Arc resistance

[hhhansen]

Hi out there
IEC60909 stipulates the principles for short circuit calculations.
It is well known that an arc is generated in case of a short circuit. In our calculation we are neglecting this in order to obtain the maximum short circuit current. But what about the minimum short circuit current to be used for relay settings.
I can not find anything in IEC about requirements for the value of the arc resistance.
Your comments are much welcome.

 

[Zogzog]

Check the 1584. An accepted industry standard for 480V systems the minimum fault current that will be self sustaining is 38% of the BF value.

 

[521AB]

Warrington formula.
The calculation of the arc resistance is an iteractive calculation, if you want to do it manually. Most of short circuit calculation software can manage that.

 

[00123456]

HI,

521AB: warrigton formula is applicable for phase to phase fault. What about the phase to ground fault? Arc resistance calculation for phase to ground fault is not easy to calculate. It depends on line lenght & fault location, tower hight, atmosphere conditions, soil resistance etc.

hhhansen:
Are you talking about arc resistance calculation for indoor swithchgear or for transmission line? If you can use ETAP/CYME/EDSA etc. power system software can takecare of arc flash study for indoor switchgear while you can RUN SHORT CIRCUIT module.

 

[hhhansen]

Hi
I am referring to short circuit on cables or connection points in industrial applications.
What about IEC requirements rather than IEEE

 

[00123456]

Hi,
Warrigton formula will not work for cable.

I will suggest some calculation for that.

Thanks

 

[521AB]

00123456...

ground faults (overhead lines) are not easy to calculate but not impossible. The earthing tower resistance is known, and the arc is calculated with the Warrington formula.
Clearly the Warrington formula contains the phisical aspects of the overhead line, and also it is possible to consider the wind effect.
But we are talking about cables, so forget about it.
I don't have the answer.
I would say that it is more important the estimation of the equivalent source impedance then the maximum arc resistance.
In the cables, there are no trees growing ( ), I have always "seen" cable faults with a very small fault resistance (disturbance recorder analysis), but if somebody has some figures, I am also interested in knowing them.