Earth grounding requirements on industrial equipment.

[cccelcj]

Can anyone tell me what NEC code article or IEEE std. spells out the requirements for earth grounding on industrial control panels? We are in the plastics extrusion business and have many machines that operate on 3 phase 480 power. All of out thermoformers, trim presses and pelletizers come from the OEM with the equipment grounding conductors installed and sized according to NEC specs in article 250. However, I seem to be having a problem when it comes to specific requirements regarding the grounded electrode system. The power comes into each machine from overhead busway fed from 3000 amp switchboard. Do I need a ground rod on each of the machine control panels? Inside the panels are relays, contactors, PLC equipment, motion controllers, power supplies, etc...
Any guidance is greatly appreciated.

 

[cccelcj]

I failed to mention the fact that each machine has several motors that are being controlled. Some are 3 phase AC up to 30hp. Some are DC up to 250hp.

 

[dpc]

I don't think you need a ground rod at all. The 480V feeder, no matter where it comes from, should include an equipment grounding conductor (green wire). This grounding conductor is sized according to NEC Article 250. This generally satifies the requirement for equipment grounding. Supplemental ground rods can be installed if desired, but must be bonded to to the equipment grounding conductor.

 

[cccelcj]

The nature of this process is one in which static accumulation is severe, so the supplemental grounds are desired. However it is my understanding that without proper bonding to the equipment ground that this earth ground is at a high impedance. I think the general assumption is that the machine steel components should provide an adequate fault conducting path and serve as an EGC. My boss seems to thing bonding is redundant and unnecessary. Am not sure how to convince him otherwise.

 

[dpc]

The NEC requires the equipment grounding conductor and also requires bonding of this conductor to any supplemental local ground. See Article 250.54.

 

[rbulsara]

ccelcg:

You should carefully review definition of GEC (grounding electrode conductor). GEC and grounding electrode is not something you install at every 'equipment'.

GEC is the conductor used to ground the system's "grounded conductor" , such as neutral of 4 wire system, to the system’s grounding electrode, such as the building steel or a ground rod. Now this connection is typically made only at one point, which is the service entrance or the secondary of a transformer or in other words the origin of the system.

From that point onwards the "grounded conductor" (e.g. neutral) is not grounded again while a ECG (equipment grounded conductor) or the "green wire" is always installed with every branch circuit. (metal raceways are acceptable by Code as ECG, but let us not use it for the purpose of this discussion).

The only connection between the ECG and GEC is at the service entrance equipment or the secondary of a separately derived system.

Now if your 480V system is grounded at its origin and your machine does not require a neutral, you need not install the neutral but you install the ECG and "bond' all non-current carrying surfaces of the equipment (enclosure, frame, etc.) to the ECG. For extra safety you can "bond' such non-carrying surfaces to supplemental grounded surfaces such as building steel but not 'required' the code.

As for the grounding the static charges goes all you need is the all non- current carrying surfaces that a person can touch or span by standing at one location should be bonded together and eventually conncetded to a effectively grounded surface or a seprate grouding electrode and more importantly the "ground" chasis of the equipment generating static. If you are bringing your power feed with ECG (green wire) you should also connect that to this 'bonded' system. Now a seprate grounding electrode, if installed for this purpose is not your system grounding electrode but a supplemetnal grounding electrode.

 

[advidana]

The earth-ground should be at the building service panel and at each sec of transformers connected to that service in that building. the earth ground is connected to equipment grounding conductor and that ground is carries to at the machines. the grounding conductor should be sized large enought to eliminate and static grounds. unless your buildingis 2 miles long it should not be a problem.
earthgrounding for nec designed buildings at the each piece of equipment is not a good idea it can cause ground loop currents that can lead to fires.

get yourseft a book on grounding per nec codes that has a lot of illustrations. have your boss pay for it. good luck

 

[HCBFlash]

Static can be REALLY wierd! Make sure that all pieces of metal on your machinery are definitely grounded (bonded) Loctite, paint, anti-sieze, and other "dirt" messes up machinery ground paths. I've worked on equipment that had strips and small "lightning rods" installed in the flow of dielectric granules or "powder".

You may want to try to eliminate the EFFECTS of your static problem by making sure that personnel are "UNGROUNDED", believe it or not!

Good luck! I hope you're able to resolve it all, ... at least so you can let us know what gives the best results!

 

[cccelcj]

I appreciate the replies as I have learned some things from some of the advice you gentlemen have given here. It seems I have two issues in particular. One is to make sure all the equipment is properly "bonded". The other is the issue of static. Earth grounding, equipment grounding and bonding are in themselves require their own techniques to ensure a safe installation as it relates to clearing ground faults and minimizing noise. Bonding it seems is a misunderstood and misapplied practice in my plant. When it comes to static however we do have some installations in which we are using ionizing antistatic devices. Are there any good reference material that outline procedures for static prevention other than what we are trying?

 

[dpc]

There should be a lot of information on static grounding. I'd start with the IEEE Green Book. The IEEE Industrial Application Society (IAS) would be the right place to start. You might do a search on the IAS Transactions and see what articles pop up.

 

[Harold53]

cccelcj
dpc is right. The Green Book is IEEE 142 Std (Recommended practice for grounding of industrial and commercial power systems) and should be complemented with IEEE 1100 Std (Recommended Practice for Powering and grounding sensitive electronic equipment).