MEPworx Wiring – NES-ES Distribution example CYW Updated with HV protection

By David Wren

The example Wiring file, NES-ES Distribution.CYW has been updated to, NES-ES system with HV.CYW.

This file now includes the addition of an HV CPD (CEF 63kA fuse, rated at 31.5A HV), which is the first HV protective device on the HV side of the transformer. This is to demonstrate an example of an LV/HV Discrimination Study.

This is designed to be a tutorial aid, where you can see results of the design checks that recognise the HV & LV CPDs as being suitably sized for the load but not sized for discrimination. As illustrated below, hence the design issue flagged up on the main supply cable.
 

In an LV/HV discrimination study, the HV device is expressed in LV terms.
E.g. the CEF 31.5A fuse at 11kV, equates to an LV rating of 866.25A at 400V.
(11000V/400V) x 31.5A = 866.25A

In this example, you can see that the total let-though of the 400A fuse at 0.49 x 10^6, exceeds the pre-arcing of the 31.5A HV fuse of 0.46 x 10^6.

Hence the failed discrimination in this example between, the upstream HV fuse and the downstream LV fuse.