72A on bonding conductor

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Kev2020

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Location
Ontario
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Maintenance Electrician
Couldn’t figure out how to upload a picture from my phone, so hear me out. In my plant there is a feeder for a 2000A switchgear that doesn’t seem right. Here are the details.
Switchgear current load is only 480A.
Feeder is DLO installed in cable tray.
Three parallel runs of 777kcmil DLO, plus three 2/0 bare bonding conductors
Cable arrangement in tray goes:
Bond Red Black Blue Bond Red Black Blue Bond Red Black Blue
Single conductors are spaced 100% apart. The first bond is ty wrapped close to the edge of the tray, but the middle and last bonds are just ty wrapped in between the single conductor spacing between red and blue phases.
The resulting conditions result in approximately 22A on the first bond, 72A on the middle bond, and 52A on the last bond.
I cut the ty wraps on the middle bond, and pulled it out of the tray, away from the single conductors, and it dropped from 72A down to 10A.
Shouldn’t this installation have only one 250kcmil bonding conductor? This is a Canadian installation but our codes are very similar.
 
I just said bond because I didn’t want to type out bonding conductor every time. It’s fed from an impedance grounded wye system so yeah, a wye with a groundING conductor and no neutral.
 
Some more important information.
The 2000A switchgear was existing and was the plant’s main switchgear at one point. It’s a 3 phase 4 wire board. The plant expanded and now has a brand new 4000A main switchgear, 3 phase 3 wire impedance grounded, no neutral. That is what now feeds this existing 2000A switchgear. Several contractors have been inside the 2000A switchgear and apparently all neutral connections have been disconnected. There were two ground rods at the existing 2000A switchgear that have been disconnected and cut flush with the floor. There are still two 2/0 bare conductors coming from the existing 2000A switchgear and are terminated on two different building columns. The existing 2000A switchgear also has a lot of vibrations and a loud 60Hz hum. I haven’t been able to power off and look inside yet myself, but several contractors have been in there and said things look fine. It doesn’t sound fine.
 
So when the 2kA gear was switched over what was done with the existing neutrals? They cant just be disconnected. It was a grounded 4 wire system and now its being fed with an un-grounded 3 wire? What are the voltages between phases? Why did they feel the need to cut ground rods and grind them flush but left the building steel? How much current is on the 2/0 going to building steel. Need more info.
 
Existing 3 phase 4 wire loads were fitted with a delta-wye 1:1 transformer in order to establish a neutral.
I believe the ground rods were removed because that was the ground electrode at one point, but not anymore since it is now considered a sub panel. I guess the building columns were left in tact because they are still ultimately bonded to ground this way. I have not measured the current in those bonding conductors.
 
Kev2020 said:
Couldn’t figure out how to upload a picture from my phone, so hear me out. In my plant there is a feeder for a 2000A switchgear that doesn’t seem right. Here are the details.
Switchgear current load is only 480A.
Feeder is DLO installed in cable tray.
Three parallel runs of 777kcmil DLO, plus three 2/0 bare bonding conductors
Cable arrangement in tray goes:
Bond Red Black Blue Bond Red Black Blue Bond Red Black Blue
Single conductors are spaced 100% apart. The first bond is ty wrapped close to the edge of the tray, but the middle and last bonds are just ty wrapped in between the single conductor spacing between red and blue phases.
The resulting conditions result in approximately 22A on the first bond, 72A on the middle bond, and 52A on the last bond.
I cut the ty wraps on the middle bond, and pulled it out of the tray, away from the single conductors, and it dropped from 72A down to 10A.
Shouldn’t this installation have only one 250kcmil bonding conductor? This is a Canadian installation but our codes are very similar.
Click to expand...
Size and how the conductors are arranged is a secondary issue. What and how the conductors are hooked up on each end is what matters. You should not have any amps on the bonding conductors in an impedance grounded system unless there is a fault and then it should be limited to about one amp.
 
ActionDave said:
Size and how the conductors are arranged is a secondary issue. What and how the conductors are hooked up on each end is what matters. You should not have any amps on the bonding conductors in an impedance grounded system unless there is a fault and then it should be limited to about one amp.
Click to expand...
I agree.

Could even be stray current from one of the mentioned separately derived non impedance grounded systems that maybe has a bad or not properly installed neutral?
 
I suggest measuring the current on the equipment grounding conductors downstream of this feeder as they branch off to panels and loads. Then maybe you can isolate where the current is coming from and hopefully come up with a fix.
 
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