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Thread: Cable Tray Bonding

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    Cable Tray Bonding

    It is my understanding that if the cable tray is not serving as an equipment grounding conductor, it should be bonded from the ground bus bar in the gear of the cables that are routed in the cable tray to the side wall of the cable tray, and not to the grounding electrode system.

    However, the image below (from B-Line manual) is confusing me a bit. Why is the bonding jumper from the MCC to the cable tray necessary? If there was a line-ground fault anywhere in the cable tray, the fault has a path to travel back to the source on the bonding jumper in the switchgear.

    Why is there a jumper between the ground bus of the MCC and the building steel? The building steel should be bonded to grounding electrodes, the GEC, or switchgear enclosure per 250.104(C).

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    Bump.
    "Electricity is really just organized lightning." George Carlin


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    Bonding of the cable tray to the building steel or to the earth would be for noise mitigation. Not a code requirement or a fault current path that I can see. It would be an accessory or supplemental path and lower impedance by being bonded liberally.

    The geometry of the wiring method, as you create space or separation between circuit conductors, there is magnetic and electric fields between circuit conductors, which also increases impedance as there is energy storage in the fields between the circuit conductors. Capacitance and inductance goes up due to the geometry. Lowest impedance is circuit conductors enclosed in steel conduit where there is sum / net zero EM field outside of the conduit. Highest would be single conductor widely spaced in air, which can be created with cable tray. The wide spacing of circuit conductors radiates the EM field as it reaches out to its counterpart to sum to net zero.

    So there are noise mitigation techniques with cable tray, tight bundling of circuit conductors to each other, liberal bonding and grounding, following manufacturer's instruction for installation, joints, grounding, bonding ...

    The tray in the gear room will also be a path for unintended ground loop current so it's not possible to predict each path. Only that it may or will happen so supplemental or accessory bonding will lower the impedance of that path as unintended current flow, or noise, seeks to equalize itself or its source.

    The EGC from the tray to the MCC busbar is easy to see as a fault clearing path for ground faults to the tray itself even if the circuit has its own wire EGC. If it shorts to the tray that will be the fault clearing path. If it shorts elsewhere and the wire EGC is the fault clearing path, the tray is still in the path of the stray (and huge) fault current EM fields. You want that bonded to lower overall circuit impedance and for noise mitigation, reducing radiated stray EMF.

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    The jumper between the MCC and the building steel, I would see that as supplemental bonding and a fault clearing path for MCC load side circuits faulting to the building structure. It is not the GEC or bonding between earth electrodes. It would be intended for supplemental carrying of fault clearing, ground fault currents onto the structural steel.
    Last edited by __dan; 08-30-15 at 01:26 AM.

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    Quote Originally Posted by MyBeardAndMe View Post
    It is my understanding that if the cable tray is not serving as an equipment grounding conductor, it should be bonded from the ground bus bar in the gear of the cables that are routed in the cable tray to the side wall of the cable tray, and not to the grounding electrode system.

    However, the image below (from B-Line manual) is confusing me a bit. Why is the bonding jumper from the MCC to the cable tray necessary? If there was a line-ground fault anywhere in the cable tray, the fault has a path to travel back to the source on the bonding jumper in the switchgear.
    As I look carefully at the illustration that is what is being shown.

    Why is there a jumper between the ground bus of the MCC and the building steel? The building steel should be bonded to grounding electrodes, the GEC, or switchgear enclosure per 250.104(C).
    It is not showing the cable tray bonded directly to the building steel. I think the illustration just continues on and shows what a solidly grounded electrical system looks like.
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    Quote Originally Posted by MyBeardAndMe View Post
    ...Why is there a jumper between the ground bus of the MCC and the building steel? The building steel should be bonded to grounding electrodes, the GEC, or switchgear enclosure per 250.104(C).
    Quote Originally Posted by ActionDave View Post
    ...It is not showing the cable tray bonded directly to the building steel. I think the illustration just continues on and shows what a solidly grounded electrical system looks like.
    Not bonded directly to the cable tray, but I believe the OP is on the right track...

    The structural steel itself does not qualify as a grounding electrode, but the lightning protection grounding likely does. Therefore, I will cite 250.68(C) as the proper section for compliance.
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    Quote Originally Posted by Smart $ View Post
    The structural steel itself does not qualify as a grounding electrode, but the lightning protection grounding likely does. Therefore, I will cite 250.68(C) as the proper section for compliance.
    If this is the case, is the lightning protection then considered an auxiliary grounding electrode per 250.54?

    I still don't see how the structural steel connected to the MCC ground bus is compliant with either 250.104(C) or 250.104(D)(2) depending on whether it is a service entrance or SDS. It should be terminated to the switchgear ground bus.

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    Quote Originally Posted by __dan View Post
    The EGC from the tray to the MCC busbar is easy to see as a fault clearing path for ground faults to the tray itself even if the circuit has its own wire EGC. If it shorts to the tray that will be the fault clearing path. If it shorts elsewhere and the wire EGC is the fault clearing path, the tray is still in the path of the stray (and huge) fault current EM fields. You want that bonded to lower overall circuit impedance and for noise mitigation, reducing radiated stray EMF.
    But the tray is bonded at the switchgear. Why could that not serve by itself as a fault clearing path for a fault to the tray and for lowering circuit impedance, noise mitigation, etc.?

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    Quote Originally Posted by MyBeardAndMe View Post
    If this is the case, is the lightning protection then considered an auxiliary grounding electrode per 250.54?

    I still don't see how the structural steel connected to the MCC ground bus is compliant with either 250.104(C) or 250.104(D)(2) depending on whether it is a service entrance or SDS. It should be terminated to the switchgear ground bus.
    The graphic isn't explicit enough to determine with certainty... but non-electrode structural steel does not qualify as a GEC for anything but grounding electrodes connected to the GES... and an auxiliary electrode system is not part of the GES.

    Non-electrode steel is only permitted to be used as a GEC/GES extension under 250.68(C). And to be compliant with 250.68(C), the GEC would have to be connected to the switchgear (or anywhere a GEC is permitted to be connected... which is not to an EGC).
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    So I just got a project where I am routing a bunch of 3/C 500 kcmil cables in three 36" cable trays for around 400 feet. The cable tray is routed from a couple of outdoor switchboards along the outside of existing buildings. Cables are going from the switchboards to feed existing MCCs. I was going to call out for equipment bonding jumpers from each switchboard ground bus to the respective cable tray sidewall.

    In the past, I have seen every other cable tray support column grounded to the grounding electrode system (ground grid), but after reviewing Section 250 I do not see where that would be necessary. Am I missing some NEC requirement or "good practice" rule? I would think if it were good practice to bond the support columns, the bond should be from the cable tray to the column, not from a ground grid to the column. Then an effective path would go from the column to the cable tray and back to the switchboard.
    Last edited by MyBeardAndMe; 09-02-15 at 06:23 PM.

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