By David Wren
Occasionally we have the situation where the calculated Short Circuit (SC) fault current at a Final Board is greater than the SC rating of the Final Board’s CPD devices e.g. a typical error message in the board’s Ways could be Short Circuit Fault is 19.52 kA. Maximum for CPD size is 15.00 kA.
As illustrated below:
This is where the design engineer can use the option to set the Upstream Breaker as a Backup (or Cascading) protective device. The Backup function is set in the Detailed CPD Entry dialog box.
For more information, run a search for “Backup and Cascading” in the Help (F1).
For this design function to work there must be data entered for your particular combination of Upstream and Downstream devices in the project’s CPD DB. If it is not present, this is how the data can be added, but first if you are modifying or customising a CPD DB, it is always a worthwhile precaution to Save the CPD DB with a different name or version number.
In this example, the Upstream breaker is a NSX100N MLgic 5.2, with the Downstream breaker an iC60H. This the result of checking on the manufacturer’s web site for details of the Backup/Cascading data.
Having established that this is a recognised combination of breakers, using the CPD DB program to check your project’s CPD DB, select the Upstream device (the NSX 100N Mlgic 5.2) and click on ‘Backup’.
Next, in the drop-down window at the bottom of the ‘Manufacturer’s Backup Tables’ dialog box, scroll down to select the appropriate Downstream device (in this scenario, the iC60H) and enter the Max Fault levels in kA for this combination of devices. Previously, having checked these values in manufacturers Backup/Cascading tables.
Next, open your Wiring project and change the project to use the newly modified CPD DB.
As can be seen in the screenshot below, the previous SC issues have now been resolved.
For the Backup function to work successfully, the SC fault level at the Downstream board and its CPDs must be less than the SC values entered in the Backup tables. In this example, the Short Circuit Fault at the final board is 19.52 kA and the reinforced breaking capacity for this combination of breakers is 40kA. Therefore, the requirements of Backup protection have been achieved.