Engineering House

They are typically two main devices able to interrupt fault currents, circuit breakers and fuses :

The circuit breakers must be associated with a protection relay having three main functions:
Measurement of the currents
Detection of the faults
Emission of a tripping order to the breaker
The fuses blow under certain fault conditions.

Conditions for protecting branch or distributed circuits for power distribution in LV
--------------------------------------------------------------------
4. 3-metre rule applied to overload protection devices

When protection device P1 placed at the head of line S1 does not have any overload protection function or its characteristics are not compatible with the overload protection of the branch line S2 (very long circuits, significant reduction in cross-section) – it is possible to move device P2 up to 3 m from the origin (O) of the tap-off as long as there is no tap-off or power socket on this portion of busbar system and the risk of short circuit, fire and injury is reduced to the minimum for this portion (use of reinforced insulation conductors, sheathing, separation from hot and damaging parts).

3-metre rule applied to overload protection devices
Figure – 3-metre rule applied to overload protection devices

Conditions for protecting branch or distributed circuits for power distribution in LV
--------------------------------------------------------------------
1. General principle for checking thermal stress

For insulated cables and conductors,
(the breaking time of any current resulting from a short circuit occurring at any point) must not be longer than (the time taken for the temperature of the conductors to reach their permissible limit.)

This condition can be verified by:
checking that (the thermal stress K2S2 that the conductor can withstand ) is greater than (the thermal stress (energy i2t) that the protection device allows to pass).

Question of the Day
_________________
What about the "Automatic power factor correction"?
Answer:
_______
➡️ Automatic power factor correction
-----------------------------------------------------
💡 In most installations there is not a constant absorption of reactive power for example due to working cycles for which machines with different electrical characteristics are used.

Figure 1 - ABB's power factor monitoring, type 'Dynacomp'

💡 In such installations there are systems for automatic power factor correction which, thanks to a monitoring varmetric device and a power factor regulator,
🔌 allow the automatic switching of different capacitor banks,
🔌 thus following the variations of the absorbed reactive power
🔌 and keeping constant the power factor of the installation constant.

💡 An automatic compensation system is formed by:
🔹 Some sensors detecting current and voltage signals;
🔹 An intelligent unit which compares the measured power factor with the desired one and operates the connection and disconnection of the capacitor banks with the necessary reactive power (power factor regulator);
🔹 An electric power board comprising switching and protection devices;
🔹 Some capacitor banks.

Figure 2 - Automatic power-factor correction panel (photo credit: eamfco.com)

💡 To supply a power as near as possible to the demanded one, the connection of the capacitors is implemented step by step with a control accuracy which will be the greater the more steps are foreseen and the smaller the difference is between them.