Ground rod.

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overamped

Member
Location
California
Is it acceptable if you don't have a grounding conductor running back to the main to use a ground rod to ground a machine? We have a Three phase 480 volt 200 amp Autoclave that does not have a ground wire and it may not be possible to run a ground wire back to the main.
 
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hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Depends on a lot of variables, bare concrete floor, vinyl flooring, wood floor. Could change the potential to any of these because the ground rod(s) will not give you a low impedance path if it is the sole equipment ground. With the metal conduit, if it is a complete, properly installed system can give you the ground path needed. You may also be grounded by the water line, though not legal equipment ground if it's properly bonded back at the service.
 
kwired

kwired

Electron manager
Location
NE Nebraska
overamped said:
Metal conduit and I'll have to check about the waterline. Why is it more dangerous?
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If metal conduit it continuous and made up tight it is an acceptable equipment grounding conductor.

On a very serious topic - assume there is no metal path to the source and your only grounding is the ground rod - what happens if a phase shorts to the frame of your equipment? In normal circumstances with an equipment grounding conductor properly installed we have a low impedance path for the current to flow and the current will rise rapidly to levels higher than the overcurrent protective device setting and the device opens the circuit stopping the dangerous situation.

Now with no equipment grounding conductor and assuming it is a 277/480 system, that frame becomes energized to 277 volts above anything that is grounded - including earth, you touch it you are subjected to 277 volts potential. You say but I connected it to this ground rod. How much higher is the resistance of the path through the rod than it is for a properly installed equipment grounding conductor? 10 ohms would be considered a pretty good resistance to earth through a ground rod most cases. That would mean if the path through the rod had 10 ohms of impedance only 27.7 amps will flow - is that going to open a 200 amp breaker, not to mention there will still be a rise in voltage on anything connected to this and there will still be dangerous touch voltages practically everywhere.


Driving a ground rod should not even be a consideration. If you can't get an effective equipment ground to the equipment don't connect it. If there is a malfunction - especially fault to ground - the likelyhood of someone getting electrocuted increases to very high probability.
 
tom baker

tom baker

First Chief Moderator
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Location
Bremerton, Washington
overamped said:
Is it acceptable if you don't have a grounding conductor running back to the main to use a ground rod to ground a machine? We have a Three phase 480 volt 200 amp Autoclave that does not have a ground wire and it may not be possible to run a ground wire back to the main.
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The NEC specifically states the earth shall not be used as the effective ground fault current path. Take an assumed ground rod resistance, and the applied voltage and calculate the resulting fault current. Will that amount of current open the overcurrent device?

Whats the wiring method? If its RMC or EMT that is allowed as an EGC.
 
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overamped

Member
Location
California
Thanks!!! I see the answer to my question is in the code book and the why is explained in some of your answers and elsewhere on this website.
 
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hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Just to give an example of how ineffective ground rods can be, I had a service call on a brand new big box store where their pylon sign was not working, I checked the voltage at the sign, and had 480 volts to ground from the neutral and from the hot. Nothing between the hot and neutral. Put my tracer on it and found the EGC was terminated to "A" phase, while the neutral and hot was terminated to two separate "C" phase breakers, none of which were tripping. This EGC was terminated to a ground rod at least 20-30 poles, and still did not have a low enough resistance to trip a 20 amp breaker.
 
kwired

kwired

Electron manager
Location
NE Nebraska
stickboy1375 said:
Im curious why the original installers were not called back as to why the sign didn't work? And don't people test anything?
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I think the gremlin theory makes more sense than anything you will get as a responce from the original installers or whoever last tampered with it:happyyes:
 
stickboy1375

stickboy1375

Senior Member
Location
Litchfield, CT
George Stolz said:
The answer is to not screw up the phasing at the wire pull, which is pretty simple.
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True, but how does one even explain the way they landed the wires even with correct phasing?

hillbilly1 said:
. Put my tracer on it and found the EGC was terminated to "A" phase, while the neutral and hot was terminated to two separate "C" phase breakers, none of which were tripping.
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Im seriously confused.... :?
 
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