Replacement of old fuses in MV Motor Starters

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umichEE

Member
I have a lot of early 1980s 5kV gear with motor starters that still operate with the original 1981 fuses for short-circuit protection. While doing motor maintenance, starter maintenance, etc. some folks at the site are pushing to replace the old fuses at a cost of approximately $1500 per starter on the basis that the fuses are "weaker" and could nuisance trip or fail on startup, which would lead to operational delays, etc.

Is there actually value in replacing the fuses? Do fuses really "weaken" over time when they are properly sized and have seen only inrush and normal operating current? I've heard some war stories, but I'm not convinced that spending $1500 per starter across dozens-hundreds of starters is a cost-effective solution. Appreciate any input.
 

Jraef

Moderator, OTD
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Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
We make MV controllers, this subject comes up quite often. For the most part, "normal" use in properly applied equipment should not cause fuses to change their protection properties as they age. But that doesn't mean you can ignore them completely. Read this publication from the NRC, they studied this exact issue and came to a set of recommendations involving TESTING to identify any potential or impending issues, but then do NOT say to preemptively replace old fuses. And keep in mind, this is for NUKE facilities!

http://pbadupws.nrc.gov/docs/ML0213/ML021360517.pdf

Salient point from this study for those who don't want to read the whole boring thing:
When a fuse is located in an environment protected from heat, moisture and caustic fumes, and the fuse is applied in a circuit with an ampacity no greater than the fuse's nominal rating, the fuse should not open unless it is subject to an overcurrent condition. However, fuses are often installed in cubicles that contain heat-generating equipment, such as transformers, resistors, and coil-energized relays. These components elevate the cubicle temperature and can cause premature aging of the fuse. Exposure to long-term, elevated temperatures might potentially affect the rated capacity and the response time (warm-up time, interrupting ability) of the fuse. Fuse contacts (ferrules or blades of the fuse) may also be subjected to environmental corrosion, particularly for plants in coastal locations, where humid, salt-air environments are common. Loose, corroded, or contaminated contacts can lead to electrical arcing across the contact surfaces and may affect their ability to maintain electrical continuity.
 

johnmeto

Banned
Location
US
Perused this production from the NRC, they considered this careful issue and went to an arrangement of suggestions including TESTING to distinguish any potential or approaching issues.
 

just the cowboy

Inactive, Email Never Verified
Location
newburgh,ny
Why would they change now

Why would they change now

You say that they are from the 1980's. They have held for 35years, I don't think nuisance tripping is going to be an issue. I would be more worried that they don't opendue to oxidation or something.
 

Tony S

Senior Member
If a particular drive is causing problems then it needs investigation. Given the reasons you state, carte blanche changing of OCPD’s makes no sense at all.

What is being proposed as a replacement because you could be jumping out of the frying pan in to the fire.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If a particular drive is causing problems then it needs investigation. Given the reasons you state, carte blanche changing of OCPD’s makes no sense at all.

What is being proposed as a replacement because you could be jumping out of the frying pan in to the fire.
Meaning, "If it ain't broke, don't fix it!"?

That's a lesson many of us learn the hard way I fear... I did.
 

big john

Senior Member
Location
Portland, ME
Somebody is just trying to make their commission. Even if, hypothetically, a fuse does single-phase then your relaying should still prevent motor daage. If the protection isn't set tight enough for that, that's where I'd focus my concern, not on the fuses.

That said, the fuses should be getting ductored during every PM, and if one is beginning to fail you will clearly see it trending badly.

I test hundreds of MV fuses per year and just about the only time I see them at risk of premature failure is if they passed through-fault curret but it cleared before they could pop.
 

Haji

Banned
Location
India
umichEE:
Premature aging of your motor starter fuses does not appear to have taken place and maintenance also appears to be good based on long years of trouble free operation of fuses. So I think there is no cause for alarm.





Keep few spare fuses at hand anyway.
 
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