What is the point of having a fused disconnect on a motor circuit – electrician talk – professional electrical contractors forum gas in back shoulder

Fuses and breakers can’t substitute for an overload relay…Overloads are very specialized with a very long curve. The overload trips overloads. Breakers and fuses trip on faults. Between them both breakers and fuses limit current although both can be modified to be more aggressive about it. Fuses are faster than breakers and can withstand far more fault current. It’s not unusual to see breakers with backup fuses in large switchgear so you can have a 35 ka vacuum breaker with a 100 ka rating with the fuses. gsBut MOST fuses have crpppy accuracy and they are single phase devices, and breakers and electronic overloads can respond to far more issues. There are smart fuses where you can supply an external relay that can trip the fuse giving you the features of both. Fuses are also often used in disconnects where they aren’t needed because price and availability often drives silly things. Unlike NEMA, if designed right, IEC starters can be Type II (no damage) but the short circuit protection must be a current limiting fuse. Finally you can just reset an overload and go back to work but by OSHA regulation you have to fault find before resetting a breaker or replacing fuses. Molded case breakers require visual inspection (NEMA AB 4) before resetting. The whole starter requires inspection in IEC Type 1 starters but not Type 2. Manual motor starters are also irritating…it’s both an MCP and an overload so you must do fault finding no matter what the cause even if it’s Type 2, and it’s impossible to tell if it’s an electrical fault or an overload.

The “OCPD” at the panel is technically the “Feeder” protection, providing Short Circuit AND Over Current protection for the conductors going from the panel to the motor starter. Because the code can’t assume what you are doing with that feeder circuit, there are separate requirements for “Branch” SC and OC protection. In a combo starter, the fuses or CB or MCP in that starter are providing ONLY** the BRANCH SC protection, the OL relay then provides ONLY the OC protection.

Can you use the Feeder protection device in the panel as the Branch SC protection for the motor circuit? Yes you can, in which case you can use a non-fused disconnect at the motor starter. But the problem is, non-fused disconnects have very low SCCR listings if no fuses are included, so the listing of the starter may get degraded to 10kA, as opposed to 65kA typical of a combo starter with a CB or 100kA with fuses. So yes, it’s possible, but often not practical and it’s usually simpler to just have local fuses or a CB in the starter.

If you were referring to using a fuses disconnect AT the motor as a LO/TO point, that’s almost never necessary and having multiple SCPDs in the same circuit just increases your troubleshooting time, which increases your down time. I highly recommend against that practice.

**Technically it is remotely possible to size a fuse to provide OL protection too, but I don’t recommend it because fuses only come in certain sizes so if you do end up falling into a size that will technically work, it’s usually a compromise that can result in either nuisance clearing or motor damage.