There are several short circuit calculation methods that can be used. In Europe, we have :<br>
- IEC909 : this is the standard by which LV and HV calculations should be made.<br>
- CENELEC guide R064-003 : this guide gives an alternative method, which is easier to use, but that gives accepted results on the LV side only.<br>
<br>
The problem is that the CENELEC guide does not apparently cater for motor contribution. <br>
<br>
For example, Ik3max = m x c x Un / (1,732 x Z)<br>
where m = c = 1,05<br>
Un is the network phase/phase voltage<br>
Z is the 'cold' phase impedance, cold meaning that no current was flowing through the conductors, and therefore, the impedance is lower, resulting in a higher short circuit level.<br>
<br>
If we consider motor contribution now, then we have to consider that the motors are running (seems logical, doesn't it ?), and therefore we shouldn't consider that the conductors are 'cold' anymore. If we assume the motor contribution to be equivalent to the start up current, then what do we add ths contribution to ? Ik3max ? Something else ? Obviously, using Ik3max is conservative ...<br>
<br>
Another issue, do we consider the limitation of the cable to the contribution ? Do we consider the effect of each motor on each distribution board in the network ? If we do, you can imagine the complexity of the calculation !<br>
<br>
IEC909 gives simplification methods (by merging into one all the 'small motors'), ignoring certain components, ... <br>
<br>
What methods do you apply ? recommend ?<br>
I am interested in mainly IEC responses, but I am very curious to the learn about the NEC approach too !<br>
<br>
Thanks !
- IEC909 : this is the standard by which LV and HV calculations should be made.<br>
- CENELEC guide R064-003 : this guide gives an alternative method, which is easier to use, but that gives accepted results on the LV side only.<br>
<br>
The problem is that the CENELEC guide does not apparently cater for motor contribution. <br>
<br>
For example, Ik3max = m x c x Un / (1,732 x Z)<br>
where m = c = 1,05<br>
Un is the network phase/phase voltage<br>
Z is the 'cold' phase impedance, cold meaning that no current was flowing through the conductors, and therefore, the impedance is lower, resulting in a higher short circuit level.<br>
<br>
If we consider motor contribution now, then we have to consider that the motors are running (seems logical, doesn't it ?), and therefore we shouldn't consider that the conductors are 'cold' anymore. If we assume the motor contribution to be equivalent to the start up current, then what do we add ths contribution to ? Ik3max ? Something else ? Obviously, using Ik3max is conservative ...<br>
<br>
Another issue, do we consider the limitation of the cable to the contribution ? Do we consider the effect of each motor on each distribution board in the network ? If we do, you can imagine the complexity of the calculation !<br>
<br>
IEC909 gives simplification methods (by merging into one all the 'small motors'), ignoring certain components, ... <br>
<br>
What methods do you apply ? recommend ?<br>
I am interested in mainly IEC responses, but I am very curious to the learn about the NEC approach too !<br>
<br>
Thanks !
