HRC Fuse failures

[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 ruptureing, allowing ionized gas into the surrounding area and causing an insulation breakdown between two phases on the fuse holder.

 

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? ;p;

 

Best regards,

[Carl]

Hello

 

Sounds interesting that the rupturing of a fuse, can possibly cause ionization of the surrounding air. I can understand the circuit trying to maintain the current – due to a collapsing field, but then explode and ionize the surrounding area sounds a bit strange, maybe it’s carbonizing the surrounding area causing “tracking”.

 

 

Regards

Carl

 

 

 

[GGOSS]

Marke,

 

Who is the fuse manufacturer? What is the part number of the fuse? Although these questions have a commercial bent, the fact is that fuses should not explode. In fact, there has been a recent recall made by one manufacturer where the cited reason for the recall was the possibility of explosion.

 

Regards,

GGOSS

[jraef]

The part about the soft starter artificially limiting current in the circuit has been a bugaboo for a number of things in my experience, such as not allowing ground faults to be detected by SCPD equipment (when no GF sensor was present). But I agree that a fuse should not rupture. Period. What if it were a high resistance ground fault instead of the soft starter that was keeping current high, but not too high, for an extended period? Fuses should clear when necessary, but exploding is not acceptable IMHO. I have had MOVs vaporize in an enclosure and deposit material and ionized gas so as to cause a flashover, but NEVER a fuse.

[jraef]

Hmmm... that said, I found this interesting tidbit, alluding to the fact that maybe it is because it is an HRC fuse. Perhaps this is something I didn't know about HRC fuses in particular.

 

The HRC-fuses serve only as a protection against short-circuit, the protection against overload is performed by other current safety devices at the secondary side (low voltage side). During the failure of these secondary sides overcurrent protecting devices or in the case of a failure of the interconnected devices such as for example distributing transformer, a hazardous gap occurs in the overall protection range inasmuch the conventional HRC-fuses cannot interrupt currents whose value is below the triple nominal value of the fuse. In order to eliminate this safety gap, the so-called full range fuses have been recently developed, all representing a combination of a HRC-fuse with an overload fuse. This known compound device, however, is very expensive.

Bold emphasis added.

 

This actually came from a patent aplications for an HRC fuse, here is the link.

 

http://www.freepatentsonline.com/4617544.html

[kens]

Hi Marke, regarding the ss current limiting causing the fuse to physically rupture all i can say is bull *beep*. The whole point of HRC fuses is to clear faults in a safe manner in a high fault current enviroment. There is no difference between an extended overload caused by a soft starter and any other sort of extended overload. The fuse clearing is one issue(perhaps undersized as you say) and the explosions are another. As much as I like explosions I dont like them much in switchboards!

Ken

 

Was posting this at the same time as you jraef. I read that as being more concerned with High Voltage so perhaps not an issue with this case?

[Carl]

HRC fuses contain an arc quenching medium would the lack of this medium cause it to explode on a surge current. Done a google on exploding fuses and there is a lot of talk on exploding fuses due to either the incorrect rating or use of an incorrect fuse.