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Old 01-03-2014, 08:34 PM   #1
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Why Equipment Needs to Be Grounded


This is sort of related to another thread....where someone mentioned that a professional electrician would not run more than one ground in a conduit.

Well...here is a good example of why maybe something needs it own ground....but more importantly, why a ground is needed in the first place.

This is a picture of the wiring inside a 200A 480 3ph disconnect..that is connected to a 150 HP motor.

Vibration from said motor caused the wires to rub against the wall of the disconnect...and after 7 years it wore it thin enough to short out.



This is on the outside.



Can you imagine what would happen if the disconnect was not grounded? And someone touched it?

Normally, this does not happen....because I know to NOT mount the disconnect on the same stand as the motor...I mount them on Unistrut feet. Second, I use flex from the disconnect to the motor...that way the disconnect is not subjected to constant vibration. This was not my project....so I can shed 'some' blame....but I did notice it a few years ago and should have done something then.

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Old 01-03-2014, 09:12 PM   #2
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Why Equipment Needs to Be Grounded


I fail to see why multiple grounds are needed?

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Old 01-03-2014, 09:22 PM   #3
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Why Equipment Needs to Be Grounded


If you're running a 150 hp motor in a residence, then perhaps it should have it's own dedicated ground. Not sure what correlation this has with the questions of your work in the other post though, unless you expect some rather serious, constant vibrations in your residence??
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Old 01-03-2014, 09:25 PM   #4
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Why Equipment Needs to Be Grounded


If you are saying that this is why you run grounds, yes you are correct. That size circuit in flex does require a ground to be run by code. So, any competent electrician would run a ground.
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Old 01-03-2014, 09:35 PM   #5
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Why Equipment Needs to Be Grounded


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I fail to see why multiple grounds are needed?
Because in the case of a fault, you could energizer a device upstream stream.

By each device having it's own ground, you reduce the chance of that happening.
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Old 01-03-2014, 09:42 PM   #6
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Why Equipment Needs to Be Grounded


There are WAY too many scenarios to consider to even make this a plausible discussion.

However, I will assume you are talking about relying on the conduit as the equipment grounding conductor. You are right, if the pipe is broken or cut you would loose your protection. However, if a romex homerun is cut, that happens as well.
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Old 01-03-2014, 09:44 PM   #7
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Why Equipment Needs to Be Grounded


I will also add that in the case of your picture, it would be very unlikely to loose your protection as the conduit is likely secured to building steel which is also bonded to ground.
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Old 01-03-2014, 09:46 PM   #8
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Why Equipment Needs to Be Grounded


The fault is going to go back to the panel where the circuit originates and trip in a very short order depending on the magnitude of the fault. Multiple grounds are not going to help it trip faster nor are the other grounds staying energized.
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Old 01-03-2014, 09:47 PM   #9
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Why Equipment Needs to Be Grounded


Quote:
Originally Posted by k_buz View Post
There are WAY too many scenarios to consider to even make this a plausible discussion.

However, I will assume you are talking about relying on the conduit as the equipment grounding conductor. You are right, if the pipe is broken or cut you would loose your protection. However, if a romex homerun is cut, that happens as well.
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I will also add that in the case of your picture, it would be very unlikely to loose your protection as the conduit is likely secured to building steel which is also bonded to ground.
In my industry...we don't depend on conduit as ground.

In this case, the disconnect was hooked up with rigid...but had it been done properly, it would have been mounted on the ground (isolated from the equipment) with flex to the motor...so a ground wire is a must.
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Old 01-03-2014, 09:51 PM   #10
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Why Equipment Needs to Be Grounded


I'm failing to see how your picture proves your point about multiple grounds. From what you are saying, the breakers/fuses would have opened even if a ground wire wasn't pulled.
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Old 01-03-2014, 10:52 PM   #11
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Why Equipment Needs to Be Grounded


Happens more than you realize. Seen it a lot onboard ships, due to the movement of the ship, and shipyard electricians that are rushed through finishing the job, but never properly secure all of the cables as they should be.
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Old 01-03-2014, 10:55 PM   #12
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Why Equipment Needs to Be Grounded


it is common practice in many industrial establishments to run a dedicated conductor for the equipment grounding conductor on each circuit, rather than relying on the raceway. it is not a code requirement, however. assuming the raceway was properly installed for an equipment grounding conductor, the additional dedicated conductor is not required and offers no benefit.

and unless that 200 amp switch is horsepower-rated for 150 hp, it is undersized.
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Old 01-03-2014, 10:57 PM   #13
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Why Equipment Needs to Be Grounded


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Originally Posted by k_buz View Post
I'm failing to see how your picture proves your point about multiple grounds. From what you are saying, the breakers/fuses would have opened even if a ground wire wasn't pulled.
Depends on how the failure happened. You could have an arcing event, as if you were arc welding, and the breakers would never fail, until they get so hot, then they may or may not fail.

It is really a roll of the dice in these type of situations. As for multiple paths for grounds, that you do not want on equipment, due to it causes more problems, then some realize.

You follow the rules for grounding, as they have been for ages, not by what the manufacturer states. Now of course, if you are using antivibration doughnuts or plates. Then you want a bonding strap from the motor to the actual ground bus for the equipment.

People see physically a ground strap at a motor, a ground strap at the rails for the disconnect, a ground connection inside the box on the motor for the electrical connections, a ground or grounds for various reasons inside the cabinet for the motor controller (low voltage grounds, cabinet ground, high voltage grounds), but they do not understand how they all inter-relate or interwork together, to make the device function.
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Old 01-03-2014, 11:43 PM   #14
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Why Equipment Needs to Be Grounded


mike holt said run the ground according to manufacturer instruction.
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Old 01-03-2014, 11:51 PM   #15
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Why Equipment Needs to Be Grounded


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mike holt said run the ground according to manufacturer instruction.
May want to go back and review what he stated. There is a part that you may have missed in his statement, regarding manufacturer instruction and grounding of equipment on site.

If you follow IEEE std 142 & the NEC, you will have no problems with grounding issues. The problems happen, when manufacturers want an isolated ground, that is not binding by the information in IEEE std. 1100.

So when the person hooking up the equipment with a computer as a part of the equipment goes to install it, they either do not properly ground, put in a ground rod that is not bonded to the existing grounding system, or end up creating a "Phantom" or "Floating" ground.

It gets really interesting when someone screws things up on a 3 phase electrical system in a building. Sparks will start flying at some point, if not done correctly.

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