Question about pole-mounted transformers

[cmoschip]

Hello all. I am not a lineman, nor do I work in the electrical generation or transmission industry. I have a quick question about best practices for the mounting of transformers on wooden poles and I need the opinion of professionals such as your selves.

At our place of business we expanded a machine shop approximately 18 months ago. An additional power entrance to the building was constructed by our sub contractor that consisted of several poles and 3 pole-mounted transformers. Being an extension of service on our property this additional entrance and its equipment is owned by us and not the electrical infrastructure provider.

Yesterday we arrived to find that we had lost 1 leg of 3 phase power over the weekend. We called the sub contractor that installed these facilities and they found the issue was that the entire assembly of 3 transformers and their mounting bracket (friction bracket) had slid down 10-14 inches and pulled one of the transformer's feeds from the primary electrode (see pictures).

My question is about best practices. Being unable to assure the exact diameter of the wooden pole, I find it odd that a lynch pin of sort was not used to assure zero movement of the transformers and their bracket. Is my installation "normal" or should there be some method to positively keep the entire assembly from movement due to gravity?

Thanks for reading - I do appreciate it.

Robert

[Lineman North Florida]

That cluster mount is what we call a banded cluster mount , we have used some of them years ago on 3 pot banks where the transformers were above 167 kva , I have never seen a properly installed cluster bracket slide down the pole , I suspect the bolts were not tightened enough which let it slide down a little , if that's what happened, the stinger coming from the cutout might not have been tight in the high side bushing of the pot and the load finally caused it to burn off, hard to say for sure just looking at pictures.

[Lineman North Florida]

After looking at the pictures again as tight as the other stinger is on the other outside pot, I do believe the whole aluminum cluster bracket holding the 3 transformers did slide some down the pole.

[cmoschip]

After looking at the pictures again as tight as the other stinger is on the other outside pot, I do believe the whole aluminum cluster bracket holding the 3 transformers did slide some down the pole.

Thank you for the response - I do appreciate it.

I googled "transformer banded cluster mount" and found lots of pictures. All appear to have smooth interior surfaces where they contact the pole (so no teeth or anything similar to get a bite into the pole). Is it accurate to say all of these relay only on the friction surface between the inside of the bracket and the pole surface for "staying put"? (and of course why having it tightened properly is so important).

Robert

[Orgnizdlbr]

I have used similar “Cluster Mount” brackets over the years. The type I’ve used had holes in the channels used to lag the bracket to the pole in conjunction with the mounting bolts to hold the bracket in place. The lags are not meant to carry the weight of the transforms, they’re just for additional stability. 43 years in the business, and I’ve never seen a bank of transformers slide down like that. Those bolts couldn’t be tightened properly.

my question, how did they rectify your issue?

[Lineman North Florida]

Thank you for the response - I do appreciate it.

I googled "transformer banded cluster mount" and found lots of pictures. All appear to have smooth interior surfaces where they contact the pole (so no teeth or anything similar to get a bite into the pole). Is it accurate to say all of these relay only on the friction surface between the inside of the bracket and the pole surface for "staying put"? (and of course why having it tightened properly is so important).

Robert

Yes that is why it is important for those bolts to be tightened properly, I am also guessing that the reason that they went with a banded cluster is the weight of the transformers, they look like 250 KVA pots, if so you probably have about 5,000+ lbs on that cluster bracket and a lot of people always said that every hole you bored in a pole in that area where the cluster is weakens the pole strength, so it always made sense to me, I'm gonna guess that they tightened the bolts and extended the stingers coming to the top of the transformers and let her stay right where she settled. Charlie.:D

[cmoschip]

Yes that is why it is important for those bolts to be tightened properly, I am also guessing that the reason that they went with a banded cluster is the weight of the transformers, they look like 250 KVA pots, if so you probably have about 5,000+ lbs on that cluster bracket and a lot of people always said that every hole you bored in a pole in that area where the cluster is weakens the pole strength, so it always made sense to me, I'm gonna guess that they tightened the bolts and extended the stingers coming to the top of the transformers and let her stay right where she settled. Charlie.:D

Exactly - tightened and added a "jumper" to the high voltage leads (sorry my syntax is likley not accurate) to extend them slightly. They originally planned to come back at a later date and replace the leads entirely but now say it "ok" like it is (with the extension). That's a different issue possibly.....

I do thank you all for giving me this feedback - it will be supportive for me when dispute the "fix" of this error - especially since it was just under 18 months old. If we had been 10 years down the road I might be persuaded to believe moisture had left the pole, etc and it *might* be slightly smaller in diamter. I think you all are correct and it was not tightened sufficiently

[rob8210]

Just looking over those pictures, myself. I have used those same cluster mount brackets many times over the years, and I have never seen, or heard of one sliding down the pole. Of course what really concerns me is buddy in his little van mounted squirt boom , up in the middle of that power bank with no rubber gloves to be seen, let alone worn!

[Lineman North Florida]

Just looking over those pictures, myself. I have used those same cluster mount brackets many times over the years, and I have never seen, or heard of one sliding down the pole. Of course what really concerns me is buddy in his little van mounted squirt boom , up in the middle of that power bank with no rubber gloves to be seen, let alone worn!

Rob, I saw that to but after looking a little closer at the pictures I believe there is an open set of group operated switches up above those cutouts , I ain't never really seen a setup quite like that and I am guessing that it could be a Delta bank and they use the group operated switches due to ferroresonance. We just hang a 4th cutout with a solid blade with the topside to the pole ground and the bottom to the floating neutral and close the solid blade temporarily grounding down the floating neutral until we can get all 3 fuses closed in and then we open the 4th cutout, but I've always heard that the same could be accomplished with a group operated switch, or three good lineman with pogo's and good timing. :D

[Orgnizdlbr]

Rob and Charlie, I saw the same, then noticed the gang switch in the second photo. I don’t know what equipment is inside the place, I still would have my gloves on. The original poster said that the contractor tightened the bolts and extended the taps, with that little squirt boom, that’s all they were gonna get. I wonder how much stress was on these bushings when the pots slid down, obviously the one tap came out. Was that tap tightened properly? Who knows, if I paid for that job, I wouldn’t be a happy camper.

[rob8210]

Hey Charlie and Labor,

I didn’t pay any attention to the second picture. Now that I do , I see that gang switch you mentioned Charlie. I also noticed that bank feeds a 4 wire service. It also looks like those cans have 2 primary and 2 secondary bushings. I don’t know the primary voltage but in my little world that bank would be feeding a 347/600v wye service. Now that would explain why the one primary bushing from each can would be tied together and should be grounded, it’s hard to tell in the pictures. All our transformers come with a single primary bushing now.

[cmoschip]

Rob, I saw that to but after looking a little closer at the pictures I believe there is an open set of group operated switches up above those cutouts , I ain't never really seen a setup quite like that and I am guessing that it could be a Delta bank and they use the group operated switches due to ferroresonance. We just hang a 4th cutout with a solid blade with the topside to the pole ground and the bottom to the floating neutral and close the solid blade temporarily grounding down the floating neutral until we can get all 3 fuses closed in and then we open the 4th cutout, but I've always heard that the same could be accomplished with a group operated switch, or three good lineman with pogo's and good timing. :D

Hi - here's my understanding of what is there - please understand I may say things that are basic or a slightly inaccurate in terminology.

High voltage feed to the top of the pole
"Air Switch" - disconnect before the feed to the transformers. our contractor used that term so I assumed it may be a proper noun/tradename.
480V output from pole transformers to our distribution inside where most of the usage is dropped down to 240v.