Calculations > Arc flash

The arc flash calculation can be started as follows. Select one or more nodes and optionally one or more branches/elements that are connected to these nodes. Selection of the whole network is also possible (Ctrl+A shortcut key). After that, start the calculation via ribbon Calculation | Arc Flash. The choice between two calculation methods (IEEE 1584 and ISSA) is possible via Options (press F11 and go to tab Calculation | Arc flash). Algorithms for IEEE 1584 and ISSA are slightly different from each other, below those algorithms are explained:

Algorithm of IEEE 1584-2002:

•first, the bolted fault current is determined using Fault Analysis module of Vision;

•after that, the arc current is calculated according to Section 5.2 of the standard;

•based on the arc current the equivalent fault resistance Rff is determined that corresponds to this current;

•then, the three phase short circuit with the fault resistance Rff is simulated and the operation of protection is analyzed (module Protection, Single Fault function);

•the incident energy for each switching sequence is determined from the currents and times of the switching sequences according to Section 5.3 of the standard;

•the total incident energy is equal to the sum of the energies of all sequences.

The steps mentioned above are executed two or four times (dependent on the nominal voltage of an object). In case of the nominal voltage up to 1 kV four situations are computed:

•maximum short circuit power of grid and 100% arc current;

•maximum short circuit power of grid and 85% arc current;

•minimum short circuit power of grid and 100% arc current;

•minimum short circuit power of grid and 85% arc current.

In case the nominal voltage is above 1 kV, only the situation corresponding to 100% arc current are computed. For the installations above 15 kV or conductor gap outside the gaps specified by the standard (G<13 mm or G>153 mm) theoretical Lee model is applied (Section 5.4 of the norm). Lee model leads often to too conservative results, therefore Vision gives a warning message in case this model is used.

Algorithm IEEE 1584-2018:

The algorithm is to a large extent the same as that of IEEE 1584-2002, but the formulas are quite different. Therefore results in some cases can differ significantly from that of the old version of the standard. Important characteristics of IEEE 1584-2018 with respect to IEEE 1584-2002 are as follows:

•Lee-model is not supported anymore, the upper limit for the nominal voltage is 15 kV;

•calculations are performed at 0.6, 2.7 and 14.3 kV nominal voltages, then the values of the arc current, arc energy and arc flash boundary are interpolated/extrapolated to the actual nominal voltage;

•the reduced arc current is no more equal to 85% of the original value, but is determined in a special manner dependent on the electrode type and the dimensions of the installation.

Algorithm of ISSA:

•first, all motors and induction generators are temporarily switched off and short circuit power of grid is set to the minimum value, then the minimum short circuit current is determined using Fault Analysis module of Vision (this is an equivalent of the minimum short circuit calculation of IEC 60909);

•the arc current is equal to the minimum short circuit current times the current attenuation factor Kb;

•based on the arc current the equivalent fault resistance Rff is determined;

•then, the three phase short circuit with the calculated above fault resistance value Rff is simulated and the operation of protection is analyzed (module Protection, Single Fault function), the results of this step are the (maximum) tripping times of the protective relays;

•for each switching sequence the maximum short circuit current is calculated according to IEC 60909;

•energy for each switching sequence is computed from maximum short circuit currents and maximum tripping times;

•the total incident energy is equal to the sum of energies of all sequences.

Detailed calculation results with details about switching sequences can be get via button Details (right mouse click on an object with the arc flash results and after that click on Details button).