When selecting high voltage switchgear, engineers often begin with rated voltage and continuous current. These are important, but they are only part of the selection process. The switchgear must also be capable of handling the electrical and mechanical stresses associated with short-circuit faults.
A short circuit can produce very high current within a very short period. The resulting thermal and mechanical forces can place significant stress on circuit breakers, busbars, disconnectors, cable connections, and other components.
Rated Short-Time Withstand Current
One important specification is the rated short-time withstand current. It describes the current that the switchgear can withstand for a specified duration under defined test conditions.
This rating is particularly relevant when evaluating whether equipment can withstand the fault level expected at its installation point.
The duration matters as well. A switchgear assembly may need to withstand a fault until the protection system detects the fault and the circuit breaker clears it.
Peak Withstand Current
Fault current is not simply a steady RMS value. The initial current can contain a significant peak component, creating strong electrodynamic forces inside electrical equipment.
Therefore, peak withstand capability is another important specification when assessing switchgear for systems with high prospective fault currents.
IEC switchgear standards specify rated values associated with short-time and peak withstand capability for applicable equipment.
Circuit-Breaker Breaking Capability
For circuit breakers, another key consideration is the ability to interrupt a short-circuit current safely.
The required breaking capability should be evaluated against the calculated fault level at the installation point rather than simply selecting the largest available breaker.
This is why a short-circuit study should normally be completed before finalizing the switchgear specification.
Don't Look at One Rating in Isolation
A proper switchgear specification normally considers several parameters together:
- Rated voltage
- Rated insulation level
- Continuous current
- Short-time withstand current
- Peak withstand current
- Short-circuit breaking capability
- Short-circuit making capability
- Internal arc requirements where applicable
- Environmental and installation conditions
The appropriate combination depends on the application and system design.
For a practical overview of these factors, see this guide on how to choose high voltage switchgear.
Choosing equipment based only on voltage and current can overlook the fault conditions that the switchgear must actually withstand.
