Using Ground to Complete Circuit

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CBuchheit

Member
Location
Portland, OR
Occupation
Controls Engineering Manager
I'm building a UL 508A control panel and the customer wants to start the motor using the thermal switch in their motor as the overload device. That is of course allowed in section 34.1.3. The issue is they want to the contactor coil to go through this thermal switch and then use the *equipment ground wire* as the return path back to the control panel for this control circuit. They say they do it all the time and I don't doubt that it would work, but I can't imagine this is allowed by code. So what UL or NEC reference can I point to prove it isn't?
 
You can not use the equipment ground as a normal current path. The thermal switch must come back to the contactor circuit as separate conductors.


SceneryDriver
 
CBuchheit said:
They say they do it all the time . . . .
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I cringe when I see that phrase.

Let's do a "thought experiment." Before backing your car out of your driveway and into the street, put on an opaque mask (i.e., a blindfold) and insert earplugs. At a random moment, back into the street. Then remove the mask and the earplugs.

If you didn't hit anything, does that mean the practice is safe? Or do you need to do it for a week before concluding it is a safe practice? A month? How many "successful" (defined as you didn't hit anything) attempts will it take to prove that you can always back out blindfolded and deaf without risking hitting something (or someone)?

Bottom line: The fact that they have never been made aware that anything bad resulted from their use of the ground wire in this type of installation does not prove that it is an acceptable practice.

"An accident waiting for a place to happen will, given time, find that place." (Famous saying from some famous guy.)
 
CBuchheit said:
So what UL or NEC reference can I point to prove it isn't?
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As is often the case, you're probably not going to find a single code to cite that says "you can't do that." You can however piece it together. Here's a start for you to get you going.

ARTICLE 100 Definitions

Grounding Conductor, Equipment (EGC). A conductive path(s) that is part of an effective ground-fault current path and connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. (CMP-5)
Informational Note No. 1: It is recognized that the equipment grounding conductor also performs bonding.
Informational Note No. 2: See 250.118 for a list of acceptable equipment grounding conductors.

Ground-Fault Current Path. An electrically conductive path from the point of a ground fault on a wiring system through normally non-current-carrying conductors, grounded conductors, equipment, or the earth to the electrical supply source. (CMP-5)
Informational Note: Examples of ground-fault current paths are any combination of equipment grounding conductors, metallic raceways, metallic cable sheaths, electrical equipment, and any other electrically conductive material such as metal, water, and gas piping; steel framing members; stucco mesh; metal ducting; reinforcing steel; shields of communications cables; grounded conductors; and the earth itself.

Grounded Conductor. A system or circuit conductor that is intentionally grounded. (CMP-5)
Informational Note: Although an equipment grounding conductor is grounded, it is not considered a grounded conductor.


By definition, if you use the EGC to carry current through normal use, then you have changed what the conductor is, and now it violates a laundry list of other codes.
 
Thanks everyone, this is exactly what I need to show them their historical practice is wrong and needs to be changed.
 
I've gotten a hit from a loose run of EMT that was being used as a neutral. That's the kind of thing that hits you out of the blue.
 
As jmelic pointed out, there is a risk of a shock from someone disconnecting the ground temporarily without realizing its a CCC. This is just a bad practice and clearly not allowed by code. Just don't.
 
I never liked how for the last 75 years starter manufacturers have run the grounded conductor ( neutral ) thru the overload block on starters that have a 120 VAC coil. Common sense tells you to use a fused or breaker protected conductor to supply power to all start stop stations, limit switches, pressure switches etc. If one was to use the grounded conductor for controls and it shorts out you would not be able to turn off equipment.
 
garbo said:
I never liked how for the last 75 years starter manufacturers have run the grounded conductor ( neutral ) thru the overload block on starters that have a 120 VAC coil. Common sense tells you to use a fused or breaker protected conductor to supply power to all start stop stations, limit switches, pressure switches etc. If one was to use the grounded conductor for controls and it shorts out you would not be able to turn off equipment.
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That's because motor-starters have been on the market longer than short circuits have. 🤭
 
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