Standing On My Soapbox!
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I agree with Bryan (BPHgravity).
If there are Circuits from separate Systems, not bonding all the EGCs (Equipment Grounding Conductors) together + to a Metallic Enclosure / Box would result in an odd Ground Fault path, as described below:
One (1) 480Y/277V 4 Wire Multiwire Circuit (20 Amp) + One (1) 208Y/120V 4 Wire Multiwire Circuit (20 Amp) in same Box.
Branch Circuits run underground via PVC to stubs at finish slab level, EMT terminates to the PVC between the slab and Panelboard or Box, each raceway includes One (1) EGC, Outlet Box is 5s (Metallic).
Ground Fault path for an L-G 277V fault with only the EGC from the 208Y/120V "side" would have to flow back to the SDS Transformer, through the case to the Primary Circuit's EGC (bonded to the case), then back to "Some" Panelboard on the 480Y/277V System side.
Not necessarily the best fault path, and may have issues if the Transformer's Primary Circuit EGC is inadequately bonded, or not bonded at all.
For an opposite Ground fault (L-G 120V Fault, only bond is the EGC for the 480Y/277V "side"), the Fault Current would have to flow back to the 480Y/277 Panelboard, "find" the EGC which is included with the Transformer's Primary Feeder Circuit, which:
* Might be fed from another Panelboard,
* Might be fed from the Service Equipment,
* Might be fed from a Distribution Section ("Switchgear"), that is different than the one feeding the Panelboard, where the Transformer is derived from.
Then, flow through that EGC to the Transformer, through the Transformer case, and to the System Bonding Jumper between the case and "X0" Terminal.
Again, this fault path relies on solid and correct terminations for a low Impedance Circuit.
Being that the total loop length may be very long, there might be enough loop Impedance to keep the fault level down, thereby not tripping the OCPD on the Faulted Circuit.
If this occurs, the Metallic Equipment / Enclosures connected to the fault Path will have > 40 Volts (something between 1 Volt and 120 Volts) Potential to Ground (Earth), and a significant level of Current behind it!
With all the EGCs bonded together, the hazards from the above scenarios are greatly reduced (never say "completely removed"! - Poo-Poo does occur!!!).
Besides, why would anyone NOT want to bond all the EGCs together?
Is there some reason why separate terminations would be preferred (normal conditions only)?
Is there a benefit to separate bonding, or reduction in a Circuit's merit if all EGCs are bonded together (they eventually are bonded at the Transformer)?
Would someone prefer to identify ALL EGCs between separate systems, and apply additional labor for separate makeup?
(Project Manager's worst-case scenario: counter-productive labor!).
As mentioned by another Member, IG Conductors is an exception to this view, as these "Dedicated" EGCs are intended to be "Isolated" from raceways, boxes and enclosures, between the Outlet and the Panelboard of origin.
In this case, there is a merit for the circuit to separate the bonding conductors.
Lastly, Bryan's final statement is something that should be noted.
All GROUNDED CONDUCTORS (AKA "Neutrals") between two different systems (or even 2 different Panelboards of the same system) SHOULD NOT BE TERMINATED TOGETHER!!!
Also, none of them should be bonded to an outlet box, or other enclosure - unless:
* Bonded at the same point where the System is bonded to the Grounding Electrode System + Metallic Enclosures (Service or SDS Transformer / 1st Panelboard if bonded there),
* "Older" Multiwire Circuit, which does not include an EGC, instead the Grounded Conductor is bonded to the load equipment's Metallic Enclosures.
Bonding the Grounded Conductor to Metallic Equipment / Raceways / Outlet Boxes at multiple points results in "Unbalanced Current" flowing on both the Grounded Conductor(s) and the EGC / Metallic items.
This not only creates an L-G shock hazard, but also a Fire Hazard when loose or inadequate Conduit Terminations are encountered.
Leaving the Soapbox!!!
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Scott35
