Reasons
Reasons
So, the problem was really that the 30a breakers were too small. Couldn't you have used the 40a breakers without the fuses?
(A) Equipment was a "robot" used to move heavy test heads among CHIP testers. It came with a UL Label, stating 208-240V 1 Phase 24A,
Max OCPD 30A (I guess because it was a combination of a large pump, smaller motors and a lot of control circuitry), and as I stated came factory wired with 10/3 SO cord and L6-30P T-L Plug.
After lots of testing for max inrush, it became apparent that when the robot was lifting weight very close to it's max. load rating, the inrush to the pump (if the other small motors were activated simultaneously) exceeded the trip curve of the 2p 30a CB, but not the 2P 40A CB or the RK5 30A Fuses.
(B) Design was orginally a 2p 30 CB, 3#10 (2 phases plus ground) to a 2P 30A NFSS (req'd by customer's own Lock-Out Tag Out Safety Regs) and then to a L6-30R T-L recept below it.
Customer wanted to be able to plug in 3-4 different types of equipment on this circuit, depending on what was being tested in the adjacent test bay.
(C) The cost of the 2P 30A FSS, 2 RK5 fuses, and a 2P 40A Bolt on cb, plus labor to wire it in, was about $250.00 (2002 prices). The cost of new 8/3 SO Cord, strain relief, 2P+grd matching pin and sleeve plug, receptacle, and back box, new 2P 60A NFSS, plus labor was a lot more, and wouldn't interface with all the other L6-30P connections on their other equipment.
Yes, you can wire a motor circuit with a 2P 40 and use #10, but I don't think all the NFSS/Recept./Plug (and is 10-3 OK for this use?) downstream devices would pass inspection on a 40A circuit.
I originally posted just to have an example of a situation where a larger CB can protect a 2P 30A FSS. I guess I gave too many details but not enough of the reasons why the choices were made.