Power Pole Fuses

[jw]

The answer to this will likely need to come from someone who has worked for an electric utility company.

The fuses on power poles before a transformer are there to protect the transformer as well as a means to disconnect that transformer from the high voltage line.

The question is this: If the transformer is rated at 500amps Max load, (for example), is the fuse rated at 500A, or is it a little lower to avoid overload, or possibly a little higher for surges? Does anyone know?

I know that house fuses/breakers are normally sized below the maximum load a specific gauge of wire can handle, yet they do allow for surges. But the power companies have different standards, and are dealing with much higher voltages on the primary side of the transformers.

[Pete C.]

Those fuses are a lot smaller, like 40-60A since they are on the high voltage side. I'm pretty sure that the fuses are sized for the transformers peak rating, not it's continuous rating. Utility grade transformers are rated to handle a 100% overload for 24 hours without damage.

[Evan]

Which rating are you asking about ?

If you are talking about the maximum output rating of the low voltage side of the transformer then you would be able to determine the size of the fuse required for the primary side if you knew the distribution voltage and the secondary voltage...

A 500 amp transformer sounds like it is inside of a building transforming

277/480v down to 120/240v... Or you are discussing a larger pad mounted transformer stepping down grid distribution voltage to sub-grid voltage for a given area... Utility mounted transformers are typically rated in KVA...

Given that there are a variety of utility company distribution voltages, there is no one magic answer to this without more information...

Common medium voltages are: 2,400v 4,160v 7,200v 13,800v

So without knowing more information besides some rating of the transformer is "500 amp" there can be no answer...

~~ Evan

[krw]

Yeah, they'll handle so much power they'll start glowing red.

[Pete C.]

Nope. 100% overload for 24 hours without damage is the rating. Over 100% or more than 24 hours and they will likely be damaged.

[krw]

WWwwwooooooosssssshhhhhhh!!!!!!......

[Evan]

:

ROFL...

A power transformer glowing red ?

Not unless it had overheated to such a point where the oil bath ignited and you were mistaking the fire/flames as a "glow"...

~~ Evan

[krw]

Go back to sleep.

[Evan]

ote:

**** you...

In my area of the country the power transformers don't have the little red pussy lights on them because everyone and their uncle would clog the power companies call centers if one lit up...

It is really easy to see which pole fuse is the one that blew, they generally literally pop in the center and there will be a visible gap in the fuse holder/gate...

~~ Evan

[RCinFLA]

------------------------------------- Primary side of pole transformer is high voltgae, typical 12.5kV or 25kV.

Secondary side to your house is 240vac, center tapped.

The pole fuse is on the high voltage line. There is a hollow fiberglass bar that is clipped in a mount with connectors between top HV line and the pole transformer primary input. Inside the fiberglass bar is a fuse wire of approximately #18 gauge braided wire. The fuse wire is stretched between connector on the ends of the fiberglass bar. When the fuse wire blows the fiberglass bar releases a latch on its mount and hinges down from the upper HV line connection.

The fuse is only 30 to 60 amps because it is at the high voltage. For example, if your transformer can support four 200 amp house feeds then it would be about 800A X 240 vac or 192 kVA transformer. The primary side current would be about 192 kVA / 25kV = 7.7 amps at full load.

[Jim Yanik]

RCinFLA wrote in news:3dc9e$4e3c71a6$45499b77$ [e-mail]:

My apartment doesn't run off a pole transformer,it runs off a bigger transformer on the ground,right outside my dining room window. when it blew a few years ago,it was quite audible.

I asked the lineman,when he was connecting the new XFMR,and he said it was fed by a 7200 volt line,the topmost wires on the power poles. I don't think they use higher voltages on the common power poles.

[Evan]

6$45499b77$ [e-mail]:

@Jim Yanik:

Not in your area, but you can get a general impression of the voltages involved by the configuration of the wires, 13,800v lines are most often found spread widely apart on the cross members with the big insulators holding them off the cross member... 7,200v often appears as a more tightly packed configuration which appears as a diamond with spacers in place on the line which "hang" it from a non electrified strand which is under tension to support the cabling...

2,400v is often a single uninsulated conductor which supplies power transformers on residential streets at the very top of the pole...

~~ Evan

[Twayne]

Or someone who goes out and looks at xfmrs with a problem and compares those to some good ones. It's not rocked science by any means. In addition, several grid companies explain it right on their own pages.

80%

Each line to the business/home is 120V ac 180 degrees out of phase. Center tap is neutral, each HOT is 120 Vac of opposing polarity (NOT 2 phase; there is a difference).

Thus, if power is not at high point, the xfmr can't work? And it's also not very often 25 kV at the sfmr input; you're off by one step-down. The analysis has gotten messed up.

lol, they often are, esp if it's a lightning hit that takes them out. But unless you get a racoon or something in there, you aren't likely to see much of a melt-down; just a sort of phzzzzzt and it's over with. It would take a dead short from a metallic ogject to make much of a noticeable noise. First lefel protection at the sfmr outputs is 600 V pk.

YUK/ think I'll stop bothering to read your posts - again, it's mixed up and a short search online would provide the correct spin on everything. Look it up; you'll get better info and more accurate geo infor also.