Theory behind pulling a ground wire with every circuit in one conduit

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Eddy Current

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The place i work is an industrial plant and they have their own standards. We are currently working a commercial type job and they require a ground with every circuit in a single conduit. Just to clarify that would be three circuits in one conduit with three grounds instead of just one.
 
130208-2345 EST

A likely reason, and it might be good if the EGC was insulated, is:

Consider an EGC that is the same material of wire and the same diameter as the Hot wire. At the end of the run short Hot to EGC. The resulting voltage at the shorted point relative to the EGC bus at the main panel is about 1/2 of the Hot voltage.

If you have only a single EGC, then some of that 1/2 voltage appears at other points along the EGC.

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From Eddy Current's description it sounds like EMT would be used. So an EGC the same size as the largest hot conductor plus the

EMT conduit, what condition would necessitate a separate ground for each circuit ?
 
Smart $ said:
Paranoia, insecurity, ignorance, etc. :p
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We agree. :)

Eddy Current said:
The place i work is an industrial plant and they have their own standards. We are currently working a commercial type job and they require a ground with every circuit in a single conduit. Just to clarify that would be three circuits in one conduit with three grounds instead of just one.
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This makes someone sleep better at night, IMO a complete was of someones money.
 
Perhaps some kind of variant of the isolated ground idea that somehow worked its way into the specs.

I have seen all kinds of strange things in specs over the years. I have learned to just shake my head and move on unless it is something that is really a bad idea as opposed to just adding cost.

I have probably mentioned this before but one project I did the spec called for a ground lug sized for a 2/0 cable in every enclosure. It looked a little odd in 6x6x4 inch junction boxes and next to a 10A circuit breaker in a small control panel, but I did it.

Grounding and bonding seems to be one of those things that attracts a lot of strange things to specs.
 
They own stock in a copper mine. :thumbsup:


Roger
 
Smart $ said:
Paranoia, insecurity, ignorance, etc. :p
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weressl said:
Ground conductors are 'passive' devices. You never know if an intermediate tap is loose/disconnected UNTIL a fault occurs. Expensive, but prudent.
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That's the reasoning pulling grounds in conduit (and tubing :lol:) got started, someone would not tighten a locknut, or set screw, so it made the path unreliable. Isolated grounds were real popular, but are kinda fading out now because they have not proven to help much. Just depends on how much you have to spend and how much redundancy you need. If you have an exhaust fan for a bathroom that quits working due to a wiring or motor issue that might have been prevented by meggering or some other extra measure, it may not payoff, but if you have a critical line motor that stops a process at the costs of thousands of dollars an hour, the extra expense maybe justified.
 
130209-0901 EST

There is no way to make a judgement on whether the required wiring of the original post makes sense or not. You need to know the application and what problem this wiring method is expected to solve. The application and problems have not been provided.

I can see this method solving some problems, and in other cases creating worse problems. Further, neither a single or separate EGCs may be a solution to the problem.

As an illustration consider a CNC, computer, and RS232 application.

The CNC machine frame is connected to its EGC that runs back to the main panel. The electronics and the RS232 common are connected to the CNC frame and thus to its EGC.

The computer is possibly 100 ft away supplied from its local wall outlet. Its EGC runs back to the same main panel as the CNC, but is a different EGC. At the computer the electronics and including the RS232 common are connected to the computer EGC.

I will use the EGC bus at the main panel as the voltage reference point.

There is an RS232 direct connection, no isolation, between the CNC and the computer. This connection consists of a common (ground or EGC or whatever you want to name it), and two signal lines, one from CNC to computer, and the other computer to CNC. With these three wires and software handshake successful start-stop communication can occur.

A voltage of greater than about 25 V between a signal line and common will destroy RS232 interface components. Voltages between +3 and +12, and -3 and -12 are considered valid data levels.

The common wire in the RS232 cable may be #24.

Suppose a short occurs at the computer between Hot and EGC. This raises the potential of the computer chassis to a peak of about 120 * 1.414 * 0.5 = 85 V relative to the EGC bus at the main panel. There is no voltage drop on the CNC EGC between the CNC and the main panel. Thus, the peak voltage difference between the computer and CNC commons is 85 V, and this is a low impedance voltage source, meaning high current is possible. In milliseconds this will destroy the RS232 interface chips and maybe more. A little longer and the current from this voltage difference will burn up the small common wire in the RS232 cable. Thus, the two separate EGCs are a bad idea in this application. This is an occurrence that is likely to occur when there is a lightning strike, or when someone puts a screwdriver from Hot to chassis.

A more common problem is noise voltages on the EGC lines that are large enough to exceed +/-3 V and thus introduce data errors. Actually there are many RS232 circuits that do not adhere to the RS232 specification and threshold much closer to 0 V, like a simple transistor input. These are more noise prone.

The noise problem may be reduced by running the computer EGC to the CNC EGC at the CNC. In other words run a 120 V circuit to the computer from the CNC.

The best solution is to use electrical isolation in the RS232 communication path. Trying to hold down ground path (EGC) noise by using very large ground conductors is not very practical, and not nearly as effective as isolation. On an isolated cable I have placed a 1000 V RMS 60 Hz sine wave and produced no data errors.

One can create applications where the separate EGCs might be useful. But I suspect a general specification has been written that may or may not be of any value for the particular application of the original post.

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trying to reduce noise by increasing the size of an EGC is an exercise that will almost always fail.

RS232 is just a very poor choice for communications these days. It is cheap to add in some kind of isolation if RS232 is all that is available.

There are some RS232 ports that now come std as isolated. It may even be the norm for all I know.
 
hillbilly1 said:
That's the reasoning pulling grounds in conduit (and tubing :lol:) got started, someone would not tighten a locknut, or set screw, so it made the path unreliable. Isolated grounds were real popular, but are kinda fading out now because they have not proven to help much. Just depends on how much you have to spend and how much redundancy you need. If you have an exhaust fan for a bathroom that quits working due to a wiring or motor issue that might have been prevented by meggering or some other extra measure, it may not payoff, but if you have a critical line motor that stops a process at the costs of thousands of dollars an hour, the extra expense maybe justified.
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Don't forget corrosion as being the reason of resistance buildup in the ground path. In the case of Tray Cable installation, where the continuous ground path is even more perilious, the addition of a ground conductor in the cable is a must, IMO.

Isn't EGC primary purpose personal safety? High resistance grounded systems are used to prevent ground fault tripping and only to give indication of a ground fault. The intergity of the GRC is important though in both cases.
 
I would question the designer/engineer as to why 3 EGC are required and debate it with him/her.

Maybe he or she is a newbie and just finished EE program and this is their first design job and is not familiar with the NEC and maybe doesn't know that one EGC will suffice.
 
hillbilly1 said:
That's the reasoning pulling grounds in conduit (and tubing :lol:) got started, someone would not tighten a locknut, or set screw, so it made the path unreliable. Isolated grounds were real popular, but are kinda fading out now because they have not proven to help much. Just depends on how much you have to spend and how much redundancy you need. If you have an exhaust fan for a bathroom that quits working due to a wiring or motor issue that might have been prevented by meggering or some other extra measure, it may not payoff, but if you have a critical line motor that stops a process at the costs of thousands of dollars an hour, the extra expense maybe justified.
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Ok i think im seeing why they might want this now but in a commercial building inside the plant why would they need this?

I was also thinking about tray cable that is used through out the plant and it does not have a ground per circuit, if it is say something like 12-3 tray cable there is only one ground in the cable.


Im not sure when the plant started making a ground per circuit required but it might have started when an employee was killed about 10 years ago from a fan casing becoming energized.
 
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edward said:
I would question the designer/engineer as to why 3 EGC are required and debate it with him/her.

Maybe he or she is a newbie and just finished EE program and this is their first design job and is not familiar with the NEC and maybe doesn't know that one EGC will suffice.
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I doubt this since some of higher ups are actually on the board that wrote the NEC code book.
 
The word "ground" has no meaning when discussing the rules of the NEC. Are we talking about the grounded conductor or are we talking about the Equipment Grounding Conductor?
 
don_resqcapt19 said:
The word "ground" has no meaning when discussing the rules of the NEC. Are we talking about the grounded conductor or are we talking about the Equipment Grounding Conductor?
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In secondary circuits, a ground wire is a common term for the EGC. I doubt it would be confused with the grounded conductor except among those that wouldn't normally be allowed to post here anyway. YMMV.
 
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