Diesel Generator Fault Levels

The main switchboard will be refitted with Micrologic 7.2 breakers with adjustable earth leakage and delay, I expect downstream all the final circuits breakers to disconnect before 0.4 (preferred instantaneously) seconds as per the regs. The Micrologics will be also set at before 0.4 seconds to provide backup. BS 7671 required the use of RCD at all final circuits when using a mobile generator not part of the fixed installation but that is not practical hence the Micrologics which are justified with a risk assessment. Once the job is done it is a matter of reconnecting the mains supply and turn off the earth leakage protection on the Micrologics without any breaker modification which is also fast.


The main risk here is loss of supply on a main circuit supplied by the Micrologics in the case of insufficient fault current downstream of those breakers. Which eventually leads us to this whole story on the discrepancy of the fault graph and the per unit values. I also note that the per unit values provided have a time constant associated with them the sub-transient and transient values seem to not be useful as they are very fast.


I also acknowledge that far final circuits will not cause a short circuit of such a degree to cause the current to raise above the overload current. In all the installation is safe, but just a matter of using the most suitable value when undertaking the calculations in Amtech


Thanks,

Mike

The main switchboard will be refitted with Micrologic 7.2 breakers with adjustable earth leakage and delay, I expect downstream all the final circuits breakers to disconnect before 0.4 (preferred instantaneously) seconds as per the regs. The Micrologics will be also set at before 0.4 seconds to provide backup. BS 7671 required the use of RCD at all final circuits when using a mobile generator not part of the fixed installation but that is not practical hence the Micrologics which are justified with a risk assessment. Once the job is done it is a matter of reconnecting the mains supply and turn off the earth leakage protection on the Micrologics without any breaker modification which is also fast.


The main risk here is loss of supply on a main circuit supplied by the Micrologics in the case of insufficient fault current downstream of those breakers. Which eventually leads us to this whole story on the discrepancy of the fault graph and the per unit values. I also note that the per unit values provided have a time constant associated with them the sub-transient and transient values seem to not be useful as they are very fast.


I also acknowledge that far final circuits will not cause a short circuit of such a degree to cause the current to raise above the overload current. In all the installation is safe, but just a matter of using the most suitable value when undertaking the calculations in Amtech


Thanks,

Mike