Short circuit ampacity of ACSR wire 1

[Blanton]

I am trying to find a reference in a textbook to the amount of current ACSR wire can carry before it is damaged. As an example, 3/0 ACSR can carry 325 amps and there is a damage curve for it but it does not consider what happens if someone loads it to 500 amps for an hour.


Thanks,

 

[stevenal]

I have not seen such a reference.
Since the designer based the clearances based on wind, ice, and loading; most likely the ACSR will sag until it contacts the conductors below.

 

[cuky2000]

The behavior of the ACSR conductor is determined through sag-tension calculations. If an ACSR overhead conductor is loaded continuously above rated ampacity as suggested, the mechanical strength of the conductor is reduced.

This will lead to a loss of mechanical life, possible, not instantaneous failure. However, permanent conductor deformation may compromise the electrical clearance to the ground that will be in violation of the NESC in the US similar to other international codes.

There is ACSR manufacturer data and commercial software available such as PSL-CADD, SAG10 software, etc that can determine the conductor behavior exceeding the thermal rating. The nomogram below is another source for illustration.

 

[Palletjack]

We had an instance where 1/0 was loaded way over ampacity on a three phase structure on arms with MGN.
The B(center) phase dropped enough until it hit the neutral, opening the breaker.
Load it enough it will eventually fall from weakened steel.

 

[7anoter4]

See IEEE 738/2006 IEEE Standard for Calculating the Current-Temperature of
Bare Overhead Conductors.

 

[7anoter4]

According to Ampacity for Aluminum and ACSR Overhead Electrical Conductors/1986 Second Edition of Aluminum Association Table 4-AMPACITY FOR BARE ACSR CONDUCTORS 325 A ampacity it is for 4/0 conductor [no sun and wind]. The ambient temperature 40oC and conductor temperature Tc=75oC.
If we consider formula from IEEE 242/ 2001 CHAPTER 9.5.2.3 Intermediate and long-time zones :
IE/IN%=SQRT(((TE-To)/(TN-To)-(Io/IN)^2*exp(-θ/K))/(1-EXP(-θ/K)*(230+TN)/(230+TE))*100
where:
IE is emergency operating current rating=500A,
IN is normal current rating=325A,
IO is operating current prior to emergency=325A,
TE is conductor emergency operating temperature 119,
TN is conductor normal operating temperature[75oC],
TO is ambient temperature,
K is a constant, dependent on cable size and installation type (see Table 9-5),
and considering the ACSR conductor as a small cable in air K=0.5 hrs. and θ=1 hr then the conductor reached TE=119oC if the load current was 325 A for long time period before and suddenly jumps to 500 A for an hour.

 

[Fischstabchen]

It sounds like you might want to review how the conductor based on ambient temp and wind or get a system to monitor the dynamic loading of the line. These are sag detectors or things like optical temperature sensors.

 

[davidbeach]

As with a whole family of question of similar ilk over the years, if you have to ask you're too close to the boundary. It stated out with, if the inspector has to measure the clearance you don't have enough clearance, regardless of what gets measured. Nobody gets rewarded for closest to the edge, but lots of peopled get reamed for being just shy of the edge. Give yourself plenty of room, room that doesn't require detailed calcs.

I’ll see your silver lining and raise you two black clouds. - Protection Operations