idea of it being a static discharge rather than a current leak.

To the OP: One way to make a determination on this point is to consider whether it happens each time you touch the saw when the saw is running and whether it will occur twice in quick succession. If so, it is probably current leakage and is, or could develop into, a dangerous situation. Correction requires a repair to either the saw's wiring harness or the internal wiring of the motor. Addition of a ground wire will mask the problem but has the undesirable features mentioned elsewhere in this thread.

If it zaps you once but immediately repeating the contact that yielded the first zap is "zap free", then it is likely a build up of static charge and grounding the frame of the saw will cure the problem. In that case, the source of the static buildup likely to be as stated in the other thread - frictional buildup from the rotating belts, a la Van de Graaff generator.

Tom Veatch Wichita, KS USA

[Nova]

My thoughts on the shock not occurring all the time is the presumed "neutral" being tied to the saw's frame and depending on how the non polarized plug is connected. Connected one way the saw's frame would be on the neutral side of the motor's windings. Connected the opposite way the frame would be "hot".

-- Jack Novak Buffalo, NY - USA (Remove "SPAM" from email address to reply)

[Scott Lurndal]

Both you and he are incorrect. Grounding an appliance (tablesaw) which has a neutral frame connection (advertent or inadvertent) will energize the ground throughout the premises while the appliance (tablesaw) is on - a potentially fatal occurance for someone else on the premises.

not necessarily true.

scott

[Scott Lurndal]

Mike, the grounding conductor is only intended to carry current for the couple of milliseconds it takes for a breaker to trip.

It must _never_ carry current routinely, as it would in the case above where the neutral is bonded to the frame of the saw (the grounding and grounded (neutral) conductors are essentially a single conductor in this case (abstractly modelled).

Look:

+-----------+

• +-----------B---- breaker

• saw +-----------W---- grounded conductor bus in breaker box

• +-----------G---- grounding conductor bus in breaker box

+-----------+

If W and G are connected at the saw, then the return path for the current from 'B' will be both 'W' and 'G'. Thus, current will flow in the ground. In such a configuration, the frames of all other devices which share the same ground path will be energized. I don't think you really want this. The breaker will _not_ trip in this case as it is not a short circuit to ground.

Not necessarily. If the grounded and grounding conductors are bonded at the device (saw) the breaker won't trip, however the grounding conductor would be carrying current.

[Andy Dingley]

We know all about American wiring - we've seen Edison's film of it electrocuting the elephant.

And Volta, Faraday and Tesla were Americans ?

[Mike Marlow]

I understand that Scott, and I apologize if I have not made that clear. I got a little carried away in response to the statement that the ground wire must never carry current. I do well understand that it is not to be wired for current but it seemed that there were people who were more interested in pointing out that it should not be wired to carry current than recognizing the fact that myself and others were only talking about this very sort of current carrying use - fault condition, very short duration.

I should have responded to one of your earlier posts which talked about neutral bonding of older motors and given recognition to the point that you made, but to be honest, by the time I saw it I really just wanted to bail out of this thread. The electrical threads always seem to get so hot - not that I don't owe my own responsibility to that. So... genuinely... thank you for pointing that out as it is indeed a factor that I had not considered. Damn - I had to learn something again today.

Thanks Scott - especially for being so patient while I suffered through my brain fart.

[Mike Marlow]

I think I read one time that the wished they were...

[Doug Winterburn]

I think a fellow named Ben Franklin did his kite experiment in 1752. I may be wrong, but I think this predates the 'lectrical contributions of the three you named.

-Doug

[Wes Stewart]

|"Mike Marlow" writes: | |>For the record, let me say that I agree that if the motor is defective |>internally then it should be repaired or replaced. I jumped past that in my |>original reply. However, adding a ground in no way presents a danger or a | |So answer me this. If he adds this ground, and the motor frame is also connected |to the neutral (not at all uncommon in older motors), what will be the |effect of this connection on the stove, dryer, and other metal appliances |in the home which are grounded?

We should ask you for the answer since it is your hypothetical. But I'll take the bait and reel *you* in.

Let's assume for a moment that the neutral is indeed connected to the motor frame. I'm not sure this is as prevalent as you and others suggest, but I'll play along.

First the two-wire case:

If Murphy's Law is on holiday and the extraordinary happens and the non-polarized plug is actually plugged in correctly, then the motor frame is connected to Earth via the neutral (grounded conductor). The neutral is a current-carrying conductor; therefore there will be a voltage drop along its length.

Assume for example that this is a 20 A circuit wired with 12 AWG and it is 100 feet to the service panel. (We'll ignore the few feet of flex pigtail from the saw to receptacle) To keep the numbers easy, let's also assume that the motor draws 10A @ 120VAC. (1.2KW or ~ 1.6 hp)

From the copper wire tables in any edition of "Reference Data For Radio Engineers" we find that 100' of 12 AWG at 20 deg C has a resistance of 0.1588 Ohm. Dusting off Ohm's law, we calculate that the voltage drop along this length of wire is 10(A) * .1588(ohm) =

1.588 VAC.

Now if this is a hazardous voltage then I better go attach a ground rod to my Tony Stewart model Mag-Lite flashlight, because it has not one, but three, 1.5 V cells in it and it's orange besides.

When the motor isn't running, there is zero current in the wire, the voltage drop is zero and the frame is effectively grounded via the neutral. Does this meet current code, no. Is this a good thing, no; but with the givens, it is *not* a shock hazard.

Now to your question. With everything as before, let's hook up the three wire cord with "grounding conductor" (a terrible term as it is too easily confused with "grounded conductor", hence my personal penchant for calling it a "safety ground.")

Circuit wise, we now have two 12 AWG wires in parallel connected from the motor frame back to the service panel. The resistance is half, as is the voltage drop. And there is the benefit of redundancy in case the neutral wire should open, which is one reason the safety ground is now a code requirement. BTW, use of a GFI will expose this connection in less than a heartbeat.

Now (absent a GFI) you want to know what happens to all of the other appliances connected to this circuit branch. Note: I said *this* branch; nothing happens to anything not connected to this branch. Other loads don't give a rat's ass what is going on in the three wires running to the table saw any more than they care what is going on in your neighbor's house.

Other loads on *this* branch with metal frames or enclosures that are grounded via the third conductor will be elevated above ground by some factor. If they are downstream (further away from the service panel) from the table saw, and don't have any fault currents of their own they will be at the same potential as the saw, ~0.8 VAC in this example. If they are upstream then they will be proportionally closer to ground potential as they get closer to the panel.

BTW, do you realize that in recent memory, stoves and dryers had the neutral tied to the frame and were "grounded" that way. And they typically had unbalanced currents in the two hot legs since lights, motors, etc were operated line-to-neutral. Thus, the chassis was elevated above ground by the voltage drop in the neutral, miniscule as it was. Did this worry you?

[Wes Stewart]

|GerryG writes: | |>There is nothing illegal or dangerous about connecting an earth ground to the |>frame, as he said below. However, a tool that discharges current through this | |Both you and he are incorrect. Grounding an appliance (tablesaw) which has a neutral frame |connection (advertent or inadvertent) will energize the ground throughout |the premises while the appliance (tablesaw) is on - a potentially fatal occurance |for someone else on the premises.

Really funny. In another post you say:

"In such a configuration, the frames of all other devices *which share the same ground path* (emphasis added) will be energized. I don't think you really want this.

Now it's "throughout the premises."

[Norman D. Crow]

Nada. IF the ground carries current, it will not affect other grounded equipment on the "line", as it will carry the current directly back to the service panel, where, as I said before, the neutral & ground are bonded together, so any ground current that makes it that far will rejoin the neutral, which is also connected to the water pipes, building ground, and in the case of the code here, where I am about to have a 200A 240V service installed, will also be connected to a 6 or 8' 1/2" copper(coated) rod driven into the ground.

I'll back up a *little* bit here. When I recommended rewiring with a 3 wire, I did say that if it popped the breaker, to have the motor checked out. This should only occur if there is some shorting from the hot to the frame somewhere. Otherwise, the ground will protect from any slight leakage in the motor windings(which has been suggested by others) causing the OP to be shocked.

Codes went from 2 wire non-polarized to 2 wire polarized to prevent "hot" chassis problems, then when people would replace polarized plugs with non-polarized, or otherwise destroy this integrity, went to 3 wire grounded, all to maintain safety.

IIRC, when I was working on computer installations & spec-ing them, EVERYTHING had to have "hot", neutral(if needed, such as a 240V plus neutral for 120 legs), and a ground wire. The reason given @ that time was that an electrical system could have unbalanced load on the "legs" and end up with slight current in the ground. Thus they insisted on a good ground wire, not depending on metal conduit, to eliminate any "ground loop" interference.

That's my story, and I'm stickin' to it.

[Nova]

Franklin's "experiment" showed that lightening and electricity were one in the same. He did not "discover" electricity.

From

:

"Around 600 BC Greeks found that by rubbing an 'electron' (a hard fossilized resin that today is known as Amber) against a fur cloth, it would attract particles of straw. This strange effect remained a mystery for over 2000 years, until, around AD 1600, Dr William Gilbert investigated the reactions of amber and magnets and first recorded the word 'Electric' in a report on the theory of magnetism."

I believe Gilbert was a Brit.

-- Jack Novak Buffalo, NY - USA (Remove "SPAM" from email address to reply)