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Old 04-20-2010, 09:04 PM   #1
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240 volt load/appliance and ground vs neutral


I have a question regarding grounding of 240 volt circuits for electricians. I have an old house that has no ground wires (2 wires to all wall receptacles). I am hooking up an old large transformer type welder to 240 input in my shop. My question is: Is it safer to have no ground to welder chassis (simply hook only black and red for 240 volt welder input) , or to connect the neutral to welder chassis as ground? In this case the 240 circuit is brought to shop via a 3 line (no bare ground) wire from main service panel (black & red w 240V and white to neutral ). Besides the welder, there is only a 240 volt compressor on the circuit. So there is no current being carried by the neutral wire anywhere along path to main panel. I have a feeling many would say I should not connect ground to neutral wire but in this case I do not see why this would not be a true equivalent to having a proper ground connection and would be a safer connection than no ground at all. The main service panel is grounded via the cast iron and copper plumbing system (no ground rods), which has proven to be a fine ground as the house went for few years with the plumbing being the neutral conductor current to city’s distribution transformer due to a broken neutral wire in service wires to the house – it was only during a severe drought (making for high resistance to transformer pole’s ground) that the bad neutral become noticeable and the problem was discovered (and fixed by city). It has been suggested (by a ‘professional’ electrician) that (in lieu of a rewire of the house) I should drive a ground rod just outside from welder and hook welder chassis to that. While I do see that would make a better circuit than I if there was a hot wire short to welder chassis, I still think my idea to use neutral wire as the ground is better 'safety/fault' circuit…
Also - If I were to add 120 volt lighting to this circuit in future (I.e. – then the neutral is conducting current) would that change the situation if I am using neutral as ground on the welder - if so, then why?

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Old 04-20-2010, 11:42 PM   #2
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240 volt load/appliance and ground vs neutral


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Originally Posted by kgize View Post
I have a question regarding grounding of 240 volt circuits for electricians. I have an old house that has no ground wires (2 wires to all wall receptacles). I am hooking up an old large transformer type welder to 240 input in my shop. My question is: Is it safer to have no ground to welder chassis (simply hook only black and red for 240 volt welder input) , or to connect the neutral to welder chassis as ground?


Connect the "neutral" wire to the chassis ground, and also re-position it on the ground bar in the panel and connect it to the ground terminals at the 240V receptacles instead of the neutral terminals. If the neutral and ground bars in the panel are not separate, leave it where it is. Re-mark it green at both ends. This is not technically code compliant - you cannot re-mark white as ground. However, it's the safest solution here. The wire will no longer be a "neutral" - it is now "ground".

Quote:
It has been suggested (by a ‘professional’ electrician) that (in lieu of a rewire of the house) I should drive a ground rod just outside from welder and hook welder chassis to that. While I do see that would make a better circuit than I if there was a hot wire short to welder chassis, I still think my idea to use neutral wire as the ground is better 'safety/fault' circuit…
Never ask that "electrician" friend for advice or let him work on your house! Not only is he wrong, he's dangerously wrong. There is an important difference between grounding and bonding. Bonding is what matters here. A bonding connection ties the metal chassis of equipment to the SYSTEM NEUTRAL, so that a fault to the chassis will trip the circuit breaker immediately. Grounding (connecting the chassis to the actual earth) is only marginally important for completely different reasons (stabilizing system voltage and lightning protection mostly) and DOES NOT PROTECT YOU AT ALL FROM SHOCK.

Why? Ohm's law. The resistance of a ground rod is on the order of 25 ohms (sometimes vastly higher, sometimes a bit less). Let's say you ground your welder to a ground rod, but do not bond it to the system neutral. Let's say there's a fault in the welder and a hot wire hits the chassis, making it hot with 120V. What's the current through the ground rod? 120V / 25 ohms = 4.8A.

4.8A through the ground rod clearly will not trip your breaker. Not even close.

What will happen? The welder chassis will stay hot with 120V, while the ground rod continuously wastes 120*4.8=576 watts of power that you're paying for. Touch the welder, you get shocked just the same as if it weren't grounded. Touch the ground wire, the ground rod, the air compressor chassis on the same circuit, or any other metal contacting any of these things and you'll get shocked, too - it's worse than having no ground at all because it makes all the "grounded" metal on the circuit hot!

There's also a "step potential" on the ground around the electrified ground rod, which can shock you or easily kill animals that go near it, even without touching the rod.

You may be thinking that the ground rod provides a better path to ground than your body does, which it true. However, that's irrelevant. Electricity does not take ONLY the "shortest" path to ground. It takes all paths in proportion to their conductivity. Whether the electricity is ALSO flowing through a ground rod has absolutely no bearing on what it will do to you. This is obvious: Connect a 100W light bulb and a 40W light bulb in parallel. Does only the 100W bulb light up, since it's the more conductive path? No. They both light, drawing current determined only by their own resistance. The presence of the 100W bulb does not affect the current in the 40W bulb at all - just like the presence of a ground rod will not affect the current through you.

Quote:
Also - If I were to add 120 volt lighting to this circuit in future (I.e. – then the neutral is conducting current) would that change the situation if I am using neutral as ground on the welder - if so, then why?
That would not be permissible by code. That's equivalent to the old style three-prong range and dryer receptacles that share ground and neutral. It's no longer allowed, and for good reason. If that shared line breaks (even due to a loose plug, for example) then the chassis becomes hot since the 120V loads will bridge between the chassis and a hot wire. Not so good. You could do it and it would work, but it's not to code and I would not install it in my house that way.

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Old 04-21-2010, 12:42 AM   #3
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240 volt load/appliance and ground vs neutral


Thank you mpoulton for the excellent response.
So in the first paragraph you are agreeing my concept to use the neutral as ground in this case is OK - and I should re-label (color) the neutral wire and remount to ground bus. Still the same circuit...but OK - I'll do that - this is mainly for future operators to know whats going on is the main point in doing that??

Regarding second paragraph - I completley understand what you are saying there. I did not intend to follow suggested ground rod to welder idea as I felt the resistance of the ground rod's path would not be much less than the resistance of my hand on welder to concrete floor - you made same point below - I still get a shock...and faulted welder would likely not trip breaker too. Although I am not sure why the compressor would shock me - it has no ground either. This "electrician' friend has worked in the maintenance department at local college for decade or two and learned his 'trade' on the job there...we often argue about such things...I would not let him do anything to my house besides have a beer or 2..you are good 'quick read' of people

Regarding the 3rd response - the equivalent of the old 3 prong dryer w neutral and ground the same - I am still not clear why that is such a bad situation? If I have say a 120 v flourecent light I add at junction box (no plugs)- now making the neutral 'live 'when light is on...and the 'neutral wire somehow broke between service panel and welder - then I have essentially conected my welder chassis to a HOT through the light (and now with no ground protection as the 'neutral' is open - I assume that is the point (??) and a good one too...I do have other 'normal' 120 circuits in the shop I can add lights to. ...So bottom line - I am OK with nuetral being my groud if only 240 conections are made on that circuit - correct??
Thanks again for your response mpoulton
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Old 04-21-2010, 08:29 AM   #4
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240 volt load/appliance and ground vs neutral


If the circuit has a neutral but is to be taken over for 240 volt only use, then the neutral conductor hooked up as a ground makes a perfectly good ground except for its color.

Leaving the neutral hooked up as a neutral but "grounding" the welder chassis to it is equally unsafe as trying to make a neutral double as a ground in a 120 volt circuit. But it will be safe if no 120 volt loads tap into that neutral anywhere in the branch circuit or anywhere before the main panel since when the neutral arrives at the main panel it is connected to the same place (terminal strip) as ground wires.

The old 3-prong 240 volt dryer with combined neutral and ground is subject to shock hazards if the neutral somehow broke. Dryers (most brands) are not pure 240 volt appliances because the motor is usually 120 volts, and the neutral will carry current.

Compressors, welders, air conditioners, etc. with no ground are safe provided that no hot or neutral electrical parts were connected to or have faulted to the cabinet or frame. In the case of a welder, this requires a "regular" transformer without a ground tap on its primary, not an autotransformer.
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Last edited by AllanJ; 04-21-2010 at 08:49 AM.
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Old 04-21-2010, 09:11 AM   #5
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240 volt load/appliance and ground vs neutral


Thank you all for your responses - I get it - makes perfect sense now
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Old 04-21-2010, 10:29 PM   #6
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240 volt load/appliance and ground vs neutral


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Originally Posted by kgize View Post
Thank you mpoulton for the excellent response.
So in the first paragraph you are agreeing my concept to use the neutral as ground in this case is OK - and I should re-label (color) the neutral wire and remount to ground bus. Still the same circuit...but OK - I'll do that - this is mainly for future operators to know whats going on is the main point in doing that??
Yes, it is still the exact same circuit electrically and the only real point is to avoid a problem with future changes. The chassis should be bonded through the ground terminal on the receptacle, not the neutral terminal, even though they end up in the same place at the panel.


Quote:
Regarding second paragraph - I completley understand what you are saying there. I did not intend to follow suggested ground rod to welder idea as I felt the resistance of the ground rod's path would not be much less than the resistance of my hand on welder to concrete floor - you made same point below - I still get a shock...and faulted welder would likely not trip breaker too. Although I am not sure why the compressor would shock me - it has no ground either.
I was assuming the compressor was wired the same way as the welder, with the chassis connected to the circuit's ground conductor and a ground rod, but not bonded to system neutral in the panel. Therefore, the compressor and welder chassis are connected to each other through the grounding conductor - if the welder is hot, the compressor is too.

Quote:
If I have say a 120 v flourecent light I add at junction box (no plugs)- now making the neutral 'live 'when light is on...and the 'neutral wire somehow broke between service panel and welder - then I have essentially conected my welder chassis to a HOT through the light (and now with no ground protection as the 'neutral' is open - I assume that is the point (??)
Yes, that's exactly it. An open neutral will energize all of the metal on the circuit.
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Old 04-22-2010, 04:22 PM   #7
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240 volt load/appliance and ground vs neutral


Thank you mpoulton - I get it - seems obvious now - but wasn't at first.
I assume that (open neutral creating hot ground) is the reason why a subpanel must have seprated ground and neutral bus bars - also wasn't immediatly obvious to me as I read through this site.
I am glad you mentioned the assumption of ground on my compressor - got me thinking that I also made an assumption and where did the 'third' wire go to on that compressor's 220 plug (there were two 240 plugs on this circuit - one like you mentioned an old dryer three prong, and then another 20 amp horizontal blade receptacle)...well it turns out that other receptacle was wired just as the topic to start this thread - the ground 'slot/hole' in receptacle was wired to neutral - and nothing was marked..
So the way I was proposing wiring my welder was already happening regarding the compressor (I.e your assumption was correct)...
Anyhow - thanks all for your expertise - and the circuit will be dedicated 240 V only and the neutral is marked and the receptacles have notes written on wall next to them
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Old 04-22-2010, 11:40 PM   #8
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240 volt load/appliance and ground vs neutral


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I assume that (open neutral creating hot ground) is the reason why a subpanel must have seprated ground and neutral bus bars
You got it. Not many people actually understand this, just like not many people understand the difference between grounding and bonding, or why a ground rod provides no protection from shock.

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