Melting temperature polyethylene insulation 1

[psycholuck]

Hello I am in charge of sizing and ordering substation batteries and chargers. I have run into a problem while trying to size the power cable between the 60 cell battery bank and the DC distribution cabinet. My company sizes the cable using the following formula.



(I/A)^2*t = .0297 * log [(T2 + 234)/(T1 + 234)] with the following values for the subscripts.





I = Short circuit current (specified by the battery manufacture)

A = Conductor cross sectional (in circular mils) (Value that is to be solved for)

t = time of short circuit (14 seconds)



T1 = Conductor Operating Temperature (75 C)



T2 = Maximum Short circuit Temperature (Temperature at which insulation is damaged)





I know by consulting NEMA WC5 that the conductor operating temperature for power cable is 75 C

I can’t find values for variable T2.

Would any forum be able to locate the melting temperature for polyethylene insulation? Is this temperature standard for most power cable manufactures?



Thank you for your assistance
y

 

[stevenal]

According to the Aluminum Association, T2 for thermoplastic polyethylene is 150C, and 250C for cross-linked PE. Your formula is different from theirs, so I guess you are using copper. Shouldn't affect T2, though.

 

[psycholuck]

yes i am using copper. That melting point matches what example given in the Company standard. Thank you

 

[cuky2000]

T2 is the maximum SC damage rated temperature. The melting temperature should not be used in this calculation. Polyethylene insulation damage temperature is 250oC

Where 14 seconds came from? Most protective devices could clear the fault faster than 14 sec.

See the enclose site for reference

http://www.acewirecable.com/pdfs/ok58.pdf

http://www.okonite.com/engineering/short-circuit-currents.html

 

[psycholuck]

The okinite site you refered to is where the standard got the 250oC maximum SC damage rated temperature. The value of 14 seconds is a very conservative time frame use by past engineers at my utility. I am investigating the possibility of lower this time to a lower but still conservative 10 seconds.

Does this seem reasonable?

cuky2000 what time value do you recomment i use.

All i am looking for is the smaller control wire to melt before the power cable. I am not sure if control cable is what you call the wire that exits the DC distribution cabinet) Please enlighted me in this regard.

 

[buzzp]

The short circuit numbers from manufacturers are sometimes a little low since reserve capacity is not used in their calculations and neither is the charging mechanism. Typically, 2% is added to their number to come up with a better approximation. Still, the calculations should still be made.

 

[sykimk]

Cable size usually selected from the largest size that calculated by the criteria based on short circuit current, voltage dips and cable ampacity calculation.

I think that cable sizing by short circuit current would not be bigger than those by voltage dips especially for low voltage system, but it would be good for ensuring your result. Anyway, I recommend you to select voltage dip criteria between batteries and the associated DC charger panel, because DC voltage generally used for switchgear control. Therefore, you should consider various conditions (e.g. battery discharging, and operating voltage of trip/closing coil of circuit breaker, etc.)