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Old 01-04-2014, 06:37 PM   #16
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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.
In a typical commercial and industrial establishment it is almost impossible to NOT BE GROUNDED, structural steel, concrete with rebar, metal ducts, sprinkler, water, compressed air piping, plus all the conduit, metal studs, drop ceilings and a slew of other metallic connections I an forgetting

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Old 01-04-2014, 06:47 PM   #17
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Why Equipment Needs to Be Grounded


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Originally Posted by gregzoll View Post
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.
Arcing ground faults are common in 480/277 and higher voltage systems to have an sustained arcing ground fault the peak of the sine wave must reach 360 VAC (277 x 1.414=360 vac)

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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.
First that statement makes no sense and is IMPOSSIBLE to arrive at in any commercial, industrial establishment.



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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.
Not sure what device you are discussing but if you mean utilization equipment, a ground is not required to operate, if you mean a OCP (Overcurrent Protection Device) then you are correct.

There are many older ungrounded systems and newer resistance or impedance grounded systems. In an ungrounded system the fault above would only set off an alarm and the process could operate as the site electricians located the short to ground. In an impedance grounded system the same thing happens. The saying goes the first short is free, should there be a second short to ground then the OCP's operate. In an ungrounded system all metallic components are bonded, grounded.
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Old 01-04-2014, 06:51 PM   #18
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Why Equipment Needs to Be Grounded


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Originally Posted by Jim Port View Post
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.
The fault returns to the source transformer or generator the OCP closest to the fault in the circuit if properly coordinated, should open to interrupt the fault current.
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Old 01-04-2014, 08:02 PM   #19
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Why Equipment Needs to Be Grounded


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Originally Posted by itsnotrequired View Post
and unless that 200 amp switch is horsepower-rated for 150 hp, it is undersized.
It is rated for the HP. It's one of those mistakes you make only once.

In fact, a few weeks ago, I made a contractor replace 3 disconnects. He used the motor FLA as the deciding factor for the disconnect size. He had never looked at the inside cover of the disconnect where it shows exactly what the HP rating is of the disconnect. Two guys with 20+ years of experience each and they didn't know about that. Not to mention failing to take into account derating on some of the wire fills in conduit. Yep....licensed professional contractors.
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Old 01-04-2014, 08:12 PM   #20
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Why Equipment Needs to Be Grounded


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Originally Posted by ddawg16 View Post
It is rated for the HP. It's one of those mistakes you make only once.

In fact, a few weeks ago, I made a contractor replace 3 disconnects. He used the motor FLA as the deciding factor for the disconnect size. He had never looked at the inside cover of the disconnect where it shows exactly what the HP rating is of the disconnect. Two guys with 20+ years of experience each and they didn't know about that. Not to mention failing to take into account derating on some of the wire fills in conduit. Yep....licensed professional contractors.
couple months ago, contractor told me the disconnect switch needs to be rated for the ampere rating of the motor branch circuit ocpd.
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Old 01-04-2014, 08:44 PM   #21
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Why Equipment Needs to Be Grounded


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couple months ago, contractor told me the disconnect switch needs to be rated for the ampere rating of the motor branch circuit ocpd.
Must be something in the water.

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