Shorting a fused switch for Arc Flash

[sobeys81]

Hi,

I am going through an arc flash remediation issue. The primary of the transformer is fed from a non fusible disconnect switch.

It is a 600 V primary system for a 225 kVA transformer.
They want to make sure that during secondary breaker operation on 208 V, arc flash hazard is limited to
Cat. 2

The suggestion was to use a fusible disconnet switch with current limiting fuse in parallel with non fusibible disconnect switch and open the non fusible switch.

Could you advise, if it is a smart and right thing to do.

 

[wbd]

What is the existing protection on the primary side of the transformer? It must be connected to a protective device somewhere.

Regardless, I don't think you will be able to get the arc flash to go to Cat #2 by using a fuse on the primary side. If the issue is working on breakers in a panelboard, you might be better off putting fuses in between the panelboard and transformer.

 

[dpc]

I agree with wbd that you will probably need to put an overcurrent device on the secondary side to get the arc-flash level down. There must be some type of primary protection (fuse or breaker), but this likely will be much too slow for secondary faults.

 

[bacon4life]

If this was strictly used for allowing work on the secondary, could the fuse be sized to the expected load rather than the transformer size? Seems like this would be similar to an alternate settings group in a relay that sacrifices coordination for speed.

 

[jghrist]

It's an interesting proposal. Normally, a high side fuse would have to be larger than needed for load current in order to withstand inrush. If it can be shorted and only in the circuit while live work is done on the secondary, the fuse could be smaller and may significantly reduce the clearing time. I don't think you would get into the current-limiting range of the fuse for a secondary fault.

 

[PHovnanian]

Consult the applicable electrical code and the AHJ. Paralleling a set of fuses with the non-fused disconnect will raise some interesting safety and code issues.

 

[Maak]

Current-limiting fuse is very fast IF the current is high, but if the current is lower than bolted-fault current, as it is often in arcing faults occurring in LV systems, ...

You could consider dedicated arc-flash protection products to guarantee high speed protection --> low incident energy.