Hi All,
Searched through the forums, internet, and various people - but can't find an explanation. I am aware that NEC 610.61 requires the fourth equipment grounding conductor for overhead cranes. We have a crane like this, powered by a 3phase 480V AC bus rail that is also supplied with a grounding conductor bar. The crane chassis is bonded to the ground conductor bar. The crane itself contains a series of VFDs, and a 120V AC control transformer with the X2 also bonded to the crane chassis. The 3phase power fed to the crane is supplied by a Delta-Wye, solidly grounded, transformer.
Due to the environment, the ground bar has gotten dirty (resulting in a high resistance/open connection to the system ground). When staff have touched the hook/pendant suspended from the crane, and fixed building steel (system ground) they have been shocked. It is unclear what the voltage and current of the shock is, and it is also unclear whether or not the dirty ground bar and the shock event are directly correlated. We are still in the infancy of collecting data/measurements.
However, in digging into this, I am trying to piece together and understand the possible causes of the shock hazard in lieu of a short circuit, ground fault, or objectionable current situation. The crane technician and other engineers can't seem to describe why the ground connection is necessary to prevent this particular shock, only that they state that this EGC is required per NEC610.61. This is correct, of course, but the lack of explanation of the mechanism is driving me crazy. All the various crane websites/publications simply restate NEC610.61 without any explanation as to why its important beyond the very basics.
Understanding, of course, that the bonded connections are to provide a fault path back to the source both on the 480v and 120v circuits, if the open ground bar connection is directly tied to the shock issue, why would that present a shock hazard without a second issue/phenomenon also being present? What is the circuit that is being completed by the person between the crane chassis and building system ground?
Explanations for the shock have, so far, included:
1. Some kind of leakage current/capacitive coupling from the VFD circuits that no longer can discharge back to their source through the system ground connection (as the 480V AC circuits are now totally isolated from the system ground).
2. Due to the bonding of the X2 to the chassis on the control transformer, this induces a potential difference by itself, between the now floating frame of the crane, and the structural steel of the building (system ground). This was an engineers explanation, and then started talking about how the chassis could become part of the circuit anyway, with no place to 'bleed off'? That current would just circulate through the chassis without a return path? I can't see how this makes sense, as unless there is some kind of fault/short circuit between the 120V AC and the chassis ground, there should be no current flowing between the chassis, and the neutral/X2 connection point. Due to the fact that the chassis is directly bonded to the X2 terminal, any short circuit condition between the 120VAC and chassis would then travel directly back through the X2/neutral connection and likely not through the pendant/crane hook?
2. Somewhere on the 120V neutral, someone bonded something twice that we haven't found, resulting in objectionable current flowing through the crane chassis. Note that this explanation would likely be independent from losing the crane chassis ground.
4. Static buildup on the crane chassis with no discharge point back to the earth, passes through the person once contact with the system ground is re-established resulting in a momentary shock, that cannot be replicated consistently.
You're thoughts are appreciated! Please let me know of any reference material/threads relevant to the subject matter. Always hoping to learn more. Thanks!
Searched through the forums, internet, and various people - but can't find an explanation. I am aware that NEC 610.61 requires the fourth equipment grounding conductor for overhead cranes. We have a crane like this, powered by a 3phase 480V AC bus rail that is also supplied with a grounding conductor bar. The crane chassis is bonded to the ground conductor bar. The crane itself contains a series of VFDs, and a 120V AC control transformer with the X2 also bonded to the crane chassis. The 3phase power fed to the crane is supplied by a Delta-Wye, solidly grounded, transformer.
Due to the environment, the ground bar has gotten dirty (resulting in a high resistance/open connection to the system ground). When staff have touched the hook/pendant suspended from the crane, and fixed building steel (system ground) they have been shocked. It is unclear what the voltage and current of the shock is, and it is also unclear whether or not the dirty ground bar and the shock event are directly correlated. We are still in the infancy of collecting data/measurements.
However, in digging into this, I am trying to piece together and understand the possible causes of the shock hazard in lieu of a short circuit, ground fault, or objectionable current situation. The crane technician and other engineers can't seem to describe why the ground connection is necessary to prevent this particular shock, only that they state that this EGC is required per NEC610.61. This is correct, of course, but the lack of explanation of the mechanism is driving me crazy. All the various crane websites/publications simply restate NEC610.61 without any explanation as to why its important beyond the very basics.
Understanding, of course, that the bonded connections are to provide a fault path back to the source both on the 480v and 120v circuits, if the open ground bar connection is directly tied to the shock issue, why would that present a shock hazard without a second issue/phenomenon also being present? What is the circuit that is being completed by the person between the crane chassis and building system ground?
Explanations for the shock have, so far, included:
1. Some kind of leakage current/capacitive coupling from the VFD circuits that no longer can discharge back to their source through the system ground connection (as the 480V AC circuits are now totally isolated from the system ground).
2. Due to the bonding of the X2 to the chassis on the control transformer, this induces a potential difference by itself, between the now floating frame of the crane, and the structural steel of the building (system ground). This was an engineers explanation, and then started talking about how the chassis could become part of the circuit anyway, with no place to 'bleed off'? That current would just circulate through the chassis without a return path? I can't see how this makes sense, as unless there is some kind of fault/short circuit between the 120V AC and the chassis ground, there should be no current flowing between the chassis, and the neutral/X2 connection point. Due to the fact that the chassis is directly bonded to the X2 terminal, any short circuit condition between the 120VAC and chassis would then travel directly back through the X2/neutral connection and likely not through the pendant/crane hook?
2. Somewhere on the 120V neutral, someone bonded something twice that we haven't found, resulting in objectionable current flowing through the crane chassis. Note that this explanation would likely be independent from losing the crane chassis ground.
4. Static buildup on the crane chassis with no discharge point back to the earth, passes through the person once contact with the system ground is re-established resulting in a momentary shock, that cannot be replicated consistently.
You're thoughts are appreciated! Please let me know of any reference material/threads relevant to the subject matter. Always hoping to learn more. Thanks!
