![]() Where duct lines enter pull boxes, manholes, and vaults, the sections of duct. However, since control, instrumentation and communication cables run in separate steel conduit which - if it will be well bonding could create, also, a grounding effective path for power cables in the same duct bank. Construct the concrete-encased ductbank with three-inch minimum cover on all. However, in this case, grounding with individual cable ground conductors (see 5.5.3.2) should be considered to avoid installation and bonding complications." When the cable is run underground, this grounding conductor should be located in the same direct burial or duct bank run as the cable. It is important that the ground conductors be run as close as possible to the circuit conductors, since this reduces the reactance of the fault current return path. The separate grounding conductor is run in the same conduit or cable duct as the outgoing power cable and is also used to ground the conduit or cable duct. Where a single cable leaves the cable tray, a branch grounding conductor is added and connected to the common grounding conductor and to the equipment enclosure or motor terminal box. It is usually a bare stranded copper conductor connected to the main ground bus and fastened to the cable tray. A common equipment grounding conductor could be used for all cables in the same tray (it may also be used to ground the cable tray). ![]() From this main ground bus, a grounding conductor is run within the same raceway as the circuit conductors. It should be noted that in systems with impedance grounding, the grounding conductors should be connected to the neutral ground and not to the transformer neutral. If your duct bank meets these conditions and or you pour a slab over the bank then your looking at 4' cover IMO. This may be the switchgear or the motor control center ground bus to which the supply transformer neutral is connected (solidly or through impedance) and that is solidly connected to the grounding grid. Under minimum of 102-mm (4-in.) thick concrete exterior slab with no vehicular traffic and the slab extending not less than 152 mm (6 in.) beyond the underground installation. "It is recommended that a main ground bus be provided. Duct banks in loose back fill or low strength CDF/FTB may flex instead of fracturing.I am sure it could be also in other standards however this is what I found in "my collection".įrom IEEE-665 GENERATING STATION GROUNDING 5.5.3.1 Grounding with common equipment grounding conductor run to the neutral ground. The bending capacity of concrete-encased underground electrical duct banks has been the subject of considerable investigation using the load-structure. There seemed to be some concern that inadequately reinforced concrete duct banks could facture during earth movements. I have heard various discussions from civil engineers about which duct protection approach is best for handling movement from earthquakes. ![]() In some applications red dye is also added as an attempt to discourage dig-ins. Sand and pea gravel have horrible thermal properties, so these materials are only used for conduits carrying instrumentation cables or power cables with negligible thermal loading.įTB and CDF both provide some protection again accidental dig-ins, while still allowing some ability to expose conduits if needed. 8 Way (4X2) Concrete Duct Bank With 4 PVC Pipe-Communication, EA. FTB is very similar to CDF but is specifically designed to have good thermal transfer. Wire must be positioned near the bottom corner of encasement to ensure it is surrounded by a minimum of 2 inches of concrete on all sides when concrete is poured. Depending on the thermal properties of the native soil, we may require the use of Fluidized Thermal Backfill (FTB) for installations with thermal constraints. Install 50 ft of bare 250 kcmil wire in the bottom of that duct bank to form a concrete-encased electrode. For large power cables, thermal considerations are really important. One important distinction is thermal constraints. ![]()
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