Switchgear assemblies play an important role in the distribution of electrical power. The function of the switchgear assembly is to control electrical power by switching and to protect electrical circuits by interrupting current during fault conditions.
For a given application, the determination of the suitable switchgear assembly follows a series of steps. The selection of the voltage rating for the switchgear assembly is the first step.
The switchgear assemblies may be categorised by operational voltage level. Broadly, there are two main categories, low voltage and medium voltage. Low voltage assemblies are those which are rated to not exceed 1,000 VAC or 1,500 VDC. Medium voltage switchgear assemblies are rated for voltages from 1 kV up to and including 52 kV.
Once the operational voltage rating is defined, the next step is to determine the suitable fault rating of the switchgear assembly. The starting point is the prospective short-circuit current for the switchgear assembly. The prospective short-circuit current is the short circuit current at the supply terminals of the switchgear assembly, when the supply conductors are short-circuited. The prospective short-circuit current is usually found from a power system study, which provides the three-phase short circuit fault level and the phase-to-ground fault level. For connections to the grid, the prospective short circuit level is provided by the utility.
The short circuit rating of the switchgear assembly must be greater than the prospective fault level. This ensures the selected switchgear assembly is able to safely withstand and interrupt a fault level up to the prospective fault level.
In selecting the short circuit rating of the switchgear assembly, some consideration should be given to the likelihood of the prospective fault level increasing during the life-time of the switchgear assembly. For instance, a processing plant may be operating from in-house generation as an island, independent of the grid, but there may be plans to connect to the grid within the next 18 months. In this instance, the prospective fault level should be determined from the worst-case scenario, with the grid connected.
By referencing the appropriate standards, according to the operational voltage level, the suitable short-circuit ratings for the switchgear assembly can be selected. For example, a low-voltage switchgear assembly may have ratings for the short-time current (ICW) and the peak withstand current (IPK). The rating may be conditional. For example there can be a rated conditional short-circuit current (ICC) and a rated fused short-circuit current (ICF) for a switchgear assembly.
BESST Engineering is able to provide the power system study and provide the determination of the suitable switchgear assembly for your application.