Although rare, the bus zone fault can occur in several scenarios, such as circuit breakers or insulation support failure. Most HV switchboards have bus zone differential protection to avoid arc flash caused by these failures. Besides the bus itself, bus zone protections cover other components of the switchboards, such as circuit breakers, disconnecting switches, instrument transformers, and bus sectionalising reactors.
The fault clearing time for a bus zone protection scheme is typically around 100ms; this includes:
Bus zone relay operating time
Master trip relay operating time
Circuit breaker clearing time
Here are two examples of different configurations in an HV switchgear.
Configuration 1: For HV Circuits without Remote Infeed
Figure 1 below illustrates an arcing fault on a radial feeder circuit breaker cleared by the bus zone protection.
Arc flash hazard incident energy (IE) is calculated based on the If1 and arcing current duration of 100ms.
Configuration 2: HV Circuits with Remote Infeed
Figure 2 below exhibits a scenario in which faulted breaker tier may contribute from local infeed and remote infeed.
Two separated incident energies contribute to the fault location:
IE1 with local fault current, If1 (blue)
IE2 with remote fault current, If2 (red)
IE = IE1 + IE2
IE1 is calculated based on the If1 and arcing current duration of 100ms.
IE2 is calculated based on the If2 and arcing current duration T2.
T2 duration depends on the time it takes for the remote current infeed to be interrupted. T2 may be less than 100ms or greater than 100ms.
An arc flash hazard analysis is performed to ensure the safety of the operators and protection of the equipment from significant damages during the event of an arc fault. Error in such analysis may result in severe injuries if an employee is not wearing the required CAT rating personal protective equipment (PPE) in the event of an arc flash.
Our engineers highly recommend performing flash risk assessment every five years or any time a modification is made to the system to avoid any hazard. Click here for more information about arc flash hazard analysis services provided by Divergent Engineering or call us on 08 6115 6370 today to discuss how we can support you.
About the author:
Senior Electrical Engineer
Vu has over a decade of experience in the resources sector. His expertise lies in power system modelling & analysis, protection coordination, arc flash hazard analysis, failure analysis and investigation and programming.