MV Cable on the ground 4

[gokulkrish2]

Hello All,

I have a question regarding the regulations over using a Medium Voltage cable (4160V or 7200V) installed by just running above ground(Not direct buried or concrete)

For our project of dewatering the pit(Yes we are a mining company) we are considering options to supply a barge which will be situated at the bottom of the pit. Currently we are using generator and we want to go for permanent power. Overhead lines are next to impossible as we have slope issues and other mining factors. We want to just run cables parallel to the pipes carrying water from down in the pit to our reservoir.

What does NEC/MSHA/OSHA say about having cable just layed on the ground. This will be a permanent installation. But we will use Double Insulated cable.

Thanks in advance for your help.

Regards,
gokul

 

[7anoter4]

First, if your cable is "on ground" it means also "underground with zero cover" then you will come against NEC Table 300.50 Minimum Cover Requirements.
If we will take it as aboveground installation then NEC 300.37 Aboveground wiring method states:
"In locations accessible to qualified persons only, exposed runs of Type MV cables, bare conductors, and bare busbars shall also be permitted."
That means the cable will be accessible only for "qualified person only" [well enclosed on its entire length].
I think the only method permitted will be a cable tray -or rigid steel conduit-at minimum 8 ft. above the ground- but more above the roads, highway, railway and so on[See NESC Rule 231B]

 

[ScottyUK]

Hi 7anoter4,

HV trailing umbilical cables are common in opencast mining applications for powering draglines and similar mobile plant. These cables are massively armoured and very heavy in relation to their current-carrying capability. With the right cable choice I would expect that this could be an acceptable installation. I'm not sure what US legislation covers mines and quarries, and whether this over-rules the NEC. There is a mining forum on Eng-Tips somewhere.

 

[7anoter4]

Thank you, Scotty. My last experience is mostly in power station. However, I think I saw a Stacker-Reclaimer medium voltage cable – armored rubber sheathed-but I think it run in a kind of open trench. The supply end termination was plugged in an outlet. But, I am afraid ,the cable was often damaged.

 

[stevenal]

Mines are specifically excluded in the scope of both NEC and NESC.

 

[podobing]

Gokul,
Mining cables (Coal mining - USA) are loosely classified in two broad categories - trailing cables and power cables. Trailing cables are cables that supply power to portable or mobile machinery. They can lay on the ground and "follow" the machinery. It does not sound like the equipment you are discussing is either portable or mobile. Power cables are supported from insulators and are not allowed to contact combustible material (typically coal in my case). A typical installation for a power cable that goes any distance is to hang a steel messenger wire to support the weight of the cable. Different companies (CAB comes to mind) make insulated hooks to hang the power cable from the messenger wire. We have hung miles of 500 kcmil, 15 kV Mine Power Feeder (MPF) or Shielded (SHD) cables with no problems. Make certain to check regulations regarding resistance grounding and/or ground wire monitoring requirements for your particular industry. The US Mining industry (MSHA) requires both on this type of installation.
Dave

 

[Mbrooke]

I do not have much experience with mining, however in the utility industries it is the norm that when MV cable be used underground or exposed it must have a concentric grounding shield. A concentric shield will take the phase to ground should the insulation be damaged, a must if the cable is at any risk of person contact.

 

[gokulkrish2]

Thanks all for your valuable feedback.

Mbrooke,

Does the utility just lay power cable on the ground if the cable has concentric ground and proper ground monitoring protective device. If so then i can implement that in my application. We typically use STARTCO relays for Ground monitoring.

Podobing,

You are right. I do not think barge pump can be classified as portable or mobile machinery and hence my confusion on whether or not we can use cables just laying on the ground to supply power. When you discuss power cables supported from insulators to prevent contact from combustible materials, is that a requirement for combustible materials alone or every power cable must be hung on insulators? Mine is a copper mine and no combustible materials. So am i good to just lay them on the floor and call in good? Remember, i have double insulation and i am going to check if it has concentric ground. It is impossible to sector off the whole cable routing area to be made accessible only by electricians/qualified personnel.

Regards,
Gokul

 

[345kv]

Gokul
There are a few types of cable and methods suited for your application; type MPF, SHD and jacketed armor interlock or CLX.
MPF is mine power feeder cable, it is a 3/C construction using copper tape shielding on each individual conductor with a smaller ground wire laid in the intercies of the three phase conductors with one ground wire being insulated. There is an overall jacket of PVC, CSPE or other suitable materiel. This cable uses class B stranding and is most similar to standard MVP 105 cable. It is used primarily as a primary voltage feeder cable in mines, pits and tunnels either plug connected or hard wired and is laid directly on the ground or elevated on supports.
SHD or trailing cable is similar to MPF in both construction and application. SHD cable uses a much finer conductor stranding, the shielding is a woven tinned copper mesh and the jacket is a rough service Hypalon, CSPE, neoprene or similar. It's application is where flexing and frequent movement is required such as at a mine shovel or TBM, it is used in conjunction with the MPF cable. The MPF cable brings power close to the shovel where there is no movement of the feeder cable, it is then connected to a section of SHD cable which is connected to the shovel, as the shovel moves so does the SHD cable. Sections of MPF cable are added as the shovel moves further from the power source.
Armor interlock or CLX cables are also 3/C MV 105 cables with the same ground wire construction, steel or aluminum armor and a PVC jacket overall. This is not a portable cable.
Which ever cable type you use you will need to have the feeder protected by ground fault relaying and a ground continuity monitor circuit aka ground check circuit. For the ground fault to be effective the system source must be a wye either solid or impedance grounded, if it is a delta than a derived neutral must be made using a zig zag transformer or similar method.
The ground check circuit monitors the ground wire continuity from the source to the load by a low voltage signal on the ground wire to the load and returns on the insulated ground check wire, an open in either ground/check wire or short between them will cause the feeder breaker to trip.

I hope this helps you out somewhat.
345KV

 

[podobing]

Gokul,
I have no experience in any mines other than coal mines. I am not sure of the local geology, but when I say that we hang cables, we drill a hole into the roof (shale or sandstone)of the mine entries (our seam is typically 6- 8 feet in height). We insert a bolt with an expansion shell to secure the bolt to the roof. The lower end of the bolt is threaded, and we install a clip through which we install a steel messenger wire. Then we install insulated hangers (every 6 - 10 feet)on the messenger wire and install the MV cable in those hangers.

I am not sure of the requirements in your situation, but it will get the cable off of the floor, eliminate a tripping hazard and better protect the cable if it is hung. All of this hardware is available through a company like Fairmont Supply or other companies that supply (coal) mining equipment. I will try to find some pictures of what I trying to describe.

Hope this helps,
Dave