Can anybody explain the discrimination overlaps in the attached protection study?
For example, a Live to earth fault in the downstream OCPD and how this may play out.
I know the current settings are not ideal, but this is a legacy issue restricted by cable sizes etc!
I think "very poorly" is how it would play out!
But the plots don't quite look right to me. For example:
- The HV fuses would be transformed from 100A at 6.6kV to 400V (presumably) at 1650A equivalent, but the plot shows them coming in at around the 10kA point.
- The first ACB is set to 1440A but seems to be starting its trip region around 8kA
- The 800A BS88 fuses at around 4kA
- The second ACB seem to trip at roughly the same threshold, but has a very aggressive time/current setting, probably an attempt at selectivity
It might be the log-scale plot is just out by some factor, as relative to each other the plots look believable.
What is not clear though is the ACB let-through and if they can possibly stop the upstream fuses from going, as most mechanical breakers take a few milliseconds to initiate opening and that normally lets through a whole lot more energy than a HRC fuse might for the same sort of conditions.
Same applies for the ACBs for any down-stream fuses, though the #1 seems to have been set with a bit of a delay to try and remain selective with the down-stream 800A fuses, but as you can see on the plot (ignoring and scale errors for now) the brown and blue curves cross each other at multiple points so there are various fault currents that would lead to having, or not having, selectivity there.
