grounding and bonding of conveyors.

[cmelguet]

Hi,

I have a grounding question again. Please refer to the attached drawing.

At the project we have an overland conveyor of 1km long. At the start of the conveyor we have a 33/4.16 kV 5 MVA transformer substation with its grounding grid designed according to IEEE 80. At the end of the conveyor we have another substation of 33/4.16 kV 5 MVA, again with its grounding grid designed according to IEEE 80. Between the two grounding grids there are 2 naked copper conductors of 4/0 AWG, buried at 600 mm depth, to connect both grounding grids and to earth the conveyor structure (equipotencial bonding). Since we have a lot of rock where the conveyor is located, the construction company is not able to buried the 2 copper conductors.

The conveyor also have a cable ladder installed with a 2/0 AWG naked copper conductor, used as a grounding conductor for the cable ladder. This conductor is only connected to the grounding grid at the start and end points of the conveyor. (red mark on the drawing).

My suggestion is to eliminate the 2 buried copper conductors and use the cable ladder copper conductor to bond the conveyor structure. Is this acceptable by the NEC? From my understanding the 2 buried conductors are used only for equipotencial bonding and not for step and touch voltage control.

Please your comments.

Regards.
Cmelguet

 

[cmelguet]

I attahced the file again. Its seems did not work the first time. Regards.

 

[waross]

Many installations connect the tray cable to the ground grid at regular intervals. Consider connecting the buried cables to the conveyor structure and to the tray bonding cable every 50 feet.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cmelguet]

Hi Waross,

Thanks for your answer, i think i was not understand correctly. What I wanted to do is to eliminate the buried conductor and only leave the tray bonding cable. The idea is to use the tray bonding cable to bond the structure.

Regards.

 

[waross]

If the ladder tray is aluminum you may want to reconsider the bare copper. Rain water on copper often generates copper salts that are corrosive to aluminum. Standard practice in this area is to use insulated copper cable and to connect it to the tray every 50 feet. The connection is painted with glyptol (About a 4" square around the connector). Also every 50 feet there is a drop to the grid. This drop cable is C-tapped to the cable running the length of the tray. The connection is painted with glyptol and then tapped including the end of the drop cable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I agree with waross, of course.
One of the grounding purpose is to reduce the voltage between the close ground and a non-live part of a electric device[which may be energized in an insulation fail case]. This is the job of the bonding to a grounding electrode. The other role it is to assure an effective path to return the fault current to the source in order to activate the protection to clear the fault in time[equipment grounding conductor].
A conveyor assembly contains the conveyor frame, low voltage power cables[ for welding machine, for instance] , outlets connected with ,control cables [for rope switches, for instance],cable tray for motor power cable[medium voltage],telecommunication and instrument cables.
If the conveyor legs are electrical connected with the reinforcing steel bar of their foundation then the conveyor frame is grounded and could be employed as grounding electrode.
An aluminum cable tray may use as equipment grounding conductor [see NEC Art. 392.6 and 250.96] if the Circuit Breaker Protective Relay Ampere Trip Setting for Ground-Fault Protection of Any Cable Circuit in the Cable Tray System will be less then permitted in Table 392.60(A).If it is not, then a grounding cable has to be run in the cable tray in contact with the cable tray metal. The cable has to withstand the maximum possible short-circuit current.
.

 

[cmelguet]

Hi Zanoter4,

Thanks for your answer. First i want to clarify some points:

- The cable ladder is galvanize steel cable ladder.
- We are running a naked copper 2/0 AWG conductor along the cable ladder to use as a grounding conductor.
- We are not running any buried cable along the conveyor. The only point where we have buried conductors as a ground grid is at the start and at the end of the conveyor. Attached is a Picture with the problem. I want to know if i am breaking any NEC rule by doing this. Thanks again, Regards.

 

[7anoter4]

In my opinion, the conveyor foundation is not a suitable grounding electrode. See NEC Art.250.52.
The equipment grounding conductor-in cable tray- has to be according Table 250.122[at least].

 

[cmelguet]

Hi Zanoter,

Thanks for your answer. The conveyor structure is grounded using a proper grounding electrode, but only at the start and at the end of the conveyor. See drawing attached. Isnt this good enought?

Regards,
cmelguet

 

[7anoter4]

NEC does not refer how to bond a long cable tray to grounding. NEMA VE 2-2000 does:
"4.8 BONDING TO BUILDING STEEL AND EARTH:
Metallic cable trays shall be bonded to building steel and earth as supplemental grounding for ground fault protection and signal grounding (“noise” prevention). The tray shall be bonded to building steel and earth, at least every 60 ft. This is only required when the cable tray system is not inherently bonded (connected) to building steel and earth through metallic support systems."
NEC art.250.54 Auxiliary Grounding Electrode permits bonding auxiliary grounding electrodes –not entirely as per 250.52 but steel bonded to earth-to the equipment grounding conductor run in the cable tray if this electrode will not used as an effective ground-fault current path.
Briefly, if all 60 ft. you may connect the conveyor leg to the foundation rebar of this leg by welding or by steel tie wires and the cable tray will be welded in the same place to conveyor frame you will conform with both-NEC and NEMA VE-2.

 

[1capybara]

ok u explainned that you want to eliminate the 2 underground copper conductors that connect the ground grids. Why? what problem is this a solution to?

 

[waross]

Since we have a lot of rock where the conveyor is located, the construction company is not able to buried the 2 copper conductors.

I don't think that the "Tomb Stone" supports will provide adequate grounding of the conveyor sections. I would consider connecting your grounding conductor to each individual conveyor section.

Bill
--------------------
"Why not the best?"
Jimmy Carter