Hi, I am looking for a source of information where I can find how to select the correct circuit protection for a control panel. I want to know how to calculate the short circuit current, when to use UL489 or UL1077 protections and when to use CB or fuses and how can I select the correct current rating of every one.
Thanks.

Arturo

 

You select the appropriate level of interrupting rating based on the available short circuit current. If you have enough information you can calculate the available short circuit current using Ohms law.

The choice of fuse or CBs is often a matter of either personal preference or cost. Fuses are often less expensive while it is more convenient to reset a tripped CB rather then replace a blown fuse.

 

The available short circuit current at most control panels is usually less than 10,000 Amps which is the standard for most CBs. I assume you do not mean a 'power panel'. If the panel is supplied by larger wire (350 MCM or larger) then the available short circuit current may be greater than 10,000 Amps in which case only fuses can interrupt a direct short.

The calculation of available short circuit current is a subject of many text books and can't be explained here.

The size of fuses or CBs are sized to protect the 'downstream' conductors so they can't overheat and cause a fire. There are recommendations for sizes in the National Electric Code (NEC).

Hope this helps.

Warren
http://www.pc-pid.com
"the PC-based Controller people"

 

Dear Sir:

We´re an Electrical Engineering Firm located at Chihuahua, Mex.

We´ve developed an Excel application to calculate this and other values using REAL short circuit contribution of electrical motors, which are main contribuitors on this kind of failure, that consideres specific impedance values and voltages to calculate final short circuit current of an specific panel.

This tool is a dedicated chart for power calculations, and I think is perfect for your calculation needs.

If interested, send us your comments to attached mail: [email protected]

Regards:
Oscar Rodriguez Mendoza