Ungrounded system question, ground rods at each machine? ( sigh )

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Tim163

Member
Location
Coldwater,MI
I have driven ground rods at machines in the past as a contractor, I have driven ground rods in the past as a journeyman. I have recently acquired my masters licence, and it has come up yet again. in the past it has been one or two , and I did not say much. but this time my boss ( a mechanical engineer ) found in the nfpa 86( 86 5.2.15 "The metal frames of furnaces shall be electricallygrounded"). we have around 150 furnaces he has purchased ground rods and wire for.

my question is. dose anyone know the reason for this grounding? is it a redundant requirement that is already covered under equipment ground?

I have tried to look up the definition of "electrically grounded" in the nfpa. there is no definition that I can find.

after thinking about it for a wile, we are on a delta / delta system ( ungrounded) no neutral, 4800v primary, 480v secondary. we don't have a grounded conductor to attach it too. we are using EMT / buss duct / IMC / switchgear so the ungrounded system requirements are meet per the NEC.

I tried to explain to him that these ground rods are not away to satisfy the " electrically grounded " requirement, these ground rods are not doing any more than the 1 inch bolts in concrete holding down the furnaces, or more that the 3/4 EMT that is attached with lock nuts are already giving you. He replied with a ground is a ground. and these ground rods make it safe so no one gets shocked. I really don't know where to start, I am thinking of hooking up a 120v hot wire to a ground rod, and watching it not blow the breaker ( I digress )

he left me today with, if you can find what is required in an official document, I will not have your crew install them.


as I said earlier, I have recently got my promotion as plant Master Electrician, and I have 1 masters, 2 journeyman and 3 apprentices all asking me to get them out of this un-needed work, I would like to help them out,and open my bosses eyes to what a ground is for, and what a ground is.


Thank you all for your help and insights, I have been a Mike Holt fan since 1996 ( when I started my apprenticeship)
 

kwired

Electron manager
Location
NE Nebraska
Ungrounded systems still require all non current carrying components to be bonded together via equipment grounding conductors and are also required to have a grounding electrode system connected to them.

This puts all those metal components all at same potential to reduce the risk of shock hazards. Only difference with an ungrounded system is the electrical system itself has no ground reference and you do need ground fault indication system to let you know when there is a fault. If you don't install indicators- the ungrounded system essentially becomes grounded system once there is a fault, but it is only desired to have this be a temporary condition, have it indicated and then to find and fix the problem as soon as practical. Should a second fault happen on another system conductor you will have high fault current and will blow overcurrent protection and have immediate shutdown - a condition usually trying to be avoided when using an ungrounded system in the first place.



I guess a more direct response to what was mentioned in OP is the system should already have a grounding elecrode system, this is your "grounding reference" and equipment grounding conductors tied to everything so those will extend that ground reference to the furnaces - adding additional ground rods doesn't really gain you much of anything.
 

Tim163

Member
Location
Coldwater,MI
we have ground fault indicators, and have had ground faults in the past, that we have had to locate and remove.

with out digging up places I can see ground wires going under ground, I can assume we have a grounding electrode system, there are several places we have used cad weld to attach ground wires to the building steel.

dose anyone have any references that I can use, about the nfpa 86? something like the nec hand book, that will give more detailed description of what this article is revering too?
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
....dose anyone have any references that I can use, about the nfpa 86? something like the nec hand book, that will give more detailed description of what this article is revering too?
I don't have any idea about nfpa 86 but I'm sure there is no way you can drive enough ground rods to keep anybody from getting a shock until you move up to the kilovolt level.

He's the engineer, make him show you the math.
 

Tim163

Member
Location
Coldwater,MI
I don't have any idea about nfpa 86 but I'm sure there is no way you can drive enough ground rods to keep anybody from getting a shock until you move up to the kilovolt level.

He's the engineer, make him show you the math.



I wish it were that easy, he has taken the stance,(prove to me that I am wrong, and I will re- evaluate)

Is there a way you can contact the nfpa, and as for a more detailed description, on one of there articles ?

I have though about asking him to watch the mike holt grounding and bonding dvd set, but Im pretty sure he will replay with, I don't have time for that
 

kwired

Electron manager
Location
NE Nebraska
AFAIK most NFPA publications are viewable on the NFPA website. They want you to register and log in, so they can stuff your email inbox as well as your snail mail box full of crap for years to come, maybe take what address you use into consideration - however if you ever want to purchase hard copy of something it will need to be sent to an address where you will actually get it.
 

Dexie123

Senior Member
Tim 163

This is considered a auxiliary electrode and is allowed but not required. Article 250.54. Mike has actually talked about this extensively.
 

John120/240

Senior Member
Location
Olathe, Kansas
In the machine tool world additional ground rods driven at the equipment is a common requirement by misinformed Engineers. They provide little if any additional protection, more of a feel good measure. All such tactics really do is line the Electricians pockets with $$.
The IAEI Soares book on Grounding is a good resource, I do not know if it addresses your situation.
 
... in the nfpa 86( 86 5.2.15 "The metal frames of furnaces shall be electricallygrounded")

Yes by the equipment grounding conductor (whether it be a wire or approved raceway) which is run with the circuit conductors and connects to the grounding electrode conductor/system at the service - like every other electrical installation. Why un-invent the wheel? The building grounding electrode system is likely a "better ground" than a lone rod driven at the machine.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Yes by the equipment grounding conductor (whether it be a wire or approved raceway) which is run with the circuit conductors and connects to the grounding electrode conductor/system at the service - like every other electrical installation. Why un-invent the wheel? The building grounding electrode system is likely a "better ground" than a lone rod driven at the machine.
To uneducated, uninformed and misinformed ground rods a magical cure that heal and protect electrical equipment.
 
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