FIRE! What was supposed to protect me from this?

[wyliesdiesels]

A fused cutout is not really fused according to the load. It is but its not.
The fuse rating on a cutout feeding a transformer is based on the amount of "fault" current a particular device can generate.

I have never in 40 years seen a overloaded transformer clear a fuse.
A burned up transformer will clear a fuse. Burned up (bad windings etc).
Load? No. At least from what I've experienced.

weird

ive seen cutouts blown for transformers. PoCo comes out, does some testing, finds no issues, re-energizes without issue. only probable issue was overload seeing as how they changed nothing other than the fuses.

also seen that happen on the 3 primary lines feeding my sub (old 50s neighborhood). 2 cutouts blew. troubleshooter drove around entire neighborhood (not large maybe 9-10 streets couple hundred older homes) with spotlight checking all lines. 30mins later finds nothing, hangs new fuses, and closes the doors. lights back on.... this was over a year ago. no outages since then. so obviously it wasnt a short or line to ground fault. only other option was an overload....

The smallest fuse we ever used was a 3T . 3 amp fuse, 7200 primary.
That was for a 25kva and lower on 12470/7200

A 25kva is good for ( 125 amps per line) 100 amps, nominal per leg, was 100% but overloaded is 125%.
A 50 kva is good for 200 amps per leg continuous. So overload would be 250 amps.
The primary fuse is rated much higher.

except the primary fuse in your example is actually rated lower than that, at 3a. what is the HV equivalent of 125a or 250a @ 240v?

EDIT: see below

 

[wyliesdiesels]

I could probably do the math. But its Monday, and I've been drinking . (Because im retired)

HA! no need. found a calculator for that

Amps at High Voltage Calculator | Power Network Design

Convert low voltage kVA and amps to the high voltage side of a transformer.

powernetworkdesign.com

looks like your example of a 3a fuse for 25kVa is about right. calc says 2.4 and i doubt you had that size....

so working backwards, a 3a HV fuse is good for about 156a on the secondary. i can see a transformer feeding 5-10 houses pulling that much current....

 

[Codyboy]

weird

ive seen cutouts blown for transformers. PoCo comes out, does some testing, finds no issues, re-energizes without issue. only probable issue was overload seeing as how they changed nothing other than the fuses.

also seen that happen on the 3 primary lines feeding my sub (old 50s neighborhood). 2 cutouts blew. troubleshooter drove around entire neighborhood (not large maybe 9-10 streets couple hundred older homes) with spotlight checking all lines. 30mins later finds nothing, hangs new fuses, and closes the doors. lights back on.... this was over a year ago. no outages since then. so obviously it wasnt a short or line to ground fault. only other option was an overload....



except the primary fuse in your example is actually rated lower than that, at 3a. what is the HV equivalent of 125a or 250a @ 240v?

EDIT: see below

I'll digress , on the blowing a fuse due to load. However the fuse doesn't "blow" violently like when there is a fault, but rather it melts out.

As to the OH primary lines and no cause...
The cause may not be found , unless it is very obvious.
We had a line fuse that would go out quite often and could never find a cause. No trees or limbs close enough etc.
Until one day we finally found the issue. There was a stand of bamboo that was very tall , maybe 35 to 40 ft high but it was a good 20ft away from the line.
While walking out the line in the easement the wind picked up a little bit and we saw the tops of the bamboo would just lean over enough to contact the line. Arrrgghh. Months of this off and on BS.

Looking at that calculation that sounds about right.
I had started to type out my theory on that but deleted it as I started drawing a blank.

I didn't know there was a calculator for that process.

A 120/240 transformer on 12470/7200Y has a turns ratio of 60:1 .
7200/120=60.
60x 3 amp fuse =180 amps possible on the 120v leg.

Then you have the type of fuse also T, E, K, H , all have a different time over current rating.
I just let the engineers figure all that out. lol

 

[BreeStephany]

It’s been a long day.

I arrived at my beach cottage in NJ about 11pm Thursday night, basically just made the bed, turned on the baseboard heaters, and went to bed. This morning (Friday), I woke up about 7:15am, made a mug of tea (electric stove), and was contemplating life when about 7:45am there was a loud Bang! Bang! Bang Bang Bang Bang Bang! from outside the house. Sounded like somebody trying to start an unmuffled badly tuned lawnmower.

Except there are no lawns here. It’s all rocks and sand.

Opening the door and looking out to see what was going on, I found my meter pan actively throwing flames and sparks, and smoke pouring out of the mast. Sorry, no pictures or video, it was a bit hectic. Neighbor called 911 while I grabbed a set of pliers to remove the do-not-disturb tag, opened the meter pan, and hit it with fire extinguisher #1.

Emptying #1 in to the box mostly stopped the fire there. It did not stop the burning wire. We watched as it burned from mast to over head wires, and dropped the burning wire on the roof. Fire extinguisher #2 saved the roof. We then watched as the wire burned all the way to the pole. At that point, the transformer down the street finally blew its “fuses”, and shut down the block.

Heres the meter pan, before the power company showed up and pulled the meter.



Old asbestos & concrete siding for the win. It has some scorch / smoke damage, but it didn’t burn.

Inside what’s left of the meter pan.



Inside the breaker panel.



Inside, there is melted Romex from the heat of the ground wire.



Electricians will be fixing this. Today, I had police, fire department, power company, electrical inspector, fire inspector, building inspector all here. The meter is gone, and the service is cut off at the pole. After electricians, and inspection, power company will reconnect. We need at least a new meter pan, mast, some wiring, and whatever else, plus new service lines and a meter.

Here is what didn’t happen. Two blocks over, a couple weeks ago, basically the same thing happened. But they didn’t catch it fast enough and the house is a total loss.



So, extinguishers work. Have several on hand, as one may not be enough. I just bought two new ones.

The initial opinion from the fire department is that salt air (beach cottage) and time caused corrosion in the wires at the meter panel, eventually leading to failure. One of the hots met the neutral, and (literally) Bang! The failure seems to be on the power company side, so not caused by anything in the house.

The electric was inspected post Sandy (2012) and the meter is new. They were all replaced after Sandy blew through. I don’t know how old the service wire is, probably is newer than 1990. A neighbor claims to be the first (1990) house to have to add a mast, previously the service wire ran to a fascia mounted bracket, and often was just laying on the roof.

From what I can see, there is a ground rod ~5’ from the panel, though I don’t know how deep it goes. It is tied to the neutral at the panel. Then there is a neutral / ground that runs across the house and is tied to the (copper) plumbing, going in to the ground. The in-house ground wire heated up to the point of melting its own sheath and the nearby Romex.

If the failure is, in fact, on the power company side of the meter, was something supposed to stop the in-house side of this from melting down? It seems like this shouldn’t have been this bad, with up to code wiring, grounding, and a modern breaker panel.

It seems that I am incredibly lucky. If this had cooked off 12 hours earlier, I would likely be filing an insurance claim for “destroyed house”. An hour later, I would have been food shopping, and it would have gone up. It picked exactly the time when I was there, quiet, and heard it go. The extinguishers worked, though they taste horrible.

Before the main breaker, there is little to stop a fault that is significant enough to start a fire but not significant enough to blow the fuse on the primary side of the utility transformer. As an electrician, I have done my fair share of panel upgrades, rewires and remodels and almost every one where we utilized the existing meter, I would thermal image the jaws of the meter, which are a common failure point, do a visual inspection of the meter base and the feeder conductors and would megger the conductors from the meter to the first point of disconnect, and based off of those observations give my recommendations and observations to both my employer and to the property owner.

Judging from the pictures, the L2 jaw likely either failed at it's insulating post and made contact with the meter pan, which is bonded to the center point of the utility transformer and began arcing, the initial arc flash or bang you heard likely disintegrated the jaw entirely and the following arc flashes were as the L2 line began arcing within the mast.

Once an arc flash begins on the unfused utility conductors, there is little that you can do beyond hope that it gets a good enough bond to ground to draw enough current to exceed the rating of the transformer and blow the fuse on the primary side. The bad thing is that more often than not, utility transformers are oversized as they often provide power to multiple homes, either currently, or in the foreseen future, so your available power at the transformer far exceeds the current required for the conductors before your main disconnect to turn to dust.

Glad you had the correct fire extinguishers on hand and were able to contain the fire without getting hurt. Arc flashes are scary as ****. As you may be aware, they burn at between 25,000~35,000 F, do not require oxygen to be sustained and are more or less a plasma ball of angry pixies going whereever the hell they want with a LOT of force. I have worked in the industrial and heavy commercial fields as an electrician for nearly 2 decades and hope to never directly witness one.

 

[Skooterj]

line to neutral short



nope. there is no protection on the line side of main breaker in the panel, that would stop that.

Shouldn't the fuse on the transformer blow? Never mind. I read all the posts explaining how these fuses work. My experience is that birds and squirrels pop them all the time. I assumed a downstream short would pop them too. Seems that is not the case.