Fuse Selection

[dwinchel]

I have a 120vac operator panel that has several multi-position switches that control the operation of three solenoids. How do I determine the internal fuse for the entire panel and components. Do I need to provide a fuse for each solenoid or does the panel fuse provide sufficient protection.

 

[buzzp]

Well I can tell you how to do it from a designers standpoint with little attention given to the NEC. Someone will likely have input on that. Typically, you would add up the currents and size main fuse according to this. The fact that your using solenoids makes it a lil more difficult. These solenoids will pull 6-10 times the current on the nameplate when they are energized so fast acting fuses would not be my choice. Then I would determine what would happen if something shorted on each of the individual circuits you are considering fusing. Sort of a failure analysis. If a resistor in that particular circuit shorts out then is there something else that could limit the current (like a fuse or another resistor). Resistors generally fail open but nothing is a given. Just analyse each circuit this way to determine where fuses would add safety or operational advanatages. Good luck, Buzzp

 

[REDDOG]

UL 508A requires each solenoid circuit to be fused.
A dual element time delay fuse of the supplemental type would be used such as Bussmann types FNQ, FNQ-R, FNM, MDL depending upon the load served.
Fusing could also be used to protect circuits leaving the panel. This feature allows quick trouble shooting to find short circuits in the control systems by isolation of the defective circuit. The size of the control wiring can be reduced in size to # 16 or # 18 depending upon the size of these fuses ie 5 or 3 amps. When you have to trouble shoot on system with one control fuse you will appreciate the above.
The main fuse and disconnect shall be sized for the load served. For a small load such as you refer the solenoids may be fused at 2 amps and the main may be fused at 10 amps class R. I assume the panel would be supplied by a 15 amp branch circuit and the panel disconnect switch would be a toggle switch. This switch would be on the line side of the class R fuse holder.
Hope this is of some help.

 

[dpc]

Separate fusing of solenoid valves is a good idea. Coil failures to ground are fairly common in these devices. With individual fusing, protection is better and troublshooting is much easier.

dpc