Crane added 4th wire

anthonysolino

Senior Member
What you described is a three phase 4 wire system. You happen to not be using the neutral for any loads but is still the grounded point of the system. All ground fault current will return to the XO terminal of the transformer regardless of what other items it finds a path through on it's way there.

All that content of 610.61 mentioned in OP is about is assuring there is a bond around the trolley in case there is too much resistance between the crane and the frame. If whatever cable method you have between crane and frame for the supply conductors contains a fourth conductor used as an EGC, that will make up the bonding jumper, they just don't want to rely on the trolley to have to provide low enough resistance to be effective ground fault path.

I am asking because I really do not know, why would one not terminate the EGC to the EGC terminal at the transformer frame along with the supply primary and secondary EGC's? I understand the concept that some of the fault current will go to the XO but would you not have loss from the xo? I was under the impression you wanted a clean path back to the source to open the breaker or fuse.?
 

kwired

Electron manager
Location
NE Nebraska
I am asking because I really do not know, why would one not terminate the EGC to the EGC terminal at the transformer frame along with the supply primary and secondary EGC's? I understand the concept that some of the fault current will go to the XO but would you not have loss from the xo? I was under the impression you wanted a clean path back to the source to open the breaker or fuse.?
Normally XO is the part of the source you would want to return to. Since OP has stated he has 240 volt service and steps up to 480/277 maybe he doesn't have an XO on the secondary, but whatever the secondary grounded conductor is connected to is where you want the fault current returning to. Most of the time on a wye secondary it will be labeled XO. If this is step up transformer or reverse fed maybe it needs to be HO that you connect to.

Whatever terminal is the "grounded conductor" it needs to have a bonding jumper installed at the source or at the first disconnecting means, from that point a connection to the grounding electrode system, and the equipment grounding conductor begins at this bonding jumper. Equipment grounding conductor may be a wire conductor it may be a metal raceway containing the ungrounded circuit conductors, it may be a little bit of both.

Add that grounded conductor terminal at the transformer is where ground fault current is trying to return to.
 

anthonysolino

Senior Member
my question is I terminate that EGC straight to that XO all the fault current is going to return back straight to the XO, you would THINK that you would go straight to the metal frame on the transformer to make the wire type EGC part of the EGC system not the neutral system any wire that leaves that XO in to a race way is infact a neutral conductor not a EGC. do you agree that third wire leaving the XO would be a neutral conductor? not an EGC?
 

GoldDigger

Moderator
Staff member
What you described is a three phase 4 wire system. You happen to not be using the neutral for any loads but is still the grounded point of the system. All ground fault current will return to the XO terminal of the transformer regardless of what other items it finds a path through on it's way there.

All that content of 610.61 mentioned in OP is about is assuring there is a bond around the trolley in case there is too much resistance between the crane and the frame. If whatever cable method you have between crane and frame for the supply conductors contains a fourth conductor used as an EGC, that will make up the bonding jumper, they just don't want to rely on the trolley to have to provide low enough resistance to be effective ground fault path.
(y)
Our terminology is on shaky ground [sic] here since the 480V is supplied by step-up transformers. If the transformers were designed for step-down use (but rated by the manufacturer for step-up use also) the H windings will be the 480V windings and the X windings will be 240V. The presence of an X0 terminal, in that case, indicates that the 240V side is a wye configuration. Most likely the 480 side is delta. That causes severe circulating current risks if the X0 is grounded or connected to the supply side neutral. And the 480V side must either be operated ungrounded or corner grounded, unless a zig-zag transformer is added to supply a balanced neutral/ground reference.
But the OP explicitly states that his transformers are 240V delta primary and 480Y/277 secondary. Which would make the X0 terminal on the secondary (but higher voltage) side, yes?
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
my question is I terminate that EGC straight to that XO all the fault current is going to return back straight to the XO, you would THINK that you would go straight to the metal frame on the transformer to make the wire type EGC part of the EGC system not the neutral system any wire that leaves that XO in to a race way is infact a neutral conductor not a EGC. do you agree that third wire leaving the XO would be a neutral conductor? not an EGC?
Let's try to explain it this way.... The Equipment Grounding Conductor is an extra neutral that only carries fault current.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Have a steel cutting plant that is having there cranes zipper rail replaced. Found several code problems that will be addressed. Crane operates on 3phase 480V The main Incoming service is 800Amp 3phase 240V. Several transformers scattered around building supply the 480V. Crane now operates on a aging 3 wire system and will be upgraded with a 4th ground wire. Art 610-61 entitled Grounding. The trolley frame shall not be considered as electrically grounded through the bridge or trolley wheels and it’s respective tracks. That’s how it is now
No problem completely understand the need for the 4th ground wire. However the last line in the article “A separate bonding conductor shall be provided” So can that be a simple jumper from the structured steel or do I need to carry the grounded XO terminal from the 480V transformer to the 4th crane ground wire. The 480 V transformer is currently bonded to the structured steel I beam. Surface lug on outside of transformer to lug on I beam. #2 jumper
If you can, I suggest putting a current clamp around that #2 bonding jumper when the plant is operating to verify that there isn't any significant current. This would be to make sure there aren't any hidden load or leakage currents through the EGC.
 

kwired

Electron manager
Location
NE Nebraska
(y)
Our terminology is on shaky ground [sic] here since the 480V is supplied by step-up transformers. If the transformers were designed for step-down use (but rated by the manufacturer for step-up use also) the H windings will be the 480V windings and the X windings will be 240V. The presence of an X0 terminal, in that case, indicates that the 240V side is a wye configuration. Most likely the 480 side is delta. That causes severe circulating current risks if the X0 is grounded or connected to the supply side neutral. And the 480V side must either be operated ungrounded or corner grounded, unless a zig-zag transformer is added to supply a balanced neutral/ground reference.
But the OP explicitly states that his transformers are 240V delta primary and 480Y/277 secondary. Which would make the X0 terminal on the secondary (but higher voltage) side, yes?
You addressed issues I had in mind when I posted there. Mostly the fact that depending on the situation XO may or may not be on the secondary side of the transformer. If it were a step down situation it is most likely to be secondary side.
 
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