Equipment sizing

Short-circuit

The calculation follows IEC 60909 and gives the initial, peak, breaking and steady-state currents, at maximum for equipment withstand and at minimum for protection. The peak depends on the R/X ratio. At 12.5 kA rms, the DC component takes it above 30 kA, and the forces on the busbars are calculated from this value.

Cables

The cross-section is determined to NF C 13-200 and IEC 60287, with the Neher-McGrath method for complex installations (heavily loaded cable layers, ducts, crossings, deeply buried cables). We also check short-circuit thermal stress, voltage drop and screen earthing.

Earthing system

We design the earthing system to NF EN 50522 (2022 edition), which covers the earthing of installations above 1 kV alongside IEC 61936-1, and to IEEE 80 on international projects. The standard assesses the installation on touch and step voltages, compared with permissible values that depend on the fault clearing time. A resistance value is therefore not enough to conclude, and the earthing system is designed together with the protection plan.

The earth potential rise of the substation is also transferred off site through fences, cable screens, pipes and telecom lines. We identify these transfers and deal with them.

Insulation coordination

To IEC 60071, we check equipment insulation levels, the selection and location of surge arresters, and clearances. When switching or lightning overvoltages have to be calculated, we use an EMT study.