HRC Fuse failures with soft starter 3

[Marke]

Hi
I have recently been asked to look at a situation where there is reported failure of the HRC fuses in that the fuses supply a motor driving a high inertia load and started by a soft starter.
I believe that the fuses may be a little on the light side for the starting current and time of the machine.

The report is that the fuses are failing at times during start, and that the fuses are then physically rupturing, allowing ionized gas into the surrounding area and causing an insulation breakdown between two phases on the fuse holder. I only have the written report to comment on, no pictures or failed fuses.

The argument put forward is that because the current is limited by the soft starter, the arc is being sustained for a long time and the energy build up inside the cartridge is causing the gas to leak. If the current was higher, the arc would extinguish quicker and the gas would not leak.
The fuses are a blade type fuse in the correct holder and rated at 120KA.

Any comment or experiences?

Bewst regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Hi Mark,

If the fuse cannot stand what the soft-starter can take, isn't there something wrong with the coordination then?

I mean; normal load currents should not blow the fuse, but overcurrent should. If there is a "half-way" overcurrent that the semiconductors can survive, but still melts the fuse elements - and causes an arc! - then something must be wrong. A sand filled HRC fuse shall melt and quench the arc. And it should do it even if the overcurrent isn't a dead short current. There are so many situations where overcurrents are in the middle range and where fuses blow reliably without causing secondary damages.

Would a fuse with a higher rated current not protect the semiconductors reliably? I mean a fuse that is selected not to blow under any normal setting of the soft-starter and any acceptable load type.

A fuse that has not lost its sand should not do this. Has it happened several times? What make are the fuses? Rated voltage and mains voltage?

Gunnar Englund

http://www.gke.org

 

[Marke]

Hi Gunnar

Yes I agree, the fuses selected are too light and I have proposed that they are upgraded.
My concern is the statement that the fuses were carrying an arc for an extended time because of the low current and the result was a higher energy dissipated within the fuse than would be the case with a short circuit and this "higher" energy was causing the body of the fuse to leak ionized gas which was then causing a failure!!

From my experience, the stresses on the fuse body are highest with the highest fault current, not the lowest, but perhaps I am wrong??

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

How about posting the ratnig of the fuses, the motor and the type load beig driven, if known, the starting current and the duration..


Is the start ramp adjusted correctly?

 

[Marke]

Hello rbulsara

The question is not about the rating of the fuses, I know that is a problem. The problem that I am looking for an answer on is : the fuses are then physically rupturing, allowing ionized gas into the surrounding area and causing an insulation breakdown between two phases on the fuse holder.
I have not experienced the fuse cartridge rupturing on fuse failure and causing a flashover between phases. I would certainly not expect this when the current is low relative to the fault current rating of the fuse. (about 1% of rated fault current)
If this was a potential problem, then it must have ocured at other times with traditional motor control.
I do not believe that I am being given the full story here and I have my own theories as to what is happening, just interested in comments from others.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Marke,
What would be the difference between an artificially limited current through the soft starter, and a high resistance grounded fault somewhere else on the load side of the fuse? I think if their theory were correct, there is big trouble in the fuse world! In a resistance grounded system where fault current could be limited, they are saying that the fuses could rupture and cause a flashover? Seems unlikely to me that the fuse mfgrs would overlook that detail. Exceeding the I^2t rating of the fuse should just be a plain and simple event, regardless of how fast or slow it got there. Once the fuse link opens it should extinguish the arc and be done with it, and the fuse itself should withstand the the forces created by any kind of fault within it's rating.

I agree with you. Lower currents should actually be easier on the fuse than high energy faults, but they build them to withstand the higher energy because they need to prevent a distructive failure, not because they require higher energy to make it work correctly.

I have seen some unexpected phenomenon that occurr only in soft starters and thyristor power controllers because of the artificially limited current, but never a fuse rupture and flashover.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[ScottyUK]

Are the fuses in question from a reputable manufacturer, or are they copies from, say, China?

There have been a number of warnings about 'bargain' fuses of Chinese origin which look the part but have no breaking capacity. Affected products range from small BS1362 cartridge fuses up to MV distribution fuses, and they are visually very convincing copies of big-name manufacturers products.


----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[Skogsgurra]

Thanks for that information, Scotty. Well desrved star.

How can one distinguish between good and bad? Any link?

Gunnar Englund

http://www.gke.org

 

[Marke]

Hi ScottyUK

Now that is an interesting slant on things, but if they rupture on low fault current, what would they do on real fault currents??
I am not convinced that the fuses are acturally rupturing. This is the report from site and I can not get there.
Evidently there has been a splash across the top of the fuse carriers and the dialogue was to prove that fuses should not be used with soft starters, the fuses should be replaced by MCCBs!!
I believe that the barriers may have been removed from between the carriers and that there has been something else that has caused the arc, and this has caused the fuses to blow, not the other way around.
If the barriers were in place, the arc would not occur between the carriers. The client insists that the barriers are in place and the ionised air has caused the arc between the phases. I believe that if there was ionisation from one cartridge or carrier, it would be channeled upwards by the barriers and would not get near the second phase.
We are missing the correct story here, but I was intrigued by the explanation for the arc, being caused by gas venting from the fuse due to the low fault current.
I guess we will never know the facts, but it was a good story!!

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ScottyUK]

If you type something vaguely like 'counterfeit chinese fuses' into Google there are a lot of hits. The specific article I'd read in the trade press was from BEAMA, and it prompted me to dig a bit deeper. The more I read, the more scary it becomes, especially for the cut-throat domestic / commercial / light industrial end of the market where price is everything to the contractor.

http://www.beama.org.uk/Newsletter/Issue2/articles/beamainstallation.htm

Identification seems to be major problem. I guess demanding tracability certification is one way, and using reputable suppliers.

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[rbulsara]

marke:

All those symptoms are the fuse being way undersized and underrated for the available for short circuit current.

scottyuk is probably right. Oneway to test is install fuses of similar rating, but higher interrupting capacity(200kA should be enough) from a known good source and of a reputable mfr and see what happens.

Also placing some barrier between the fuse holders helps too, as many assemnblues put the fuse holder too tight together. Barrier only helps from flash over, fuse will still open.

 

[PWR]

There are many fuse designs both for low voltage and medium voltage current limiting fuses that have no ability to interrupt low level faults or overloads. Contrary to what would seem obvious, current limiting fuses have an "easy" time clearing high level faults (within their rating) and as the fault current gets lower can reach a point where they simply cannot successfully generate sufficient voltage to "clear" the arcing that is occurring internally. I have seen many cases of fuse failures where rupture of the enclosure is followed by phase to phase and phase to ground faulting. Take a look at some current limiting fuse characteristics and you will find many that have no characteristic shown in the overload/low fault region. It isn't there because they are never intended to operate in that region (the motor overload must operate or some other device must protect the fuse in that region).

 

[jraef]

PWR,
I knew that MV current limiting motor fuses must be coordinated with the overload device because they may not interrupt a low level fault, but it is news to me that this is the case in LV as well and I have never heard the rupture outgassing issue before. This means that more carefull consideration must be made when selecting fuses, not just for the interrupting capacity and let-through, but also in coordination with the overload device since it will be required to interrupt the low level faults without help from the fuses. Is that only on current limiting fuse designs do you know?

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[rbulsara]

PWR:

Low current faults will not rupture the fuses. The very reason they do not operate is because there is not sufficient current and fuses or a breaker will carry low currents within their rating safely, whethe be it due to a fault or normal operation.

Arcing taking place at the location of low level (high impedance) fault has nothing to do with the OCPD. OCPD are not supposed to open low level faults.

Also statements like "not having enough voltage to clear the arc" etc. do not make any sense. Voltage if present help establish the current, not quench it.

 

[PWR]

Rbulsara: "Low current faults will not rupture the fuses"
Perhaps you can furnish the operating characteristic in the region where the manufacturer does not deem to offer one?

I repeat: I have seen many cases where current limiting fuses have ruptured at low levels of current. They may do so because they were previously exposed to a current-time combination that almost caused them to blow (like a transformer inrush) and thus caused some of the elements to be damaged. Others were simply misapplied as to their continous current rating. Another scenario I have seen, and that is perhaps the case at hand, where they encounter an unintended operating point with current above their continuous operating range but below the values at which they were intended to operate.

While by no means am I a fuse designer, I am aware that fuses do generate sometimes harmful voltages in excess of the system voltage in the process of an interruption. I also am well aware that arcing can take place internal to the fuse. Perhaps you might want to investigate those phenomenon further so that they will no longer "not make any sense" to you.

 

[Marke]

Hi Guys,
In this particular instance, the fault is certainly being interrupted, so there is no problem there. The question that I have is the statement from site that due to the low current being interrupted, the fuse body is rupturing. Is this a vaild claim?

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

marke: No to your last question.

 

[rbulsara]

pwr: you are just confused..

If you have a fuse rated 400A, and fault current of 350A, it will not open, nor should it...

OCPD do not open becuase they detect the 'fault' but becasue they detect overcurrent, for whatever reason..

You can always have a fault below rating of an OCPD and it will not open. That is why other sensitive schemmes such as GFP or GFCI or newer arc interrupting devices are emplopyed where it is considered helpful.

 

[rbulsara]

marke: since you provide or have only limited information, it precludes many other possiblities too..

There can be a phase to phase flash over because of contamination on the fuse blocks..etc. May be the starter was not set right and it may be starting the motoro at full or near full voltage and causing much higher inrush than the rating of the fuse, this together will high inertia load will cause the effect of a short circuit..

As I said..there may be much more or other than what you think....

 

[PWR]

Jraef: I have only seen the problem surface with current limiting fuses.

Marke: What Cat. No. fuse is being used and what is the operating voltage?

As I reread your original post the thought occurs to me that perhaps the extended start is damaging the fuses and they are then opening (and sometimes rupturing) at low load. The most common rupture mechansim that I have observed is damaged fuses(due to approaching the operating point and then backing off) later overheating at light load. If that is occurring, you should see some of the fuse bodies overheated. Also, to determine if a fuse has been damaged, measure the resistance with a digital low resistance ohmmeter (DRLO). Fuses have remarkably precise internal resistances (often a group of three will read within a few microhms of each other). Any internal damage is easy to spot due to a significant increase in resistance.