PoCo Primary Fuses - Rating and Pupose?

[kenjsil]

I have never known much about power transmission - what happens on the primary side of the PoCo transformer has always been a mystery. So, I was just wondering:

Consider a residential 240/120 3 wire service supplied by a 25 kVA Tx with %Z 1.58. I don't know what the primary voltage is, but I'll guess 4,800V (is that reasonable?) The secondary FLA is 25000/240 = 104.17 Amps, and the primary (assuming 100% efficiency) should pull 5.21 Amps.
As I understand it, if there's a bolted L-L short in the service drop, the SCA at the secondary terminals would be (104.17*100)/1.58 = 6,592.83 Amps, and so the current at the primary terminals would be 329.64 Amps.
If this is all approximately correct, then I wonder what the rating of the primary fuses is. It's been drummed into me over the years that the primary OCPD won't open under these conditions and the service drop fault will have to burn itself clear - but I'm curious about their rating and purpose. I assume it's to clear a short on the primary side.

Are there any PoCo engineers out there who can enlighten a humble residential electricain?

[ August 19, 2004, 02:33 PM: Message edited by: kenjsil ]

 

[charlie]

Re: PoCo Primary Fuses - Rating and Pupose?

The purpose of our primary fuses are to protect the line and keep our customers in service. Your calculations are OK as far as they go; however, the primary voltage is usually in the 15 kV class (13.8 kV, 13.2 KV, 12.47 kV, and 12 kV). Our primary distribution voltage is 13.2 kV and the fuse we use for a 25 kVA transformer is 15T. A T rated fuse will hold 150% forever so it will take something more than 514 kVA to blow the fuse. Keep in mind that there is normally dynamic impedance introduced into the fault circuit since you hardly ever have a bolted fault. Your calculations are also assuming an infinite bus. Normally, there is a significant amount of voltage sag during a fault.

The bottom line is that the primary fuse is not there to protect the transformer but to protect the line that feeds it. Do not ever think the service entrance conductors are protected by the upstream primary fuse.

The above concept changes gradually as the transformer sizes increase.

 

[ron]

Re: PoCo Primary Fuses - Rating and Pupose?

Charlie,
I follow and agree with most of your statement. Although I thought that the transformer would be protected by a Class E fuse in that voltage range, or is that only on larger xfmrs?

kenjsil
Either way, if you look at characteristics for the fuse, you will see that it takes quite a bit of overload current for a long time to melt one of the medium voltage fuses. Most medium voltage protection in this type of application is not for instantaneous protection (short circuit), but large overload protection.
E rated fuses (actually most medium voltage fuses), do not reach their trip value for a very long time. See the web link http://www.bussmann.com/library/bifs/6701.pdf
you will see for example that the total clearing for a 10A fuse is not affected by a 30A overload for at least 100 seconds. It probably will not provide protection for a 15A overload for 2 hours or so (although we can't verify this because the published curves do not show more than 100 seconds).

[ August 19, 2004, 10:10 AM: Message edited by: ron ]

 

[charlie]

Re: PoCo Primary Fuses - Rating and Pupose?

Ron, my answer reflects the way we do it for pole mounted transformers. We use T rated fuses for all of our pole mounted transformers and most of our taps and risers. E rated fuses are used in our switchgear in underground distribution loops.

 

[kenjsil]

Re: PoCo Primary Fuses - Rating and Pupose?

The bottom line is that the primary fuse is not there to protect the transformer but to protect the line that feeds it. Do not ever think the service entrance conductors are protected by the upstream primary fuse.

Charlie,
Thank you. Your response is very clear. I looked up the Minimum Melting and Total Clearing times for 15T fuses. It appears that the curves "flatten-out" at around 1000 seconds, where the charts end. The minimum melting time at about 28 Amps appears to be infinite. With a 13.2 kV to 240 V step-down, that corresponds to about 1540 Amps on the secondary.

Here in LA, the usual residential service is supplied by a pole-mounted 25 kVA Tx which feeds secondaries tapped for 3 or 4 houses with 100 Amp 240/120 services. There is usually (but not always) a set of primary fuses just before the Tx.

I'm completely convinced that an arcing fault in a service pipe will burn clear well before the fuse. So, what sort of "event" would produce sufficient current for sufficient time to melt the fuse? The only things I can imagine would be a short in the primaries between the fuses and Tx, or within the Tx itself, or perhaps a lightning strike.

 

[hurk27]

Re: PoCo Primary Fuses - Rating and Pupose?

kenjsil

I'm completely convinced that an arcing fault in a service pipe will burn clear well before the fuse

This is one reason I can't understand why LA wants the service entrance conductors inside the outside wall?

As some have posted here from LA and even showed a diagram they are to follow when installing a service.

 

[shocker3218]

Re: PoCo Primary Fuses - Rating and Pupose?

A side note on this topic.

I was trenching in a new service the other day and caught the secondaries of a utility transformer that fed a cell tower. 200 amp 1 phase sevice. (locators "forgot" to mark this line)

I heard two "pops" (not very loud) Long story made short, it blew the fuses in the utility transformer (primary side) and shut down a auto repair shop fed from the same transformer. Based on what you guys are posting here, it sounds as if this would be the exception rather than the rule.

 

[kenjsil]

Re: PoCo Primary Fuses - Rating and Pupose?

This is one reason I can't understand why LA wants the service entrance conductors inside the outside wall?

As some have posted here from LA and even showed a diagram they are to follow when installing a service.

hurk27

The PoCo (LA Dept of Water and Power) has a 254 page manual on electric service requirements. They allow risers for overhead services to be in the wall or exposed. As someone observed in a five page forum debate on this topic last year: The flush mount service boxes and in the wall risers tend to be found in "good" neighborhoods (I guess they have a more finished appearance).

Most modest residences here have exterior risers and surface mount boxes. For myself, I would prefer to have a rigid or IMC riser supported on unistrut 1" outside the 1" of stucco on the building exterior. We have earthquakes here, as I'm sure you've heard and they can put a severe stress on the mechanics of any installation.

 

[hurk27]

Re: PoCo Primary Fuses - Rating and Pupose?

Shocker Yes it would be and might have been because of the load the shop was pulling at the time.

Ken this is why I made that statment as if these wire short out in the panel or in the riser and the riser is inside the wall next to wood and they burn through the conduit wall, :roll:

 

[iwire]

Re: PoCo Primary Fuses - Rating and Pupose?

As foreign as that method sounds to me it must have a good record of safety or I would think they would stop doing it.

Many people are horrified that services in my area are done with SE cable, again it has a good record of safety.

If it ain't broke why try to fix it?

 

[caosesvida]

Re: PoCo Primary Fuses - Rating and Pupose?

I have seen a number of pole fuses blow, I don't know what caused it exactly. Sometimes there is a short in the service, but that might have been coincidence. I was always told the fuse would never clear before the house burned down. It always amazed me that the fire department used cable cutters on the service entrance to cut the line. With the transformer fuse not failing, you now have a live service hit (overhead) floating around a bunch of guys spraying water. It seemed to me that (electricians usually were part of the force) they would just have a pole on the truck to pull the fuse on the transformer.

 

[charlie]

Re: PoCo Primary Fuses - Rating and Pupose?

It depends on the size of the transformer (larger transformers are fused closer). the electric utility (we don't all do it the same way), and the type of fault (bolted or arcing).

If the cut is on URD service cable, the conductors will be larger and the fault will be nearly a bolted one. Under those conditions, the fault current will be significant and the primary fuse may let go. If it is an arcing fault, the dynamic impedance of the fault will let it burn open and keep the other customers in service.

 

[hurk27]

Re: PoCo Primary Fuses - Rating and Pupose?

Hey Charlie I have always thought that it was this way everywhere. But I was on a downed pole mounted service that had two 100 amp service on it and we decided to run both meter feeds from the drop in the same riser pipe to allow the POCO to tie on to them. well some how two of them got crossed marked and we had one hot and one neutral that was crossed, this caused a dead bolted fault at the transformer secondary on the #4 ran for each service. when the service man went to slap the fuse back in it blew imeditly. I was set back as I asked him what are they fused at. and his reply was that a 10 kva transformer would have a 2 amp fuse that act's quickly. he also said that they prefer to protect the transformer and the secondarys as it cost to much to replace them.

 

[charlie]

Re: PoCo Primary Fuses - Rating and Pupose?

Wayne, not all electric utilities are the same. Some (mostly municipals and REMCs) will fuse to protect the transformers. Most of the investor owned electric utilities have to answer to the regulatory bodies and are held to tighter and tighter standards. That forces us to keep the outage minutes as low as possible. The municipals and REMCs have the luxury of protecting their transformers. We intend to burn free tree limbs and small animals, burn open secondary and service faults, and section off pieces of our distribution system with overcurrent protection.

At one time, our smallest fuse was a 6T for smaller transformers. We now use a 15T as our smallest fuse so we don't have fuses blow easily and lose customers.