equipment grounding

[mull982]

I'm working on a project that requires equipment grounding of portable mine equipment ( conveyors and crushers) The way that most of our equipment is grounded here at our plant is through the ground grid with bare 4/0 copper cad welds from the ground grid, or a ground rod. Because this equipment will be portable it would not make sense to drive ground rods so I was looking at the next best way for doing this.

Located by the equipment will be a portable skid that has both 4160V and 480V switchgear. The switchgear on the skid will be fed from stationary switchgear in our plant so this feed will have a ground as well as a ground check monitor. Can I equipment ground all of my portable equeipment by running ground conductors from the ground bus of my switchgear on the portabe skid, as well as ground the skid from this switchgear sitting on it? How do I determine how many/size equipment ground conductors are necessary? Should I take resistance measurements?

Each piece of equipement will have a 480V motor on it which will have a ground along with its feeders. Is it possible to use the ground wire that is pulled to the motor and bond this to the piece of equipment (conveyor) on which the motor sits?

Looking for some ideas? I appreciate the help.

 

[steve66]

All I can say is that it doesn't look like the NEC covers your installation. 90.2(B) Not Covered:
(2) Installations underground in mines and self-propelled mobile surface mining machinery and its attendant electrical trailing cable.

That sounds pretty close to what you are saying.

 

[tryinghard]

Located by the equipment will be a portable skid that has both 4160V and 480V switchgear. The switchgear on the skid will be fed from stationary switchgear in our plant so this feed will have a ground as well as a ground check monitor. Can I equipment ground all of my portable equeipment by running ground conductors from the ground bus of my switchgear on the portabe skid, as well as ground the skid from this switchgear sitting on it?

I understand that you have 4160 sharing a skid with switchgear that is 480V fed from your plant switchgear, then the individual conveyor motors distribute from the switchgear on skid, probably an MCC. If the skid is a duel fed service then notice 250-24(A)(3). To qualify your installation you really need to back up to the source and the same time keeping in mind what the destination needs.

The feed from your plant can be one of the three following:

1) 4w feeder: A, B, C, N -- if it?s a grounded system -- with a common neutral/grounded conductor as qualified in 250-32(B)(2) by NOT having an equipment grounding conductor (EGC) or conductive path from its supply. In other words the neutral bonds to the equipment grounding and the neutral is used in a duel purpose of ground fault path as well as current return path, but only if it not paralleling. This is the same set up as if this were a service disconnect. You will also need an electrode at the skid (250-50) with an electrode conductor at least size #6cu (250-66(A))

2) 4w feeder: A, B, C, N, & EGC -- if it?s a grounded system -- (EGC may be a conductor sized from 250-122, or it may be mechanical from 250-118, or both). This neutral/grounded conductor is qualified in 250-32(B)(1) by HAVING an EGC or conductive path from its supply. In other words the neutral is NOT common or bonded to equipment grounding and is used in the single purpose of current return path only because in this case it would be dangerously paralleling. Sub panels are often fed this way (250-24(A)(5)). You will also need an electrode at the skid (250-50) with an electrode conductor at least size #6cu (250-66(A))

3) 3w feeder: A, B, C, & EGC -- if its an ungrounded system -- (EGC may be a conductor sized from 250-122, or it may be mechanical from 250-118, or both). An ungrounded system is only used to disable ground fault protection for critical machinery by allowing one short to remain (that must annunciate/alarm for repair) but a second short would initiate the ground fault protection (open the circuit). Again these are used typically industrially for critical machinery qualified as being dangerous if it faults and stops. An ungrounded system is one that does not have a grounded conductor from the source, you cannot have an ungrounded system from a grounded system (wye) unless you route through another separately derived system like a transformer; conversely to this you cannot have a grounded system from an ungrounded system unless you route through another separately derived system like a transformer. You will also need an electrode at the skid (250-50 & 250-32 (C)) with an electrode conductor at least size #6cu (250-66(A))

Each piece of equipment will have a 480V motor on it which will have a ground along with its feeders. Is it possible to use the ground wire that is pulled to the motor and bond this to the piece of equipment (conveyor) on which the motor sits?

Absolutely, this is the purpose of the equipment grounding conductor (EGC) that is with the circuit to the motor; this enables an effective ground fault path 250-4(A)(5), you do need to make sure you have an true fault path from motor (destination) to source (ultimately XO of a transformer or gen set neutral/grounded conductor).

Keep this clich? in mind: shorts do not go to ground (earth), shorts only try to return to source and they do this through bonding!
Rods do not provide any ground fault protection and they do not give you a safer installation for this purpose. The only reasons for ground rods are described in 250-4(A)(1), ?Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines?

If you have multiple motors on one conveyor you only need to bond it once according to the largest circuit and use table 250-122 to size it. If you?re using cable or cord for each motor branch circuit and it already includes EGC bond them as well. Notice this picture for a visual of effective ground fault path:

 

[boboelectric]

M.s.h.a. A.h.j.?

 

[mull982]

tryinghard:

I really appreciate the good information. If I correctly understand what you are saying, there are basically two parts to a ground system. The first part is for ground faults, which creates a ground fault path through the EGC's back to the source (transformer). This is done through bonding and through the service EGC's. The second part to a grounding system is for voltage surge protection from either lighting, line surges, etc. This surge protection is handled through the grounding electrode (ground rod etc..) and the grouding electrode conductor. When the surge voltage strikes a piece of equipment of object the grouding electrode takes this surge straight to the earth. The grounding electrode does not act as a ground fault path for ground faults.

Now that I think I have the theory correct let me explain a little more about my situation. As I mentioned I have a portable skid down in our quarry which consists of three main pieces of electrical equipemnt. A 4160V incoming section, a 4160 to 480/277 transformer (resistance grounded), 480V switchgear (2 Air breakers) and a 480V MCC. The 4160V feed is coming from a 1200A 5kV breaker up in our plant and is fed with parallel 3/c 4/0 with each parallel feed having a 4/0 EGC (No nuetral with this feed so I guess its considered a 4w feed) The 4160V switchgear on the skid then feeds the 1500kVA transformer with a 3/c 250MCM having a #4 EGC. The transformer which has the secondary nuetral resistance grounded then feeds the 480V switchgear with (6) parallel 500MCM's with EGC's. Another (6) parallel 500MCM feed with EGC's is then run from the switchgear to the 480V MCC.

Hopefully that will give you a little better idea of the system I have in question. I believe that on the 4160 side of the 1500 transformer I have all the necessary EGC's in place for handeling ground faults, and where this 4160V service comes into the skid I will drive a ground rod per 250.32. Now I am guessing that the secdondary side of the 1500kVA transformer is considered a seperately derived system. Any special considerations since the nuetral is resistance grounded? I will have the EGC's run from the secondary of the transformer making their way to the MCC as described above. Each feed coming from the MCC will then have an ECG going to its motor wicch will then be bonded to the surrounding piece of equipment (conveyors etc..)

Based off of what I described, does it sound like I covered everything? Any need for additional ground rods? Should I tie the ground rod I mentioned I was going to drive, onto the ground bus of the 4160V switchgear?

Thanks for the help.

 

[tryinghard]

You?re getting a good perspective on grounding but I recommend you involve an electrical engineer to qualify the installation.

You will need to apply Article 250 part X for ?grounding of systems and circuits of 1 KV and over (high voltage)?. Specifically 250-186 for impedance grounded neutral systems but you secondary is 480 so 250-36 applies.

It sounds like your 1500 transformer (4160/480) is the beginning of the premises wiring and therefore the secondary side of your transformer is separately derived. You may also need other electrodes depending on the installation.