Is a voltage overrated fuse ok?

[veritas]

I have a client who has 27kV fuses on an 11kV system (on overhead lines). I am not a fuse expert and would like to know if this is a good idea. The 27kV fuse will be longer in length than an 11kV one, and I am wondering how this affects fuse performance?

Thanks.

 

[Marmite]

It doesn't affect performance. Utilities like to standardise and reduce the variety of materials they have to purchase and hold on stock. I would speculate that your client has a largely 22kV system and that 11kV is a legacy voltage. If this is the case then the lines could have been built insulated for 22kV with 22kV rated fuses, with an eye on a future uprating project. I work in WA and the two local utilities follow that practice.
Regards
Marmite

 

[veritas]

Marmite, it is actually a mine site in NSW. They have no 22kV but a few 33kV and mostly 11kV reticulation. Actually, I am puzzled as to where the 27kV comes from as none of the networks in the area are 22kV. Will follow up out of interest sake. Thanks for the feedback.

 

[itsmoked]

Since these are outside I'm going to guess the higher voltage fuses are to help with creepage in exceptionally dusty mining conditions.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[EddyWirbelstrom]

When an arc is interrupted in a current limiting use, a transient arc voltage is created ( Ldi/dt ).
Fuses designed to interrupt the arc quickly produce a higher voltage than those which interrupt the arc slower.
This arc voltage must be limited to the system withstand voltage.
The arc voltage produced by the 27kV fuse would be greater than that of an 11kV fuse and may exceed the 11kV system withstand voltage.

 

[krisys]

EddyWirbelstrom,
Please note that the fuse characteristics will only provide the current and time curves, without mentioning the voltages. So how it can give rise to higher voltage?

 

[veritas]

I suspected that all may not be well by simply inserting a higher rated fuse, as seems to be the common perception. However, I found a text on fuses that stated the following:

"Peak arc voltage is dependant upon the number of constrictions in an element, because of the arcs in series. This gives a minimum value of peak arc voltage irrespective of the applied voltage up to a certain point. When this point is exceeded, the extra applied voltage can force the arcing to persist and produce burn-back and other effects which may increase with each incremental voltage, thus causing a larger peak arc voltage. It is therefore clear that a fuse of higher voltage rating should not be used to replace a blown fuse of lower voltage rating unless due cognisance is taken of the fact that its peak arc voltage will be greater. Peak arc voltage must not exceed the dielectric withstand of the system in which the fuse is placed."

The above is based on the fact that HV fuse designs (is not just simply a single solitary wire) but consist of a fuseable element with constrictions along its length. It is at the constrictions that melting first occurs, resulting in a series of small arcs along the length of the fuse. The arcing voltage can be around 600V/inch and is independent of the applied voltage as stated above. Note that this is a very generalised description as alloys are sometimes introduced to delay the melting time (using the M-effect) thus resulting in a fuse with a delayed characteristic.

 

[Marmite]

Are these drop out expulsion fuses, which is the norm? If so then the fuse element doesn't break the arc. The fuse element is under slight tension. When it blows the loss of tension releases the fuse latch mechanism and the top contact of the fuse moves away from the top fixed contact in an arc with the bottom contact as a pivot point. The arc current is interrupted as the top contacts separate. It is not interrupted by the fuse element. The length of the fuse tube and the insulation of the fuse mount has more to do with BIL and wet flashover voltage than nominal system voltage.

Regards
Marmite

 

[veritas]

Marmite, so you are saying then that a higher voltage rated expulsion fuse is ok? No problems with peak voltage? Is it a certainty that the peak voltage does not manifest itself prior to the fuse opening? I'm not a fuse expert, and yet I still have my doubts to tell someone, yeah go ahead no problems with using a 27kV fuse in an 11kV system. Maybe I have nothing to worry about! Not sure.

 

[jghrist]

There is no problem with a higher voltage expulsion fuse. There could be a peak voltage problem with current limiting fuses.