Fuses burned out

[lume7006]

Hello friends,

At one industrial facility, there are some capacitors which include their tunning reactor (at 4th component) to compensate PF, they are located at medium voltage.

However, very oftenly the fuses of the "capacitor+ reactor circuit" are burn out, I mean, they are not blown, after some hours they are burned, even the outside!
We have already done some measurements, and harmonics levels are quite low, in voltage and current.

Has anybody experienced something similar?
Any ideas will be welcome!

Best Regards,

 

[rbulsara]

Loose connections, bad holder are the most likely reason. This assumes that fuses are properly rated and of right type.

Can you post pictures?
Have done a thermographic scan when in operation?

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

As Rafiq said, loose fuse holders.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I too agree.

Too many times people experience a series of blown fuss and just keep replacing the fuses without inspecting the clips. The heat can weaken the pressure springs, which increases the contact resistance so the fuse starts heating up faster every time.

So at least inspect the fuse holders but if it were me, I would just do a prophylactic replacement now. Then you should try to determine why they kept blowing in the first place. My first suspicion would be resonance and/or high harmonics. Although you may think you have already eliminated that, is it possible that some piece of non-linear equipment, such as a VFD, comes on when you are not looking? Did you do a recorded analysis over time?.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[lume7006]

Hello everybody,

I thank your kind answers and comments!

The final customer has done some measurements over time, despite having some "inconsistencies", they do not show a significative amount of harmonics but, fuses keep burning.

We had previously done a resonance analysis and found the system prone to several resonance frequencies, however, even with installed relays we have taken the values of harmonic registered and nothing excessive.

I remember that even some capacitors have been replaced by the manufaturer and the problem continues.

If you had another clue or recommendation, please, let me know it.

 

[rbulsara]

Did you read the previous comments? Resonance will blow the fuses not burn as you describe.

Rafiq Bulsara

http://www.srengineersct.com

 

[lume7006]

Dear Rbulsara,

I thank your post!

Yes, I did read the previous posts, maybe I was wrong writing my last post because, it may be not correct at "this stage" of the conversation.

 

[oldfieldguy]

If the fuses are visible in normal operation, view them with an infrared camera. Good fuse clips will be relatively cool. compare like objects. This might give a bit more data on the failure mode.

old field guy

 

[waross]

Some possible causes of burned fuses:
Loose fuse holder clips.
Loose switch blade clips heating and conducting heat to the fuses.
Improper cable installation. I have seen installs with parallel feeds where one conduit contained "A" phase and part of "B" phase and the second conduit contained part of "B" phase and "C" phase. The unbalanced magnetic flux heated the conduits and the cables conducted the heat to the switch. The switch was destroyed several times by heat accelerated corrosion, and the fuses would fail from time to time.
Using the old "Renewable" fuses. These were bad news. Failures were common and burning of the fuse barrel was not uncommon. Use one-time fuses.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TurbineGen]

Can you take the equipment out of service and perfrom a contact resistance test? Check the contact resistance betwen the cable and the fuse holder as well as the fuse to the fuse clamp. I don't know what rating the fuses are, but anything above 250 micro ohms contact resistance would be a red flag to me.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[waross]

You can do a voltage drop test across each contact point (each end of the fuse holder and the switch pivot point and jaws) with the equipment in service. Check the current and calculate the Watts loss at each point.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[lume7006]

Thank you all for your ideas.

I will talk to the responsible in order to perform some tests and in that way evaluate what you politely have recommended.

Regards,

 

[oldfieldguy]

lume-

Let us know what you find. WE all learn much from the misery of others.


old field guy