Fuse cutouts / expulsion drop-out fuses

[Bung]

I am interested in hearing others experiences with single vented drop-out fuses (cutouts in US parlance, I believe). There is a perception in our organisation that the failure rates (number of non-fault related element breakages as a proportion of total operations) is too high, and that the overall failure rate (failures per year) is also too high. About 50-60% of all operations are classified by the Operators as "mechanical", "unknown", "fatigue" etc and are viewed as non-fault operations. The overall failure rate due to non-fault operations is around 0.5% per year on a population of over 50,000 individual fuses ranging from 5A to 100A rating.

The majority of the fuses are on three-phase Dyn 11kV/415V distribution transformers, upstream of the surge arrestors (which are mounted directly on the transformer tank).

What I am looking for is other people's experiences with these types of fuses.
* Do you have similar failure rates?
* Have you identified any particular weaknesses in the design?
* Do you "mix and match" elements / links, carriers and bases from different manufacturers?
* What happpens to the certification if you do mix 'n match?

As a contrast, I can't recall even one non-fault operation of a powder filled (fault current limiting) fuse in the last couple of years (in a population of well over 15,000).

Thanks in advance for your input!


Bung
Life is non-linear...

 

[Rocketir]

50-60% unknown cause is awful high. We occasionally have a weak link that just falls open or one that clears and doesn't fall down, but that is not the norm. We use a lot of fuses on our overhead tap lines and transformers. After a fuse operates we generally find a cause by patrolling the affected area. Also if we can't find a cause we have the patrol person take an amp reading on the line after they re-energize.
Are you guys keeping a log of where and when all the particular fuses operate? This could help locate problem areas for such things as animals (those poor squirrels just take a beating), excess slack in the line, etc.



Jasjit Khangura P.E.
Electric Distribution Engineer

 

[wbd]

Your unknown failure rates do seem high. The suggestion by Rocketir is a good one to try to correlate problem areas - such as squirrels, environmental (sea spray), windy areas with slack lines, etc. Also don't overlook your tree trimming program - come budget time that is usually a tempting cut (no pun intended).
You might look into what your practice is when finding a cut out blown - is the area surrounding the pole inspected for obvious causes, is the line patroled, etc.

Since your issue is mainly on 3 ph txf banks, is it confined to banks feeding a particular type of customer? Are the txfs load balanced? What is your fusing philosophy for txf's?
For instance: a fusing ratio can be calculated by dividing the fuse minimum melt current by the txf full load current. Then if a high ratio is used, it protects the system from a damaged txf but provides limited overload protection. On the other hand a low ratio provides maximum overload protection but the fuse can be damaged by inrush and surge currents.
wbd

 

[Bung]

I think we've got most of the obvious stuff covered. Animals, birds, trees in lines, blown surge arresters etc I class as genuine faults - they cause a short circuit which has to be cleared. Our philosophy is to protect the healthy system from damaged equipment, so as long as the fuse can see an LV terminal fault we're happy. We use 40E on 300kVA transformer (11kV), for example. Load picked up when transformer is re-energised is also recorded. Many times it is only one fuse of the three which breaks.

I have many examples of fuse links which have just broken for one reason or another. Some have had so little current in them at the time they opened that the element has not melted as it parted. On some you can see necking of the fusible wire, a bit of oxidation and that's about it. On others, the load current is a bit higher, so there is a bit of molten metal splashed inside the tube of the link. The elements seem to be a predominantly tin alloy (I think around 95% Sn from what I can coax out of the manufacturer). Sometimes the strain element slips out, sometimes it breaks (or is cut at the point where it is swaged into the link).

I suspect poor design of the link, but before I start to get too hard on the local manufacturers and start paying double the price for imported elements, I want to be sure I'm not just going to end up with a similar problem.

If I can get an idea of others failure rates, that would be very helpful. I don't want to know manufacturer names - I don't want to blacken anybody's name without a water-tight case.


Bung
Life is non-linear...

 

[jghrist]

See Field service evaluation of distribution fuse links; Westrom, A.C., Law, S.W., McKenzie, R.J.P., Patterson, R.C., Livesay, B.R., Kearney Nat. Co., Atlanta, GA; IEEE Transactions on Power Delivery, Oct 1990; pp 1855-1865; Volume 5, Issue 4

Abstract:
The real-world condition of one make and type of tin-element distribution fuse link after years of service on one system is explored to obtain a judgement regarding fuse-link deterioration as a cause for nuisance blowings. The microstructural analysis of all fuse links evidenced no structural changes typical of a fuse damaged by high-current pulse tests. The results of the melting current tests showed only 2% with a melting current reduction in time exceeding 10%. The occurrence of high rates of nuisance blowings reported on some systems may therefore be due to other causes or by the use of fuse types not studied. The authors postulate that recently published studies showing higher than expected magnetizing current inrush and transformer core saturation caused by low-current continuing lightning-current discharges may also account for nuisance fuse blowings. If so, the use of an ultraslow fuse link may be very effective

 

[Bung]

Thanks jghrist - sounds like a very interesting paper.


Bung
Life is non-linear...