ECN Forum Forums General Topics of the Trade General Discussion Area Pole Pig Fault Current

Wow, thanks Scott, great explanation

I just paced off the drop- 216' from my panel to the pole. There is #4/0 Al run into my box, but the aerial drop looks to be significantly smaller; going with the worst-case scenario, I'll assume #4/0, which is 0.1 Ohm/1000'. Resistance for this line would be 0.0432 Ohms. Coupled with 1.5% impedance gives a complex impedance of 0.045 Ohms at 19° (PF .95).

So, 120V bolted short on my panel bus would give a fault current of 25kVA/120V/.95PF/0.045 Ohms=4896A. 240V fault would give half that. In reality with a smaller conductor, resitance would be higher, and fault current would be lower with a PF closer to 1. Did I calculator that right?

In the Queens power outage thread, it was explained that the NYC grid is designed exactly like gfretwell described, with the secondaries tied together, with no means of disconnect in between. Strikes me as a dangerous situation, too! I'd hope there are egg insulators on the poles and the wires are there only for tension... Though, in theory, there are fuses on the primary side of all the pole pigs which should open if overloaded, right?

[This message has been edited by SteveFehr (edited 08-12-2006).]

Quote:
"In the Queens power outage thread, it was explained that the NYC grid is designed exactly like gfretwell described, with the secondaries tied together, with no means of disconnect in between. Strikes me as a dangerous situation, too! I'd hope there are egg insulators on the poles and the wires are there only for tension... Though, in theory, there are fuses on the primary side of all the pole pigs which should open if overloaded, right?"

It is dangerous, that's why we don't do it anymore. It's called "underbuilt" (at least it's called that here)and it not only provides more fault current, it allows for backfeeds from one pot to another. VERY dangerous for lineman.

As Scott35 noted, many factors go into fault calculations. Once you get outside the city, many of the distribution lines are #4 ACSR and quite a few older lines are #6 harddrawn. Depending on how far the customer is from the substation, the fault current can drop dramatically in wires like these.
As for core saturation, I thought that was more a factor of high voltage than load.

I thought it was bad that in my parents neighbourhood they have 13 houses all on a 50 KVA pig..
Their place is only 150' from the pig and the service drop is a nice short run of #6 copper triplex.. We had a line to line short in a motor and ended up blowing the tops off a pair of glass fuses!

A.D

Magnetic fields are genereated by flowing current, and core saturation is related to magnetic flux. A given cross-section of iron is only capable of sustaining a particular magnetic field before it saturates and is incapable of any more- at this point, any additional energy put in from the primary is wasted as heat, and no additional electrical energy is transferred to the secondary.

Manufacturers being cheap, transformer cores are usually sized for the kVA rating and no more, or so I thought- if these transformers are capable of 300% load, they must have cores that are 3x larger than they need to be for the nameplate rating. This also creates some inefficiencies, as too-large-cores are more lossy than properly sized cores.

Frequency also factors in- for instance, an american 60Hz transformer will saturate at a lower current at 50Hz than 60Hz, and has to be derated to 80% of its nameplate data if used on a 50Hz circuit.

[This message has been edited by SteveFehr (edited 08-12-2006).]

It would be grossly inefficient for power companies to size transformer as we do.

The power company would be paying for all the losses caused by the 'over sized' transformers.


Yes and no.

There are fuses on the primary side.

Will they open when the transformer is overloaded by our standards?

No, not likely. If I recall the fuses they use will sustain 150% of their rating indefinitely.

They will open when the transformer primaries eventually short out protecting the rest of the grid from failure.

On another forum a member posted the following and it has stuck with me as I think it is very true.

The NEC is concerned with turning the power off

The NESC (Power Company rules) are concerned with keeping the power on under most conditions.



[This message has been edited by iwire (edited 08-12-2006).]

It was mentioned (forgot where or by who) that Con Ed (NYC's POCO) has fuses inserted at various points thruout the tied together seconaries, so that if only one of many transformers is receiving power from the primary, that these linking fuses would open so that one transformer doesn't try to supply the entire neighborhood. Presumidly the linemen could pull those fuses to split up the secondary network when they need to fix something.

Though that TV news footage of secondary overhead feeds literally burning up seems to indicate that they didn't fuse that particular neighborhood. ?

Totally not true around here!! We average 7 - 20 houses on a lateral serviced by anything from 2 - 6 pole pigs. My apartment building (5 units) is served by 1 pig (right outside my 2nd story bedroom window - and I thought having a breaker panel in there would be bad...) along with 4 other houses and a few streetlights. Primary voltage is 4Kv, with a 66Kv circuit above.

By the way, a recent flashover in the underground vault a block away had those 66Kv lines dancing a bit!

However, in Beverly Hills and other areas with mansions, it is not uncommon to find one house serviced by an individual pig, or more often, padmount. If you want to talk about some serious available fault currents.....one 55,000 sq. foot location I worked at experienced a short in an air handler motor in the pantry area. (277 volt 3 phase) (no, not a mistype.) The explosion was heard throughout the house!! The breaker tripping in the mechanical room above (through a concrete slab floor) sounded like someone dropped a mini car above us!!

The whole house service was 4,000 amps!! Despite current limiting, series rated breakers throughout, I still saw an in-wall dimmer get literally blown across the room by a shorted light fixture!


Not necessarily. As seen many times here on this site, and in many accounts of downed/faulted powerlines, arcing faults may not generate enough fault current to blow a fuse, trip a breaker or melt a cable limiter. Especially on overhead lines which fall, the substation breaker may only see a higher-than-normal load, but not above the trip setting.

edited to add some dropped words..

[This message has been edited by mxslick (edited 08-12-2006).]