Comments about bonding on the supply side of services
from
electrician2.com
I have been reviewing Section 250.92 of the 2008 NEC. This is one of my favorite sections that covers bonding on the supply side of services. Services are unique in that the supply side is supplied from a utility transformer that has no secondary protection. The only protection, in most cases, are the fuses on the pole for the primary side of the transformer and these are set substantially high at 300 percent in some cases. This means that if there is a ground fault or short circuit on the supply side of the service sufficient current must be conducted to blow the primary fuses and in some cases this never happens. Of course such faults are rare and this design is practical if not practicable. Therefore the bonding on the supply side of services is the most stringent found in the NEC and does not allow the use of double locknuts or sealing locknuts to be the sole method of bonding. Bonding locknuts are acceptable if there are no concentric or eccentric knockout rings left in place. But the best bonding technique is to use a grounding bushing equipped with set screws with a properly sized bonding jumper. The UL standard that addresses the stringent test for these types of bonding fittings is UL 467. I wrote an article that references UL 467 at http://www.electrician2.com/electa1/bond.html if anyone is interested.
When the bonding requirements are delineated in classified areas in Sections 501.30, 502.30, 503.30 and for the installation of intrinsically safe systems in Section 504.60 the requirements in Section 250.92 are referenced as meeting the requirements in these areas. Now one might ask, “ Why on earth does an intrinsically safe circuit raceway require the same bonding as the supply side of service?” Section 504.60 refers to Section 250.100 that sends us right back to Section 250.92(B)(2) through 250.92(B)(4). The reason is as I found out many years ago was that when a fault occurs on a power circuit that may be in the vicinity of the Intrinsically safe circuit the fault current takes many paths and one of those paths may be the raceways used for the intrinsically safe circuits. Additionally, the bonding insures less arcing and sparking from static electricity. It should be noted that the NEC is not written to fully address static electricity or lightning either. For static electricity UL standard NFPA 77-2007 is referenced and NFPA 780-2008, for lightning is referenced in Section 504(B) Fine Print Note 3.
The bonding in a hazardous location must be installed all the way back to the source of supply although the supply may not be in the hazardous location. This requirement is often overlooked at gasoline dispenser installations when the raceways, boxes, gutters, and panelboards that are located in nonhazardous locations between the service and the dispensers are not bonded according to section 250.92. The stringent bonding used for the supply side of services is required all the way from the dispenser to the service for most installations. Since 250.92(B)(4) does not permit the use of standard locknuts or bushings as the sole means for bonding, EMT connectors with standard locknuts do not satisfy the bonding requirements of this section.
electrician2.com
I have been reviewing Section 250.92 of the 2008 NEC. This is one of my favorite sections that covers bonding on the supply side of services. Services are unique in that the supply side is supplied from a utility transformer that has no secondary protection. The only protection, in most cases, are the fuses on the pole for the primary side of the transformer and these are set substantially high at 300 percent in some cases. This means that if there is a ground fault or short circuit on the supply side of the service sufficient current must be conducted to blow the primary fuses and in some cases this never happens. Of course such faults are rare and this design is practical if not practicable. Therefore the bonding on the supply side of services is the most stringent found in the NEC and does not allow the use of double locknuts or sealing locknuts to be the sole method of bonding. Bonding locknuts are acceptable if there are no concentric or eccentric knockout rings left in place. But the best bonding technique is to use a grounding bushing equipped with set screws with a properly sized bonding jumper. The UL standard that addresses the stringent test for these types of bonding fittings is UL 467. I wrote an article that references UL 467 at http://www.electrician2.com/electa1/bond.html if anyone is interested.
When the bonding requirements are delineated in classified areas in Sections 501.30, 502.30, 503.30 and for the installation of intrinsically safe systems in Section 504.60 the requirements in Section 250.92 are referenced as meeting the requirements in these areas. Now one might ask, “ Why on earth does an intrinsically safe circuit raceway require the same bonding as the supply side of service?” Section 504.60 refers to Section 250.100 that sends us right back to Section 250.92(B)(2) through 250.92(B)(4). The reason is as I found out many years ago was that when a fault occurs on a power circuit that may be in the vicinity of the Intrinsically safe circuit the fault current takes many paths and one of those paths may be the raceways used for the intrinsically safe circuits. Additionally, the bonding insures less arcing and sparking from static electricity. It should be noted that the NEC is not written to fully address static electricity or lightning either. For static electricity UL standard NFPA 77-2007 is referenced and NFPA 780-2008, for lightning is referenced in Section 504(B) Fine Print Note 3.
The bonding in a hazardous location must be installed all the way back to the source of supply although the supply may not be in the hazardous location. This requirement is often overlooked at gasoline dispenser installations when the raceways, boxes, gutters, and panelboards that are located in nonhazardous locations between the service and the dispensers are not bonded according to section 250.92. The stringent bonding used for the supply side of services is required all the way from the dispenser to the service for most installations. Since 250.92(B)(4) does not permit the use of standard locknuts or bushings as the sole means for bonding, EMT connectors with standard locknuts do not satisfy the bonding requirements of this section.
posted by Gerald Newton at 4:40 PM
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