SC protection of Transformer secondary

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gmtt

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I just visited a site, where a 50 KVA transformer was on the ground (POCO supplied). From there, by underground cable, it went to a fenced area (locked) where I could see a meter followed by a disconnect (don't know if it fused or non-fused). My question is, if there is a short circuit between the transformer secondary point to the disconnect, how it will be protected? Shouldn't a fused disconnect was warranted within close proximity to the transformer secondary? What does the code say?. The distance from the transformer to the meter/disconnect is not much, but they are in a locked area.
 

augie47

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State Electrical Inspector (Retired) DINOSAUR
Possibly under POCO jurisdiction, but even if not, look at 240.21(C)(4). As long as the conductors are outside, Code folks don't seem to worry much about them.
 

iwire

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Location
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I just visited a site, where a 50 KVA transformer was on the ground (POCO supplied). From there, by underground cable, it went to a fenced area (locked) where I could see a meter followed by a disconnect (don't know if it fused or non-fused).


How is this different from the service supplying you're home?:)
 

gmtt

Member
How is this different from the service supplying you're home?:)

Good question. Unless POCO puts out a cutout fuse in their secondary side of the transformer, my original question stands. The question again is, if there is a short to ground in the secondary side of the cabling, unless there is any fuse disconnect means, or there is any OC protection, why the transformer secondary will not fry? Anybody knows the technical answer to this?
 

Cold Fusion

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Location
way north
...The question again is, if there is a short to ground in the secondary side of the cabling, unless there is any fuse disconnect means, or there is any OC protection, why the transformer secondary will not fry? Anybody knows the technical answer to this?

I'm going to guess the transformer primary protection opens.

cf
 

iwire

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Massachusetts
In almost all cases power company secondary conductors are not protected at all.

If they short sparks and smoke are sure to follow.
 

jim dungar

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Retired Electrical Engineer - Power Systems
Good question. Unless POCO puts out a cutout fuse in their secondary side of the transformer, my original question stands. The question again is, if there is a short to ground in the secondary side of the cabling, unless there is any fuse disconnect means, or there is any OC protection, why the transformer secondary will not fry? Anybody knows the technical answer to this?

Yes, the secondary will "fry".
Primarily, because it belongs to the POCO, but also because it is 'outside' the building it is not a concern according to the NEC.
 

iwire

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Massachusetts
I'm going to guess the transformer primary protection opens.

I would not make that guess, I have learned that can depend on the power company but many power companies size the primary protection above what would protect the secondary conductors.

Charlie E has pointed out that the POCO he works for sizes the primary protection to carry 150% of the transformers rated current indefinitely
 

Cold Fusion

Senior Member
Location
way north
edit: I didn't see iwires post 10 before I posted this one. But this answer still fits.

gmtt said:
...My question is, if there is a short circuit between the transformer secondary point to the disconnect, how it will be protected? Shouldn't a fused disconnect was warranted within close proximity to the transformer secondary?
A lot of the stuff I work with has the xfm secondary protection away from the xfm - maybe even 500 (+) feet.

Regularly someone will question what protects the secondry conductors between the xfm and the secondary OCP.

The Answerer: "Protect from what?"

Questioner: Well, overload.

A: The secondary OCP. What ever current is going in to the xfm end on the secondary conductors is coming out the OCP end.

Q: What protects the conductors from a heavy current draw between the xfm and the secondary OCP? (Yep - someone actually asked that)

A; Ahhh - like a backhoe attack? Well, the secondary conductors are mostly protected by concrete and steel. After the backhoe attack there aren't any conductors to protect - they are toast. The issue now is to put out the fire with minimal damage. So the next protective device in line is the xfm primary protection. That should open to prevent the xfm from turning into a fireball.

Q: But the utility can set the primary way high.

A: Yes they sure can. And if the primary OCP doesn't open on a xfm secondary conductor fault, it generally will open when the xfm internally faults. And if the primary OCP is still set too high, then eventually the distribution line will fault and go down.

cf
 
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iwire

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Location
Massachusetts
Here is what can happen when service condutors fault in RMC.


NB2.jpg


NB3.jpg
 

Cold Fusion

Senior Member
Location
way north
I would not make that guess, I have learned that can depend on the power company but many power companies size the primary protection above what would protect the secondary conductors.

Charlie E has pointed out that the POCO he works for sizes the primary protection to carry 150% of the transformers rated current indefinitely
Yes - or much worse. 600% sticks in my mind as an acceptable NESC number for the primary protection.

Some years back I was involved in a repair to a 200hp firepump feeder. Service was a 750kva, 34kvY/480 grounded Y, POCO owned xfm, in our yard. Except for the xfm primary bayonet fuses there was no over current protection on the firepump feeder. This was as per design and as directed by the state fire marshal.

Unknown to us, the feeder had faulted underground and taken out two of the the transformer primary bayonet fuse.

The POCO changed the fuses and energized. Seems as though they did not have the correct fuses, so they used ones that were, "only a bit bigger"

Several seconds later, the upstream protection eventually cleared, and the smoke drifted away. We had burned up switchgear, conduits, and cable. Amazingly the POCO transformer was okay and the UG feeders from the xfm to the switchgear were okay.

POCO lineman said they would be back with the correct fuses - just give them a call when we were ready.:confused::confused::mad::mad:

cf
 

Cold Fusion

Senior Member
Location
way north
Bob -
That's real similar to what I dug up on the firepump feeder. I knew right where to dig. There were three parallel runs to the firepump. One had melted locknuts and bushings. We pulled on one end with a boom truck until it broke off and came out. Pulled on the other end untill it broke off. Laid the broken off pieces on the ground over the path. Dug up the 20 foot section that was missing. yuk

cf
 

iwire

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Location
Massachusetts
Actually we where never sure what caused it. It could have been a bullet, a door and some RTUs had bullet hole damage. Or it could be because the conductors had been installed on a very cold day.
 

mattsilkwood

Senior Member
Location
missouri
Actually we where never sure what caused it. It could have been a bullet, a door and some RTUs had bullet hole damage. Or it could be because the conductors had been installed on a very cold day.
I would vote cracked insulation due to the cold installation. It would take a high powered rifle to pierce a piece of rigid that size imo.
 
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