Grounding a transformer

[guinne55pub]

I work for a small contractor and we did a building shutdown this past weekend for maintenance. We grounded a transformer with "spiders". Why are you supposed to ground it? Is it a NETA or OSHA requirment?

My journeyman is kind of a jerk so I didn't ask him why.

Thanks



Moderator edit, please keep it clean.

 

[zbang]

I work for a small contractor and we did a building shutdown this past weekend for maintenance. We grounded a transformer with "spiders". Why are you supposed to ground it? Is it a NETA or OSHA requirment?

"spider"?? is that a temp. power distribution box (AKA "spider box") or something else? Anyway, generally you ground a transformer case because it's a metal box around live wires (art. 250.4) and usually bond ("ground") one terminal of the secondary winding because it forms a Separately Derived System (art. 250.30).

 

[augie47]

Was it a high voltage transformer (>1000v) ?
The use pf grounding "spiders" is standard practice in this area for high voltage work.
"If its not grounded, it's not dead !"

 

[Smart $]

OSHA doesn't specifically require grounding the transformer, but in a lockout/tag out scenario, the blocking or removal of stored and residual energy is required. Grounding of all transformer terminals is an effective means of compliance.

 

[jeremysterling]

I've used them from time to time as the conditions dictate. Usually the condition is a boilerplate MOP (Method of Procedure) that is just plain overkill. I'm good with a tag and lock or de-terminating in almost all situations. I suppose a bus could run into the pole, flip the cut-out shut, and the spider would cause the fuse-link to blow.

The downside of the spider is forgetting to take it off before you re-energize.

 

[big john]

Are you describing a protective-ground cluster?

I'm not aware of any standard that requires grounds to be applied for low-voltage equipment (<1000V).

For medium voltage equipment at the very least it's part of establishing electrically-safe work conditions per NFPA 70E 120.1(6).

The stored static charge in long medium voltage conductors can be hazardous, and in conditions where your deenergized circuit is run near energized conductors you can have voltage induced into the dead line. The higher the voltagr, the more serious these risks become.

Grounding helps ensure that everything is at zero potential prior to work.

Further, if there is even a remote possibility that a line might be accidentally reenergized (like through unintended contact during overhead work) then the grounding can reduce the voltage the workers will experience, and quickly clear the fault.

The only time we ground in 480 situations is if the system is very complicated, increasing the risk of an unknown backfeed.