Ground and neutral tied together

[GG]

A home owner wanted to tie his neutral and ground together at his clothes washer because he only has 2 prong outlets in his house. I told him not to do it because then he could have neutral current on the frame of the washer because its bonded to the ground wire. He then asked why this was not a hazzard when done with a 3 prong clothes dryer where the neutral and ground are tied together and I didnt have an answer.

 

[skeshesh]

How is that okay in a 3 prong? I'm surprised because you stated the reason yourself: the neutral current resulting from phase unbalance can energize the metallic parts of the appliance. You never want to connect the neutral to ground at any point other than service entry.

 

[mcclary's electrical]

A home owner wanted to tie his neutral and ground together at his clothes washer because he only has 2 prong outlets in his house. I told him not to do it because then he could have neutral current on the frame of the washer because its bonded to the ground wire. He then asked why this was not a hazzard when done with a 3 prong clothes dryer where the neutral and ground are tied together and I didnt have an answer.

Your answer should have been IT IS!! That's why it was outlawed in 1996.

 

[GG]

I meant to say a 3 prong oven, but I guess its the same reason. So are there any cases of people being injured when it was allowed to tie the N and G together at an oven or dryer?

 

[winnie]

'Code' provides for 'practical safeguarding', not perfect safety. The standards for what is a 'safe enough installation' have changed over time, as technology has improved and materials have become relatively less expensive, labor relatively more expensive.

Using the relatively large and relatively unloaded neutral conductor of the dryer circuit as the frame ground was previously considered 'safe enough', although this permission was removed in a mid 90's code revision.

Using the neutral of a receptacle circuit as the ground is similar in concept, but IMHO rather different in magnitude of danger, to the shared neutral/ground of a dryer circuit. I recall hearing a story of the electrical standard in a European country around World War 2, where circuits consisted of a single insulated wire in metal conduit; the metal conduit serving as the grounded conductor. I guess that when materials are scarce enough, 'safe enough' can be pretty scary.

-Jon

 

[skeshesh]

I recall hearing a story of the electrical standard in a European country around World War 2, where circuits consisted of a single insulated wire in metal conduit; the metal conduit serving as the grounded conductor. I guess that when materials are scarce enough, 'safe enough' can be pretty scary.

I've actually seen that in another country!

I meant to say a 3 prong oven, but I guess its the same reason. So are there any cases of people being injured when it was allowed to tie the N and G together at an oven or dryer?

Over time understanding of the magnitude of the neutral current has changed. The loads in homes have also changed over time with the addition of many new electronic loads (PCs, TVs, etc.) hence the change in 1996. You should have more confidence in your knowledge and correct your clients when they are wrong. Some may dislike it but at least they'll be alive!

 

[yired29]

A home owner wanted to tie his neutral and ground together at his clothes washer because he only has 2 prong outlets in his house. I told him not to do it because then he could have neutral current on the frame of the washer because its bonded to the ground wire. He then asked why this was not a hazzard when done with a 3 prong clothes dryer where the neutral and ground are tied together and I didnt have an answer.

250.140 Frames of Ranges and Clothes Dryers.
Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.
Exception: For existing branch-circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met.
(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.
(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.
(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.
(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

 

[LarryFine]

. . . circuits consisted of a single insulated wire in metal conduit; the metal conduit serving as the grounded conductor.

Hey, power coax.

 

[Cold Fusion]

I asked for some science or physics on why this is dangereous on a similar thread a little bit back. All the answers I got roughly translated to, "The code says so". And when I questioned the physics, I didn't get any answers. Hummm Interesting - makes me think there aren't any

Personally I think the safest is to keep the system 4W to the transformer and not tie the premisis (sp?) neutral to the distribution. And if we are not going to do that, then MGN works just fine

Yes I realize I am crosswise with the Wise Men of Gotham that write the code - as a few of them have explained to me, generally using one-syllable words.

cf

 

[iwire]

I asked for some science or physics on why this is dangerous on a similar thread a little bit back. All the answers I got roughly translated to, "The code says so". And when I questioned the physics, I didn't get any answers. Hummm Interesting - makes me think there aren't any

OK, heres one and it has nothing to do with what the code says or the little bit of VD on the neutral due to current flow.

If you use the neutral as the grounding means for a circuit and if that neutral opens between the source and the load all the metal objects connected to that conductor become hot (live)

That is reason enough for me.

 

[skeshesh]

I'm surprised because you stated the reason yourself: the neutral current resulting from phase unbalance can energize the metallic parts of the appliance.

what Bob (iwire) said plus my quote above. What we're talking about is physics for sure. Unbalanced phase current results in neutral current flow + if you connect neutral to ground, residual current will flow into ground wire, hence into bonding onto equipment body. What part of this don't you get?

 

[cschmid]

older dryers no electronics all 240 volt gear dryer came with frame bonded wire ran back to panel and fastened to to netral/ground bar no big deal. new dryers are 240 heater elements and drum motor only with 120 volt ciruit boards for electronic and lights, Now we have need for nuetral. 3 wire okay ciruit for antique equipement but now using 3 wire on dryer is hazardous for the euipements life and also hazardous to humans who use dryer, especially with wet laundry. :grin:

 

[LarryFine]

older dryers no electronics all 240 volt gear dryer came with frame bonded wire ran back to panel and fastened to to netral/ground bar no big deal. new dryers are 240 heater elements and drum motor only with 120 volt ciruit boards for electronic and lights, Now we have need for nuetral. 3 wire okay ciruit for antique equipement but now using 3 wire on dryer is hazardous for the euipements life and also hazardous to humans who use dryer, especially with wet laundry. :grin:

Actually, dryers have used the neutral for 120v loads for as many years back as I can remember. The drum and timer motors (and light, if there is one) are connected from L1 to N. Only the heater element is connected to L2, which is why they worked on 208v (albeit at 3/4 the heating power).

That's why the requirement for the neutral to be insulated or the bare of an SE-type of cable has been in place for all these years. Remember, too, that this allowance was for the grounding to be done by the neutral conductor, and not the EGC carrying neutral current.

 

[flashlight]

In older NYC apartments a lot of people other than electricians got at the wiring. It's not uncommon to see the neutral tied to the ground in old remodel jobs where they did not have a grounded conductor available at a box for a switch. In these cases, when the apartment is gutted and the wiring is visible, I have observed sparking between the metal casings of bx cable. Scary !

 

[Cold Fusion]

...If you use the neutral as the grounding means for a circuit and if that neutral opens between the source and the load all the metal objects connected to that conductor become hot (live). ...

Yes, I agree, that one works. Under those conditions, touching the dryer and the laundry faucet would give one a 120V shock. That could really hurt.

Let's go a little farther. What happens when the 7200V, 1ph, MGN fed xfm feeding your house, looses its neutral/ground?

cf

 

[kbsparky]

Speaking of physics, the problem in tying the grounds to the neutrals downstream from the main service is you are connecting a current-carrying conductor to the outer shell of your equipment.

This can set up a parallel path of returning current over the (bare) equipment grounding conductor(s) instead of the insulated neutral conductor.

The hazard is electricity will seek all available paths back to its source (usually the utility transformer), and since some of that returning current will be traveling over the bare outsides of your equipment, shocks can occur when one touches said equipment while also standing on or touching another grounded surface.

You can measure this difference across the ground and neutral conductors that are fed from a substantially loaded circuit or sub-panel. The more Amps traveling over that neutral, the greater the potential difference (read: VOLTS) will be present between the 2.

While those shocks may or may not be lethal, it is in the best interest of everyone to minimize that risk.

Keeping the equipment grounding conductors separated from your current-carrying conductors is one step to accomplish that.

 

[hardworkingstiff]

That's why the requirement for the neutral to be insulated or the bare of an SE-type of cable has been in place for all these years.

I never understood why a 10/2 (Cu.) romex was insufficient and an 8/3 (AL.) SE cable was adequate (yes I know the Code said it was OK). Anyone know? (was it all political?)

 

[Cold Fusion]

Speaking of physics, the problem in tying the grounds to the neutrals downstream from the main service is you are connecting a current-carrying conductor to the outer shell of your equipment.

This can set up a parallel path of returning current over the (bare) equipment grounding conductor(s) instead of the insulated neutral conductor.

The hazard is electricity will seek all available paths back to its source (usually the utility transformer), and since some of that returning current will be traveling over the bare outsides of your equipment, shocks can occur when one touches said equipment while also standing on or touching another grounded surface.

You can measure this difference across the ground and neutral conductors that are fed from a substantially loaded circuit or sub-panel. The more Amps traveling over that neutral, the greater the potential difference (read: VOLTS) will be present between the 2.

While those shocks may or may not be lethal, it is in the best interest of everyone to minimize that risk.

Keeping the equipment grounding conductors separated from your current-carrying conductors is one step to accomplish that.

This is where I disagree on the magnitude of the hazard. Up until the 97 code it was okay to use the range neutral to bond the the range frame. The same was true for the dryer.

If the few millivolts of VD on the neutral was such a problem with people being shocked all the time, how come we don't have a body count? I think it is because as Bob said, it's mostly a problem if the neutral at the range gets loose allowing the hot wire to light up the frame.

Now lets put this in perspective. If there is a 2V drop on the distibution neutral to the house transformer then that 2V will show up between any bonded metal to ground - and you can't get rid of it (unless you install something like a Ronk blocker). Here's how you check. Go out to an outside faucet and measure from the faucet to an isolated ground rod. That's the voltage you are getting from the distribution system. If you house is wired correctly that will be a lot more than any drop you will get from the VD on the branch circuit neutral.

I am not in favor of multiple bonding of the house neutral/ground. I'm not in favor of any bond between the neutral/ground - especially the one the utility provides us with free of charge. I think that one also provides the most hazard.

Oh yeah, as for the earlier question: What happens if the house transformer looses it's connection to the 7200V distribution neutral? The house neutral/ground go up to 7200V to ground.

cf

 

[iwire]

Yes, I agree, that one works. Under those conditions, touching the dryer and the laundry faucet would give one a 120V shock. That could really hurt.

Let's go a little farther. What happens when the 7200V, 1ph, MGN fed xfm feeding your house, looses its neutral/ground?

cf

Gee I don't know, never thought of that. :roll::grin:

But if I was to think of it I might think that they are not going to redesign the entire country's distribution system as easily as changing the range and dryer requirements.

 

[Cold Fusion]

Lets try another one:

You are climbing out of your swimming pool and step on the grass while holding on to the metal railing. The metal railing is tied right to the house earthing system exactly per code. And you get a tingle.

Is the tingle because your neutral/ground are inadvertantly bonded at the pool subpanel? Nope. It is because the utility is delivering you with a 5V drop on the utility distribution neutral and that utility neutral is tied right to your neutral/ground and that puts 5V on your grounding system. There's nothing in the code that fixes that.

cf