This article was posted 04/21/2006 and is most likely outdated.

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Grounding vs Bonding: 250.96 Bonding Other Enclosures

 April 21, 2006  

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PART V. BONDING   Hi Res - Cable/DSL [2486Kb]
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250.96 Bonding Other Enclosures.

(A) Maintaining Effective Ground-Fault Current Path. All metal parts intended to serve as the effective ground-fault current path, such as raceways, cables, equipment, and enclosures must be bonded together to ensure they have the capacity to conduct safely any fault current likely to be imposed on them [110.10, 250.4(A)(5), and Note to Table 250.122].

Author’s Comment: Bonding jumpers are required around reducing washers (donuts) because they do not provide an effective ground-fault current path [250.4(A)(5)]. Figure 250–136
Nonconductive coatings such as paint, lacquer, and enamel on equipment must be removed to ensure an effective ground-fault current path, or the termination fittings must be designed so as to make such removal unnecessary [250.12].

Author’s Comment: The practice of driving a locknut tight with a screwdriver and pliers is considered sufficient in removing paint and other nonconductive finishes to ensure an effective ground-fault current path.

(B) Isolated Ground Circuit. A metal raceway containing circuit conductors for sensitive electronic equipment can be electrically isolated from the sensitive equipment it supplies by a nonmetallic raceway fitting located at the equipment. However, the metal raceway must contain an insulated equipment grounding (bonding) conductor to provide the effective ground-fault current path to the power source in accordance with 250.145(D). Figure 250–137

DANGER: Some digital equipment manufacturers insist that their equipment be electrically isolated from the building or structure’s grounding (earthing) system. This is a dangerous practice, and it violates 250.4(A)(5), which prohibits the earth from being used as an effective ground-fault current path. If the metal enclosures of sensitive electronic equipment are isolated or floated as required by some sensitive equipment manufacturers, dangerous voltage on metal parts can remain from a ground fault. Figure 250–138

Author’s Comment: For more information on how to properly ground sensitive electronic equipment, visit www.MikeHolt.com, go to “Technical,” then to the “Grounding” link.

 

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Figure 250–136
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Figure 250–137
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Figure 250–138
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Copyright © 2006 Mike Holt Enterprises,Inc.
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Comments

  • Do you still have to use a grounding bushing if you pull a ground wire and bond it to the box

    frank lukacs
    Reply to this comment

    • Reply from: Ryan Jackson   
    Generally speaking, no. If you don't have an effective ground fault current path, yes you would. For example, if you have concentric or eccentric knockouts that are damaged, yes, you would need to bond, with or without an equipment grounding (bonding) conductor.

    Ryan Jackson Electrical Code Consultant Mike Holt Enterprises
    Reply to Ryan Jackson

  • British wiring rules require that metal conduits in ALL occupancies have a redundant equipment grounding wire. I have opened up a wall and reconnected a conduit enough times ( once ) to know why they require it.

    Yes, you do have to bond the metal conduit to the box if there is a redundant grounding wire inside. Theoretically, the large cross sectional area of any steel conduit is a better path than a ground wire but a ground wire does protect you from broken conduit joints.

    A so called isolated ground should be renamed a SHIELDED ground because the idea is to avoid picking up radio stations and the noise currents that electric discharge bulbs inject into the equipment grounding grid. I one time missed a date on the matter of removing WQMX-FM from a public address system.

    Mike Cole mc5w at earthlink dot net

    Mike Cole
    Reply to this comment

  • Hi Mike, I thought we discussed the matter of reducing washers as an approved means of bonding/grounding in one of your classes last year. I thought it had been determined that a pair of reducing washers was UL approved for bonding/grounding in circuits of less than 250 volts to ground. By the way, how is the family? How is the sailor doing? Ed

    Ed
    Reply to this comment

    • Reply from: jim bosshart   
    The answer for reduceing washers begs another statememt and question.

    Statement; If the enclosure manufacturer states that the concentric rings left after inserting a lesser than maximum size conduit are NOT approved for any fault current ratings, an installer should knock out any remaining rings and use a reducing washer if on the LOAD size of the Main Disconnect. Question; If on the Supply side of the Main Disconnect, an installer wants to use a Grounding LOCKNUT with reducing washers, is that combination approved via UL and with the Grounding Locknut manufacturer?
    Reply to jim bosshart

  • I have a 3000Amp 480V3Ph service. Under grounding am I required to size this in accordance to 12.5% "bonding jumper" or is 3/0 cu the largest size required?

    Jason
    Reply to this comment

    • Reply from: Ryan Jackson   
    If you are (earth) grounding you would use 3/0 maximum. If you are sizing a main bonding jumper, you would use 12.5%

    Ryan Jackson Electrical Code Consultant Mike Holt Enterprises
    Reply to Ryan Jackson

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