LOW VOLTAGE SWITCHGEAR TERMINOLOGY: –
Before understanding LV switchgear, one must know the terminologies related to the Electrical
Switchgear:- General term covering switching devices and their combination with associated control, measuring, protective and regulating equipment, also assemblies of such devices and equipment’s with associated interconnections, accessories, enclosures and supporting structures, intended in principle for use in connection with generation, transmission, distribution and conversion of electric energy
What is switchgear?
A device or a combination of devices, primarily intended for the purpose of making, carrying and
breaking electric currents in circuits under normal conditions as well as under abnormal (faulty) conditions.
General term covering switching devices and their combination with associated control, measuring, protective and regulating equipment, also assemblies of such devices and equipments with associated interconnections, accessories, enclosures and supporting structures, intended in principle for the control of electric energy consuming equipment
The LV Electrical Switchgear Terminologies are generally with respect to the following parameters:
- Behaviour under Normal & Overload Conditions
- Behaviour under Short Circuit Conditions
- Utilization categories
TERMINOLOGIES RELATED TO VOLTAGE
Rated Operational Voltage (Ue): The rated operational voltage of equipment is a value of voltage, combined with rated operational current, determines the application of the equipment and to which the relevant tests and the utilization categories are referred.
This is the voltage that would normally be available continuously across the terminals of different phases of the switchgear. Unless otherwise specified, this rating will be 415V for LV Switchgear. This implies that even while 415V @50Hz is present between the terminals, there will not be any flash-over (i.e.) the insulation between phases is sufficient to tolerate a potential difference of 415V continuously.
Rated Insulation Withstand Voltage (Ui):-
Then, one might ask, at what voltage the inter-phase insulation would fail, if it would not fail for 415V. There are two possibilities (viz.) transformer tap changers & the system fluctuation on the upper side. One can go by the permissible fluctuations in the supply voltage (+10%) vide IER 54 – and the worst probable tap ratios in transformers (+/-12.5%). Considering that the worst condition of the transformer taps in the maximum position and at the same time, the system voltage too fluctuates on the plus side, there could be a very high system voltage on the switchgear.
If the Switchgear is exactly rated for 415V, then under the above conditions, it would fail. So, another rating Called ‘Ui’ specified. Normally this would be 690V for LV Switchgear. Even under the worst condition as described above, the 415V system voltage would not go beyond 595V. So, swithgear rated for a Ui of 69OV would very well withstand the above condition.
Rated impulse Withstand Voltage (Uimp): –