Recommended air clearance between a donut type CT and 11 KV bare busbar 1

On anything above 1kV system voltage, you would only ever use a ring CT such as that over an insulated bushing, and then, more often than not, also with an earthed metallic (non-ferrous) sleeve between the bushing and the CT, such that there is almost no chance of a flash-over to the CT.

7-8" of clearance is about the minimum clearance to ground at 11kV. The window would have to be somewhere around 15" plus the wide bus dimension for me to even think about doing it.

Treat the 600V CT like it is at ground potential, so whatever the ground clearance you need from 11kV would be the clearance needed to the CT...this is since the bus bar has no insulation.

The CT you have shown has a 1 inch diameter window, so hopefully that is not the CT you intend to use

Dear Mr. edison123
Q1. " What is the recommended air clearance between a donut type CT (rated for 690 V like shown below) and a 11 KV bare busbar? ..."
A1. It is NOT sensible/practical; ( to use ... a 690V CT ... on 11 kV bare busbar).
A2. It would need a very large [air clearance distance] i.e. needs a very "impractically" large window.
A3. Possible solutions would be:
a) having insulation plus earthed screen between the bare busbar and the CT,
b) Insulate the bare busbar.
Che Kuan Yau (Singapore)

Thanks guys.

Below are the existing donut CT's on a 11 KV, 1000 KVA alternator terminal board. The cooper bus is 30 mm x 4 mm. The CT bore is 80 mm, which means a 25 mm minimum air clearance from 11 KV bus. It has been working fine for years but I am not comfortable with a 25 mm air clearance.

I have seen in 11 KV motors, the air clearance between phase to phase at terminal bushings is around 125 mm and phase to terminal box side clearances is even less at 75 mm. Hence, my question about any standards that specify these minimum air clearances.








What if I insulate the cooper bus to 11 KV up to and just short of the connection point? Will that help in reducing the air clearances?

Thanks again for your inputs.

Muthu

NESC/2007 Table 124-1—Clearances from live parts
PART A—Low, medium, and high voltages (based on BIL factors) 95 kV BIL
15 kV maxim. Vertical clearance of unguarded parts=2.69 m
Horizontal clearance of unguarded parts=1.07m

Dear Mr. 7anoter4
7anoter44. " NESC/2007 Table 124-1—Clearances from live parts
PART A—Low, medium, and high voltages (based on BIL factors) 95 kV BIL
15 kV maxim. Vertical clearance of unguarded parts=2.69 m. Horizontal clearance of unguarded parts=1.07m "
Che. Wrong frequency?! The data have nothing to do/is irrelevant to the question asked.

Dear Mr. Edison 123
Q1. "...What if I insulate the cooper bus to 11 KV up to and just short of the connection point? Will that help in reducing the air clearances? ..."
A1. No. From the images it appear that there is a (very serious violation) in terms of [creepage and clearance] distances .
Q2. " ...It has been working fine for years but I am not comfortable...."
A2. Strongly suggest an [immediate action] to be taken to rectify the violation. A potential time-bomb!
Che Kuan Yau (Singapore)

UL 347 5.14.201 has clearances. It's for switchgear but a connection box is a similar type of gear. 4" clearance and 7" creepage except to enclosure walls which is 6" clearance and 8" creepage.

Insulation to the connection point would help, but I'm not sure that is enough. We have insulators that look like 2-piece thimbles where the CT goes onto it and the bus goes through.

Why not just buy 11kV/15kV class window-type CTS, or block type CTs, and be done with it?

Dear Mr. edison123

Mr. scottf suggests
Q1. "...Why not just buy 11kV/15kV class window-type CTS, or block type CTs ..."
A1. Agreed in full, as there seems to have room for it. The only consideration is the "cost" which may be 1000% times. It is NOT "costly" consider that the present setup is a big time-bomb!.
A2. An alternative is to remove all the LV CTs from the fiber board, sleeve them over the b]insulated[/b] cables, [observing the creepage and clearance distances].
Caution: See following learned advice:
a) Mr. ppedUK " ..On anything above 1kV system voltage, you would only ever use a ring CT such as that over an insulated bushing, and then, more often than not, also with an earthed metallic (non-ferrous) sleeve between the bushing and the CT, such that there is almost no chance of a flash-over to the CT...."
b) LionelHutz " ...UL 347 5.14.201 has clearances. It's for switchgear but a connection box is a similar type of gear. 4" clearance and 7" creepage except to enclosure walls which is 6" clearance and 8" creepage..."
Che Kuan Yau (Singapore)

Thanks Lionel for that standard.

scottf - I will look into the window type CT's, thanks.

Muthu

che12345-

The cost of a block-type 11kV indoor CT should be in the range of $300-$400/unit. That would not be "that" much more than 600V window-type CTs of that diameter.