Forums » Electrical Engineering

    • 5 posts
    August 15, 2019 1:21 AM PDT

    What is the general rule to size a fuse. 200kVA, 525A, 220 Volt. The minimum SLG fault is 950A?

    • 33 posts
    • 103 posts
    August 15, 2019 9:23 AM PDT

    The primary concern is that the conductors are protected against over current. The fuse should clear before the conductor exceeds its insulation temperature rating.

    The next thing to consider is the characteristics of load. You do not want the fuse to open unnecessarily.  For instance, with a motor or transformer load the fuse must time after time withstand the over current during inrush.

    • 156 posts
    August 15, 2019 10:33 AM PDT

    if you want to use time delay fuse, time-delay fuses may be rated at 125% of transformer secondary current

    • 75 posts
    August 19, 2019 10:07 AM PDT
    Matt Franks said:

    What is the general rule to size a fuse. 200kVA, 525A, 220 Volt. The minimum SLG fault is 950A?

    The general rule is to size the fuse such that the trip curve is outside of the start up currents, inrush, locked rotor, steady state currents.  The conductors are sized such that the conductor damage curve is outside of the fuse trip curve.

    Under the NEC, most of this work is done for us in form of tables - probably true of the IEC as well.

    Generally speaking (or typing as the case may be) the fuse is not there to protect the end device.  The OCP purpose is to protect the cable insulation if possible, and if not, get the circuit open to stop feeding energy to the fire.  One might say the fuse protects the structure.


    As for specifics, as Steve and Hameedulla discussed, the context matters:  Transformer secondary, motor feeder, xfm primary, industrial heater, rectifier for metallic plating, alien containment field in Area 51.

    A bit more information will get you a lot more constructive assistance.


    This post was edited by Carl Coulter at August 19, 2019 10:08 AM PDT