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

 

[Marke]

Hello rbulsara and PWR

The fuses can certainly be failing due to the extended start current, I have no problem with that and have recommended increasing the fuse size or trip time.
I am told that the fuses are rated at 400Amps and the start current is in the order of 1100 amps for about 20 seconds. Everything is, I under stand working as it should. There have been two instances where the fuses have failed with an obvious flashover across the top of the fuse carriers.
The fuses would normally be carying somewhere around 250 amps maximum.

One possibility is that there was something that caused the flashover and this caused the fuses to fail, or the other is that for some reason, when the fuses failed due to the starting current and perhaps a fast restart, the fuse bodies failed and this caused the arc.

Proveded that the barriers are fitted between the fuse carriers, contamination would have difficulty causing the problem, but so would the scenario of a ruptured fuse body.
I suspect that the barriers have been removed and this is the real problem with the arc.
I am still curious as whether and fuse that fails at say three times it's rated current should be expected to liberate ionised air into the surroundings.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

Yes marke..it can..1100A starting current for 20s with 400A fuse is NO reduced voltage start...what is the HP and voltage of the motor? What is the FLA?

There is not mystery here..u need to up the fuse or find a way to reduce starting current or time..

20 is too long a time to come to speed, you may be better of DOL starting this load with say slightly larger fuse, to reduce the start time...it appears that reduced voltage is not developing enough torque..

Flash over would be the result of the violent fuse blowing, not otherway around..in this case..it appears..