Ruptured 3.3kV fuses 1

[THD]

We have recently been rupturing 3.3kV fuses (not just striker pin but full flash over with shattered fuses and partially vapourised busbar spouts!!)

This happens when two 375kW DOL motors start (off the same 3.3kV fused contactor) for a conveyor at a coal depo.

This has happened intermittently and usually in the evening when its colder and quite humid. We have been scratching our heads for ages on this one...

Kevin Bosch
Rainbow Technologies

 

[itsmoked]

With head scratchers I find that often you have two problems which is why there is no obvious single cause.

Perhaps your fuses are truly under capacity for the interrupting currents. AND. Your conveyor is having aging problems that manifest themselves when the hardware is cooler. Like perhaps the belt is much tighter causing much greater drag. Or is less flexible, or bearing grease in a million rollers is stiffer. This is causing the fuses to blow but during the start current period the fuses are under rated in interrupting capacity.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[HienN]

May I ask? Only one fuse ruptured or the 03 fuse did?

Just check again the ampere rating of the fuse then compare with the starting current of the two motors. You can easily view it if the starting time is long enough. If the starting current goes beyond the fuse rating. As a result, these fuses may blow up. Can these motors start in succession?

I have one experience that blow up 22KV fuses. That is during the rain, water get into the core of the cable via connection. Then spears out in the switchgear and blow the fuses. Apply to this situation, it might caused by the dense moisture mixed with the coal dust (during the morning).

Hope this help.

Regards,

Hien

 

[alehman]

What Keith said. Fuses shouldn't rupture unless fault duty or votlage exceeds their rating. One bad fuse is understandable. Multiple ruptures = inadequate rating.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[RRaghunath]

THD,
Could it be that the fuse flashover is occuring if there is open command for the 3,3kV contactor while the motors are still running up!
If true, this is to do with switching surges and you may need to install surge suppressors in order to oevrcome the problem.

 

[THD]

To all, thank you for valuable posts. What is happening here is as follows: Striker pin operation on one or two fuses. Fuses replaced (not all) and then on start up ... BANG!!! HUGE damage. This has been happening since 2006 about once every 6months, and now more frequently (
About 18fuses later along with numerous changes of underpants ... no solution. We have checked the cables and motors with 7kV AC along with Tan Delta (all fine). This paralell arrangement is repeated 5 times in the coal terminal with the exact same cable length and sizes (50mm2). We also checked that the busbar spouts had melted but the cable spouts were fine?? Both motors start at the same time as they are fed off the same contactor (not unusual as its is done on many conveyors as well as dual motor mills on mines).

Kevin Bosch
Rainbow Technologies

 

[ScottyUK]

Are you sure you are buying genuine fuses? There have been some Chinese copies in the UK market which were essentially under-rated links with very limited breaking capacity. Visually they were quite convincing.



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

 

[THD]

Fuses are Bussman made in UK and guaranteed to be each individually xrayed before shipping...

Kevin Bosch
Rainbow Technologies

 

[ScottyUK]

Unless you buy them direct from Bussmann or X-ray them yourself I think you are overly confident - some of the big electrical distributors have been caught out by some of the fakes. They are *very* convincing.


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

 

[Kiribanda]

THD,

Fuses sometimes blow if a current little greater than its minimum melting current is allowed to pass through the fuse for a prolonged time.In your case it may happen during the motor start up.

Therefore, for everybody's interest, could you please post the full fuse details and motor data?

 

[HienN]

They said that if we do not have any trouble. We are going the wrong way. Sometimes,trouble itself is so irritating for a long time. I have some words as follows:

18 fuses replacement was not something leading to the fakes. Not only the strike pin but the fuse cover shattered, that could be a short-circuit or a ground fault happened somewhere in the power line running to the motor. If the problem caused by slightly overload or over-current, this would be clear by the contactor itself via protection relay.

I wonder what could make these fuses ruptured except the very high currents? Did you try with other higher rating fuse to avoid the heating effect during starting. I mean the characteristic curve of the fuse must be above the starting curve of the two motors.

You have checked carefully the cables, motors, tried every solution and nothing solved. I have one last and temporary solution. That is, replace the fuse with another kind of link (welding cable or something similar or the 3KV cable itself) if the fuse problem happened mostly in one phase. This would help you to sleep well for a time and think of the better troubleshooting. Over here, we are doing the same because we cannot find the suitable fuse to replace the shattered one.

Hope it helps,

Regards,

 

[Marmite]

I don't think I'd be sleeping very well if I'd knowingly replaced a fuse, which is a protective device, with a solid link.
The contactor probably will not be rated for fault interruption. That's why the fuse is there. You could end up with an even bigger bang and possibly loss of life. Don't do it.
Regards
Marmite

 

[HienN]

Marmite,

I understand what you mean. We still not sure about the system, but over here in our case, we use one switchgear (contactor type) (provide with protection relays – over-current and ground fault) and also provide with second protection by fuse links at the incoming that control the 6.6kV Compressor.

Due to the necessaries of operation, we need to seek for any solution that help to start up the plant. And that leads to using the solid one although I know that is not concern to engineering solution. As you predicted Marmite, I got a bigger BANG at the 22kV side. But it not caused by the solid wire, it caused by miss-operation (the earth switch is closed at the incoming side so lead the 22kV 03 phase down to earth. So…BANG… It was over 1 month with the solid wire… We have to change the brand-new switch if we still cannot find the replacing fuse.

If it is protected (at the outgoing of the transformer) by any protection equipments (fuse or relays…). The bang may still happen and also bring the heavier damage (power interruption happens all over the system).

If he confirmed that everything is fine. There is no way to show that why the fuse blew.

Anyway, don’t do it.

 

[rbulsara]

You can't expect different results by keep doing the same thing over and over again.

Hire some good consultant and have everything reviewed. There are things not obvious to readers here. Nor do I see any useful data posted so far to make comment of any substance.

Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

What makes anyone think they have the right fuses to start with? I see no data posted for motor srat up or running currents, no testing has been done, nothing.

Please do some testing or get someone in there that can and do not just put a shorting bar in place of the fuse, very dangerous advice.

 

[waross]

Is this installation the closest to the supply? It may be that these motors have more available current than the others by virtue of shorter feed cables. Don't look for contactors to open on instantaneous over currents. Their job is to monitor overload conditions over time and they don't act instantaneously.
Do they blow every time or occasionally?
How do they blow?
> Damage inside from more fault energy than the fuse barrel can dissipate?
> Flash over damage over the outside of the fuse barrel?
> Both of the above?
Is it possible that this location gets more coal dust and/or dew condensation than the other locations leading to flash overs?


Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

"What makes anyone think they have the right fuses to start with?" Agreed, although with fuse bottles breaking open I'd suspect a problem with the fault level rather than thermal rating. Bottles shouldn't rupture.


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

 

[PWR]

Medium voltage current limiting power fuses that rupture do so in most cases because they are attempting to open with too LITTLE current rather than too MUCH current. Most such fuses are intended to operate on short circuits only and must be protected below their minimum current operating point. In a typical motor starter the relay operating characteristic provides this protection. This application sounds like one where the fuse is undersized and the relay is not protecting it. The fuses are probably sustaining damage during each successful start and eventually fail in service (the smaller the load at the time the more likely rupture will result) or on a subsequent restart.

 

[LionelHutz]

You can't expect different results by keep doing the same thing over and over again.

Yes, 3 years and 18 fuses should prove it's time to review this switchgear and fix the problem.

Besides testing the cables, what has been done to try and find the solution? It's not a head scratcher until every possible cause of the problem has been investigated.

We had to deal with a customer years ago that had a motor starter problem and ran through 11 fuses trying to get the motor to run - he stopped when the company fuse stock ran out. He then wanted us to replace his blown fuses since our starter had the problem. Ya, that was going to happen. If something is wrong - stop and fix it.

Medium voltage current limiting power fuses that rupture do so in most cases because they are attempting to open with too LITTLE current rather than too MUCH current.

I would agree that it's just as likely this is a case of not enough current as too much current. Most medium voltage fuses are not full-range and if you don't hit them with enough fault current then they burn.