Input protection for 460V 3-Phase DC power supply

[rmore]

We have a 460VAC 3 Phase DC power supply in a design that has internal fuses protecting the line inputs. We have a 20A D characteristic breaker on the incoming power. Additionally, we have 3A fuses external to the power supply. The installation instructions for the supply state the following:

"The unit is protected with internal fuse (not replaceable) at L pin and it has been tested and approved on 20A (UL) and 16A (IEC) branch circuits without additional protection device. An external protection device is only required if the supplying branch has an ampacity greater than above. Thus, if an external protective device is necessary, or, utilized, a minimum value of 13A B- or 6A C- characteristic breaker should be used."

Are the fuses necessary? If not, can we get rid of the fuses if we use a different breaker?

 

[itsmoked]

Typically no. The fuses limit current during a fault whereas breakers don't. Breakers will eventually disconnect the system but probably not before something grenades inside. If the fuses blow the system will likely still need some serious work, the fuses may limit the damage unlike the breaker.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

The time difference between a fuse and a breaker is around 5x in speed. Vaporizing a teensy piece of fuse material is much quicker than flipping a mechanical switch into an OFF position. Your OP also shows a vast difference between the breaker and power supply ratings vs. fuse (20A vs. 3A), so for whatever reason you chose the 3A fuse, it might provide additional protection, particularly if the internal fuses are non-replaceable, and replacing an external fuse is much easier than replacing the entire power supply, or even repairing a non-replaceable fuse therein.

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