Is a ground rod needed for industrial machinery?

[cccelcj]

This may fall under the category of NFPA 79 which is
"Electrical Standard for Industrial Machinery". If so, can anyone reference it or tell me if there is a problem with the following scenario? A thermoforming, injection molding or any other industrial machine which is fed from a 480v busway feeder with equipment grounding conductor. Is there a problem with driving a ground rod 8 ft. into the ground and bond this rod to the machine? Can anyone who has access to NFPA 79 or from any other source tell me if this is necessary? Can there be any advantage or disadvantage to this installation?

 

[OperaHouse]

One of the advantages that isn't talked about much is that it brings the local ground (the stuff you walk on) to the same potential as the machine. Just think of the birds that can land on high tension lines. Most grounds really can't handle the potential fault currents. This at least help protect people in contact with the machine.

 

[cccelcj]

So then, does not the equipment ground conductor do the same thing? The EGC runs with the 3 phase supply conductors and is bonded to the machine for the purpose of fault clearing. And since this conductor originates at the service to the service grounded conductor,which in itself is bonded to the grounding electrode by means of the grounding electode conductor, is not the machine, which is sitting on the ground, grounded any way? This additional ground rod which I have heard may be considered by some to be a supplemental ground rod and by others an isolated ground rod seems to be redundant and serve no purpose other than to possibly create a ground loop.

 

[ScottI2R]

cccelcj,
In the past, we have installed machinery that was functioning in perfect order when it left our factory. Then, after being installed at the customer's location, the system seemed to be buggy and have all sorts of strange, intermittant issues. Normally, driving a 4-6 foot ground rod into the concrete floor right next to the machine and connected by big arse copper cable about 3-6 inches long, to the base of the machine, solved the brain damage that seemed to occur during transit of the system. Cheap, easy to do, and I have seen it work. Why not?

Scott

In a hundred years, it isn't going to matter anyway.

 

[itsmoked]

Huh, I thought you were only spozed to have a single system ground point.?? Is that not true in industrial settings?

 

[Skogsgurra]

This discussion (the ground only once vs ground as often as you can) pops up more and more. See thread237-119903 for more arguments.

Almost all equipment is grounded inadvertently. The machine rail that a paper machine stands on is grounded (it would be extremely difficult to isolate it). Same goes for most large machines - and very often small ones as well. As soon as a tube for cooling water is connected, you will have a ground connection (yes, know about isolated sections). So, even if one tries to have a single grounding point, you will have many. Most of them unknown to you.

My personal feeling is that you shall ground as much as you can and the reason is that cases like the one ScottI2R described usually only can be solved by either driving a hefty ground rod (if possible, machine not always on ground floor or in cellar) or connecting pieces together with an equipotential conductor. The latter looks very much like an extra ground connection. And is.

Also remember that there are many different systems in play. A review of the TN, IT and TT systems is often an eye-opener.

 

[itsmoked]

I see.

Never thought of that.

Thanks skogs.

 

[MakeItSpin]

skogsgurra makes some good points. I might also add that NEC requires "any" power source to a machine have it's own equipment ground. Example would be a machine that has a 480v 3 phase supply plus a 120v circuit for an immersion heater, controls, etc.

Don't confuse bonding the grounding electrode system to the neutral (grounded conductor) with equipment grounding. NEC requires the electrode system/neutral be bonded at only one point regardless of occupancy.

NEC does not allow the earth to be the sole grounding conductor. I've seen equipment that wasn't connected to an equipment ground from the source, but connected to a local ground rod. This is a NEC violation. We used to call this "peg grounding", but connect the proper equipment ground from the source and the rod is fine. Whether it's good practice or solves any problems is dependant on the particular installation.

 

[dpc]

I don't have much to add to this discussion, but I do want to emphasize MakeItSpin's comments regarding NEC requirements. The discussion of various grounding options and methods around the world is interesting, but for those of us in the US, the NEC dictates what must be done.

The power feeder to the equipment must contain an equipment grounding conductor and the equipment must be bonded to it. If a supplemental ground rod is desired, this can be added, to your heart's content, but each one must be bonded to the equipment grounding conductor.

I only bring this up because in the past, many control system suppliers in the US attempted to solve EMC issues by insisting on a separate "quiet" grounding system, with ground rods separate from the main plant grounding system. This is not only an NEC violation, but it is unsafe.

Of course, the situation is different if different grounding methods are employed, but in the US, the NEC dictates the grounding and bonding of equipment (and the inspector has the final word.)

 

[cccelcj]

dpc,
Your point is well spoken. As well as the rest of you, thanks for your input. I think in my facility there is a misinterpretation between the terms grounding and bonding. Some think of the the earth as a black hole for electricity. In reality the earth has high impedance. According to the NEC there are two grounding systems. One is earth ground. The other is equipment ground. In some of the equipment I have measure as much as 2 amps on the equipment grounding conductors. It is in my case a delicate issue to try and inform my superiors of the NEC requirements and am not sure how to proceed.

 

[rbalex]

NEC 2002, Section 250.110 and following decribes what must be grounded.

NEC 2002, Section 250.130 and following decribes how.

Pay attention to service and separately derived systems. A local rod in neither necessary nor sufficent - it is permitted as a supplementary method.

 

[itsmoked]

Makeitspin: You cleared that up for me. I was thinking of the many times I've seen, in this forum, the subject of bonding only at the service entrance. I was thinking this local grounding as the same thing. Thanks for clearing this up.

 

[RadSr]

Hi cccelcj,

I work in a factory that has a lot of thermoformer machines. That also means U probally have extruders. Hopper systems. And Vaccume systems. With the demands on heaters and the static from exposure to high plastic static. U should have a independant ground on each peice of equipment. Most Echo Gramms and Hopper systems are very sensitive to static as well.

 

[dogleg43]

It is OK to put in this separate ground rod at the machine but you must also make sure to connect a ground wire from that rod to the plant system ground. According to the NEC everything should be tied together.

 

[resqcapt19]

itsmoked'

Huh, I thought you were only spozed to have a single system ground point.?? Is that not true in industrial settings?

As long as you don't connect the system grounded conductor (neutral) to earth in more than one location you still have single point grounding system. Multiple connections of the equipment grounding conductor to earth does not change this.
Don

 

[jsummerfield]

A lot of the correct words exist in this thread but some confusion may still exist. For clarification, the references to the NEC apply to NFPA 70, the National Electrical Code. This topic could also be addressed in the "Electrical Motors" or IEEE electrical other topic sections.

It is appropriate to ground the equipment frame, perhaps from two points, to a rod and if available connect the rod to the grid. This is in addition to the power source grounding. An electrical "power" engineer typically can clarify lots regarding grounding systems. BTW, this all addresses solidly grounded systems. Some large equipment may be resistance grounded for fault current limitation. Talk to the big boys for these details.

Separating the control system issue from the equipment issue, the isolated ground issue pertained to ignorant people, mostly within one of the larger manufacturers. The equipment cabinets and shields are grounded at a different point than the 24 VDC return. One practice includes a triangle with individual rods, one for the cabinet safety and one for the signal ground. BTW, I failed to mention an intrinsic safety ground. Same concept. However, all of these individual ground rods are tied to together and to the grid if applicable. Most control systems require one-ohm earth resistance.

John

 

[bigbillnky]

This is a very important topic for all concerned. Their is a growing movement to better clarify the use of "grounded" and "bonded" as addressed by the NEC. Most of the time it is taken for granted that the two words are interchangeable, and that leads to confusion and misunderstanding. Supplemental ground rods are not in violation of the NEC, provided they are also bonded to the equipment grounding conductor. Space ground rods properly to prevent shell effect.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[mysterylectricity]

Is the equipment grounding conductor in the raceway sufficient to bond the proposed additonal grounding rod into the grounding system as a whole? What is the requirement?

 

[mc5w]

An equipment ground that is Code is not necessarily effective at ultrasonic and radio frequencies. When a ground wire is 1/4 wavelength long at a particular frequency it acts as an open circuit at that frequency. Also, an equipment grounding wire that is in steel conduit is mostly ineffective at high frequencies.

If what you have is say an induction furnace you need both a local ground rod and bonding to the reinforcing steel in the concrete floor, what is know as an equipotential mat. You also need bonding to local building steel and so forth.

In the case of a CAT scanner the manufacturere often species wires sized for 50 Celsius operation, an equipment ground wire that is 100% of the phase conductors, and a local grounding electrode conductor same size as the phase conductors for the local disconnect switch. The oversize wires are a bit of paranoia in case the electricians do not bother to remove copper oxide causing some of the wire strands to not conduct. The big fat equipment ground is to limit return path voltage drop during a ground fault. They want the redundant grounding electrode conductor in case the one at the service fails.

Part of the purpose of an isolated equipment ground is that the grounding path back to the service has to shielded from picking up radio stations yada yada. A Magnetic Resonance Imaging room is built a grounded as a single isolated ground appliance to keep radio stations from messing up the MRI and to keep the MRI from messing up radio stations.