Recently I was testing some 12.5kV switchgear and came across a fuse that differed in resistance compared to the 5 others on site. The resistance of Phases A, B and the three spare fuses were around 20500 micro-ohms. The Phase C fuse was found at 7200 micro-ohms. I replaced the Phase C fuse with one of the spares. I could not find what the resistance should be in the manufacturer’s manual, however, I assume it is around 20500 micro-ohms since 5 of 6 fuses were around that value.
A co-worker asked me why I would replace the fuse. Since, technically, it could have been left and would have run with no issue.
Other then NETA’s recommendation of 15% difference between fuses, I didn’t have an answer that I was sure of.
I guessed that it must be an indicator of the fuse being on its way out and that the time and current it would blow at would be different then of what it was designed, which in turn would affect the coordination study including this fuse.
Please let me know if my thoughts are correct or other reasons why the fuse should be replaced?