Short Circuit Capacity of PILC Cable Sheath and Vintage Equipment 1

[rbauerpe]

A campus has a significant amount of underground distribution consisting of PILC cable in ductbank. Distribution feeders at 13.8 kv each serve groups of buildings with each building having a secondary unit substation with fused interrupter switch and medium voltage transformer. Distribution feeders that serve older sections of the campus (50 years or more) have current limiting reactors. The short circuit current from the utility is 25,000 amps. After going through the reactor the short circuit current is about 6,000 amps.

There are a number of S&C interrupter switches with SM-4 fuses that are rated at 12500 amps interrupting There are some interrupters that I could not get any information, brand or fusing. Manholes with PILC cable do not have separate grounds with the PILC cable, so the primary ground return is the lead cable sheath.

We will be replacing the main switchgear that was installed about 1965. When the switchgear is replaced we will inspect each reactor and determine if it should remain in service or it should be replaced. If the reactors are replaced I would like to install smaller reactors. Reduce short circuit to below 12500 amps.

I have not been able to determine what equipment is being served that required the short circuit current to be reduced to 6000 amps. I am thinking that the limitation might be the lead sheath on the cable. I have not tried to calculate the maximum allowed current on the sheath yet. The PILC cable is 4/0 awg and 350 kcmil.

Any suggestions as to what I should be looking for as the limiting factor that required the fault current to be reduced from 25000 amps to 6000 amps.

 

[dpc]

I don't know, but check with Okonite. They still make PILC. Also maybe check AEIC CS1-90.

 

[7anoter4]

Usually up to 1965 the short-circuit conductor maximum temperature for paper [oil-impregnated] insulation was:
160oC low voltage 140oC up to 20 kV and 120oC for above.
If the cable is not so old PILC maximum temperature could be 200 oC.
If a conventional 1 sec fault clearing time was then-for copper conductor cable- the minimum cross section area for 6 kA short-circuit current current will be:
Scu[mm^2]=IK * SQRT(A / B * tsec) * DF where:
For 1 sec DF=1 A=ALPHA * RO * 10 ^ 4 / TCAP where:
ALPHA=0.00393 RO=1.724 TCAP=3.422
B=LN(1+(TA-TB)/(K+TB)) where :
TA=120 ,TB=90 and K=234
Scu=89.8 mm^2 [more than 3/0 then 4/0 minimum required]

 

[7anoter4]

On the other hand if the lead sheath will be used for ground fault current return and limited to the 3 phase short-circuit current 6 kA , 1 sec fault clearing time and maximum 170oC -I take the lead sheath thickness as per modern standard-95 mils-the lead sheath could withstand 7.3 kA rms.- the minimum 572 kcml is required for 6 kA[4/0 PILC is provided with 700 kcml lead sheath that means it could withstand 7.3 kA rms]. I did not take into consideration the heat evacuated from conductors in the same time, indeed]