Industrial switching & protection systems (Application guide)

Short circuit currents

A short circuit current is a current triggered by a negligible impedance fault between points of an installation normally having a potential difference.

3 levels of short circuit currents can be identified:

1. Peak short-circuit current (I_sc peak) corresponds to the top of the current wave, generating heightened electrodynamic forces, notably at the level of busbars and contacts or equipment connections,
2. RMS short-circuit current (I_sc rms): rms value of the fault current which leads to equipment and conductor overheating, and may raise the potential difference of the electrical earth to a dangerous level,
3. Minimum short-circuit current(I_sc min): rms value of the fault current establishing itself in high impedance circuits (reduced section conductor and long conductors, etc.). It is necessary to quickly eliminate this type of fault, known as impedant, by appropriate means.[/info_box]

Harmonics

Definition

Harmonic current or voltage are mains “stray” currents or voltages. They distort the current or voltage wave and lead to the following:

• an increase in the current’s rms value,
• a current passing the neutral being higher than the phase current,
• transformer saturation,
• disturbance in low current networks,
• intemperate tripping of protection devices, etc.,
• distorted measurements (current, voltage, power, etc.).

Harmonic voltage is caused by harmonic current passing through mains and transformer impedance.

Harmonic voltages

For a measurement period of one week and value set to 95%, the averaged 10 min harmonic voltages should not exceed the values given in the following table Total voltage distortion rate should not exceed 8% (including up to conventional number 40).