Utility transformer

[The electron man]

Is the utility transformer fused ?

Like what would happen if you have a phase to phase short before the main ocpd would it just blow up the transformer?

 

[Elect117]

It depends and sometimes.

 

[MD Automation]

It's very common to have POCO transformers protected by "expulsion" fuses.

They can be replaced and reset from the ground using a "hot stick".

Here is the one for my house, showing what happens when an internal fault in the transformers trips that fuse...

 

[busman]

It's very common to have POCO transformers protected by "expulsion" fuses.

They can be replaced and reset from the ground using a "hot stick".

Here is the one for my house, showing what happens when an internal fault in the transformers trips that fuse...

View attachment 2573754

Correct. The "door" of the cutout can be lowered with a hot-stick and re-fused. The "horns" on the top of the cutout are for a "load buster" device to extinguish an arc from opening the door under load. The cutout should be mounted "angled down" so the door wants to fall open with the expulsion mechnism.

Mark

 

[don_resqcapt19]

At a fire scene, I watched as about 25' of 4" RMC with four 500 kcmil copper conductors pretty much vaporized. It was fed by a 750 kva transformer in a vault. When the utility guy got on scene, he opened the vault but the secondary conductors were smoking a lot. He went to the substation, dumped the 12.8 kV feeder circuit, came back and cut the faulted secondary conductors loose and went to the substation and re-energized the circuit to supply power to the customers served by that transformer.

 

[hillbilly1]

Pad mount’s usually have fuses.

 

[Elect117]

Pad mount’s usually have fuses.

Not as common as you might think.

1000kVA usually doesn't. Depending on the primary voltage, 750kVAs might not either. It depends on the primary FLA.

Some utilities fuse the primary at the riser instead.

Transformers may or may not be protected by them. They could be to protect the line from tripping. on a GF at the customer. They usually are not going to provide overload protection. They are also typically higher on the trip curve than the conductor. Depends on the transformer size and if it feeds multiple people.

 

[jim dungar]

Transformers may or may not be protected by them. They could be to protect the line from tripping.

Yep.
Removing a faulted device from service is usually a higher priority than preventing the fault in the first place.
But, this is even true for most non-utility branch circuits also, it is unlikely that a 20A 120V breaker protects my kitchen blender with its #18 cord.

 

[Hv&Lv]

Is the utility transformer fused ?

Like what would happen if you have a phase to phase short before the main ocpd would it just blow up the transformer?

Transformers have some OCPD.
Some like a CSP transformer have a secondary breaker that trips just like the one in your house.
Look below. The switch with the hole for a long stick is connected to the secondary breaker.


Others like the picture MD Automation posted have fuses and are conventional transformers with two bushings on the primary.

Here is a chart that substantiates what Elect117 posted.

 

[hillbilly1]

Not as common as you might think.

1000kVA usually doesn't. Depending on the primary voltage, 750kVAs might not either. It depends on the primary FLA.

Some utilities fuse the primary at the riser instead.

Transformers may or may not be protected by them. They could be to protect the line from tripping. on a GF at the customer. They usually are not going to provide overload protection. They are also typically higher on the trip curve than the conductor. Depends on the transformer size and if it feeds multiple people.

May be a regional thing, in the south, most I’ve ran across on commercial, are either fused at the rear of the padMount, or fused at the pole, yes, they are all overfused, but fused all the same. Had one in South Georgia that was a 3000 amp service, lighting hit the transformer, poco threw some more fuses in the pole, 15 minutes later, transformer went up in flames!

 

[Elect117]

May be a regional thing, in the south, most I’ve ran across on commercial, are either fused at the rear of the padMount, or fused at the pole, yes, they are all overfused, but fused all the same. Had one in South Georgia that was a 3000 amp service, lighting hit the transformer, poco threw some more fuses in the pole, 15 minutes later, transformer went up in flames!

I meant that they may or may not be fused at the transformer. The context was lost my reply lol.

Don't forget the occasional PME/PMH as well. Fuses at the riser and branch line fusing on a primary line through a industrial complex that might have more than one customer on it.

 

[garbo]

Years ago I asked a PECO lineman what size fuse they use to feed the single 7200 volt lead feeding area 75 KVA residential transformers. It was at least 5 times higher what an electrician following the NEC transformer over current rules. Told me they rather have your house catch on fire then have to spend a few hundred dollars to have a person come out to replace transformer fuse..One time while doing a service upgrade had my college big younger brother working with me a couple of summers. Forgot to pack my temporary power cord that had two large alligator clips wired thru a 30 amp safety switch feeding two GFCI receptacles. Told my brother to get a roll of 12/2 NM cable & wire in a duplex receptacle so that I would power for my hammer drills & drop light. He misunderstood what I told him and connected the white wire to the black wire. When I climbed up on my fiberglass ladder I connected them to Ultility Power and got a nice flash but never cleared the mega fuse on transformer primary. Beat you would be able to weld off secondary of Ultility transformers. ( just using the 240 volts ).

 

[Elect117]

Years ago I asked a PECO lineman what size fuse they use to feed the single 7200 volt lead feeding area 75 KVA residential transformers. It was at least 5 times higher what an electrician following the NEC transformer over current rules. Told me they rather have your house catch on fire then have to spend a few hundred dollars to have a person come out to replace transformer fuse..

The lineman's statement is a little misleading. The transformer is fused to protect the primary circuit from tripping and might have multiple customers on it. It isn't normally meant to protect a single customer. The NEC does that job. The NEC cares to protect each individual service.

There are instances where they will, like large customers that could cause issues on a primary feeder.

 

[LarryFine]

And you wouldn't want other careless customers to be able to easily kill your power either.

 

[Frank DuVal]

conventional transformers with two bushings on the primary.

Around here conventional transformers (single phase primary to 120/240 secondary) are 19.9 kV and a single bushing. Neutrals are connected together inside. Maybe I better go look again... All the new subdivisions are pad mounts.

 

[Hv&Lv]

Around here conventional transformers (single phase primary to 120/240 secondary) are 19.9 kV and a single bushing. Neutrals are connected together inside. Maybe I better go look again... All the new subdivisions are pad mounts.

Terminology.

When utility people say conventional transformers they are generally talking about two bushing transformers that require a fuse above for primary protection. We won’t bank CSP transformers and are switching to all conventional transformers. It may take us a few years to dwindle down our supply of CSPs.

Duke also is in the process of switching over and not buying CSP transformers

Some utilities put a fuse above CSP transformers, (those with one bushing where the N is connected together internally) but it's generally not needed. Those transformers have ann internal fuse link and a breaker inside the casing. Look for the ring I was talking about in my earlier post.
There are some manufacturers that make a single bushing conventional transformer but you can't really bank them for every bank we build.

 

[electrofelon]

It may take us a few years to dwindle down our supply of CSPs.

Duke also is in the process of switching over and not buying CSP transformers
.

Here there are quite a few CSP's, but I don't think the Poco is using them anymore because I haven't seen a new one go in in a long time. So why is that? It saves the fused cutouts and that extra installation, seems like it would be a good thing no?

 

[Hv&Lv]

Here there are quite a few CSP's, but I don't think the Poco is using them anymore because I haven't seen a new one go in in a long time. So why is that? It saves the fused cutouts and that extra installation, seems like it would be a good thing no?

I can't speak for all utilities on this.
Around here though the additional fuse above is aggravating to install, but during a storm they are extremely beneficial.
A CSP has a weak link primary fuse that can't be replaced in the field.- this means during a lightning storm (most are during bad lightning) the internal fuse may blow and it requires a changeout.
Sometimes the internal secondary breaker goes out, again, requiring a changeout. Linemen break the breakers slamming the handle up and down. They think the harder you slam it the better chance it will stay in..
With a conventional transformer it's lighter, all the protection is outside the tank, and easier to replace a blown fuse.
With that being said, we have had a tap with 7 transformers on it and the mainline fuse was blown, along with 4 other fuses above conventional transformers. A man in a pickup can replace the fuses and get everyone back on. No need for a bucket truck that's busy changing a CSP transformer.
It's a total ownership cost issue, and maintenance has to be taken into account.

I don’t climb or get in the bucket anymore, but in the 90s early 2000s I've changed many CSP damaged by lightning. Never changed a single conventional transformer from lightning.

One issue here though..
Say you have a small mill that's about 45kW load.
So we build a bank and fuse the bank for (generally) twice the bank output.

We have had instances where a motor got locked up and blew fuses in the mills. Electrician or maintenance man comes to fix it and ups the fuse, or worse yet sticks a piece of copper pipe in there. The motor locks up, and blows out primary fuse.
Some of the linemen that won’t call me have been known to put a bigger fuse in to stop it from blowing.
Fuse too high on the primary and the transformers gonna blow before long.

Here is a link to look at the CSP breakers

CSP Circuit Breaker - Single Phase - The Ardry Group

www.ardry.com

Oil Immersed Circuit Breaker

www.transguard.in

 

[electrofelon]

I can't speak for all utilities on this.
Around here though the additional fuse above is aggravating to install, but during a storm they are extremely beneficial.
A CSP has a weak link primary fuse that can't be replaced in the field.- this means during a lightning storm (most are during bad lightning) the internal fuse may blow and it requires a changeout.
Sometimes the internal secondary breaker goes out, again, requiring a changeout. Linemen break the breakers slamming the handle up and down. They think the harder you slam it the better chance it will stay in..
With a conventional transformer it's lighter, all the protection is outside the tank, and easier to replace a blown fuse.
With that being said, we have had a tap with 7 transformers on it and the mainline fuse was blown, along with 4 other fuses above conventional transformers. A man in a pickup can replace the fuses and get everyone back on. No need for a bucket truck that's busy changing a CSP transformer.
It's a total ownership cost issue, and maintenance has to be taken into account.

I don’t climb or get in the bucket anymore, but in the 90s early 2000s I've changed many CSP damaged by lightning. Never changed a single conventional transformer from lightning.

One issue here though..
Say you have a small mill that's about 45kW load.
So we build a bank and fuse the bank for (generally) twice the bank output.

We have had instances where a motor got locked up and blew fuses in the mills. Electrician or maintenance man comes to fix it and ups the fuse, or worse yet sticks a piece of copper pipe in there. The motor locks up, and blows out primary fuse.
Some of the linemen that won’t call me have been known to put a bigger fuse in to stop it from blowing.
Fuse too high on the primary and the transformers gonna blow before long.

Here is a link to look at the CSP breakers

CSP Circuit Breaker - Single Phase - The Ardry Group

www.ardry.com

Oil Immersed Circuit Breaker

www.transguard.in

Thanks, good info.

 

[garbo]

The lineman's statement is a little misleading. The transformer is fused to protect the primary circuit from tripping and might have multiple customers on it. It isn't normally meant to protect a single customer. The NEC does that job. The NEC cares to protect each individual service.

There are instances where they will, like large customers that could cause issues on a primary feeder.

Saw an old u tube video where the old service cable wires shorted out before the service head and caught the side of the house on fire with several sets of shorts producing a large amount of sparks. Have to wonder if the utility company had a smaller primary ampere fuse or better yet overload protection on the secondary side would the house have not caught on fire.