Someone help me get something thru my thick head. I inspected a 1200 amp 480/277 service today. The main has GFCI protection built-in and a C.T. on the neutral buss. The bond strap is factory installed from neutral to can/equipment grounding bar. I questioned the connection point of the grounding electrode conductors. My thinking was that they should be ahead of the CT so any currrent flow would not show as an inbalance. The factory advises the gr. electrode are always connectd AFTER the gf CT. I can't get thru my head why that is.
ground-fault service/gr. electrode
- Thread starter augie47
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brian john
Senior Member
- Location
- Kilmarnock, Va
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- Retired after 52 years in the trade.
The Main has GFP (Ground Fault Protection) Not GFCI (Ground fault Circuit Interrupter)
I assume this is a residual GFP system? CT's are built in the CB for the GFP with a 4th CT for the neutral?
In this case the GFP CT is to be placed on the downstream side of the neutral to ground bond.
If this is a ground return system (Single CT no CT's in the CB) the CT is placed on the neutral to ground strap.
It is not uncommon for manufactures to occasionally mis-locate the CT's (well I have run across it several times in 37 + years)
The picture below is a GFP with Zero Sequence (one large window CT in lieu of 4- seperate CT's), but the idea is basically the same with regards to the neutral to ground bond as the residual GFP.
Not sure how or why I came up with th current reading listed in particular the neutral and ground current readings, but I guess it had something to do with a GFP relay set at 1200 amps and why it tripped.
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electrodes
electrodes
Thanks Brian...I knew GFP..sorry just get used to typing GFCI so much..
I understand your diagram and appreciate it. That is the way I thought it should be connected. This particular switchboard has a neutral buss...with lugs labeled "Line" and the bond at that point as you have shown...then thru the CT to the neutral assembly. The factory is asvising to connect the gr electrode conductors to the neutral bar..downstream of the CTs...unlike where you show the connection....
electrodes
Thanks Brian...I knew GFP..sorry just get used to typing GFCI so much..
I understand your diagram and appreciate it. That is the way I thought it should be connected. This particular switchboard has a neutral buss...with lugs labeled "Line" and the bond at that point as you have shown...then thru the CT to the neutral assembly. The factory is asvising to connect the gr electrode conductors to the neutral bar..downstream of the CTs...unlike where you show the connection....
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
augie47 said:
A ground-neutral connection on the load side of the CT would mean that any current flowing on the normal ground path would also flow through the CT and the GFP would not notice a "fault". The only fault the CT would see would be current flowing through an unintended ground path (i.e. dirt).
A neutral to ground connection on both sides of the CT will result in the GFP nuisance tripping as well as not providing the intended fault protection.
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