3 Phase Fusible Disconnect

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ken44

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I was asked if it is permissible to use to different types of fuses in a 3 phase disconnect?, the disconnect in question has two FSR-400's and one TSR-400. I would say that as long as the AIC is the same, then it would be permissible.
 
I don't know the terms FSR and TSR. But my inclination is to say that you cannot mix types. The real answer should be found in the disconnect manufacturer's documentation.
 
I'm with Charlie - don't know fsr or tsr.

Could they possibly be "FRS" and "TRS" ??
Those are Bussmann and Shawmut designations for their Class RK5 fuses.
As long as they are all Class RK5 with the correct amp rating, it should be ok.
You could even use a Littlefuse and get the trifecta.

db
 
I don't see any issue as long as the disconnect is rated for the fuse type that was installed and the fuse is suitable for the load. However, this could raise some arc-fault protection issues as each fuse has different curves, and the replacement of one brand with a different brand, even of the same class, may require different PPE to work on the downsteam parts of the system.
 
FRS-R is a 600V Bussmann type RK5 fuse.
TRS is a 600V Ferraz Shawmut type RK5 fuse.

As long as the ampacity are the same they will provide similar conductor overload and device protection.

However, their behavior may be different in their current limiting region. this difference means that the coordination of the circuit is compromised. Also, the difference may affect a fault clearing time which might impact the arc flash incident energy.
 
ken44 said:
I was asked if it is permissible to use to different types of fuses in a 3 phase disconnect?, the disconnect in question has two FSR-400's and one TSR-400. I would say that as long as the AIC is the same, then it would be permissible.
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Absolutely and unequivocally not.

The fuse manufacturers will not stand behind their product and each will blame the other. When replacing fuses it is also recommended to replace all three even in the case of a single phase failure.
 
weressl said:
When replacing fuses it is also recommended to replace all three even in the case of a single phase failure.
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Playing devil's advocate for the moment, how many people do replace all three fuses when one blows? I can't recall seeing anyone do that. Sometimes it's a challenge just to find a single good one :D.

BTW, I don't necessarily disagree with you on this. Just sayin' that I think it's uncommon.
 
zbang said:
Playing devil's advocate for the moment, how many people do replace all three fuses when one blows? I can't recall seeing anyone do that. Sometimes it's a challenge just to find a single good one :D.

BTW, I don't necessarily disagree with you on this. Just sayin' that I think it's uncommon.
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You may be right as far as small commercial and institutional users go, but not industry or larger systems.:roll:

See my sig....
 
weressl said:
You may be right as far as small commercial and institutional users go, but not industry or larger systems.
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My experience is that the practice is not uncommon anywhere. Yes, there may be specific facilities that are very rigid about this, but I have seen examples of mismatched fuses in maybe 10% of the systems I have studied.
 
When you get up to the 400amp size, fuses get a little pricey - the purchasing department may have some input.
It is recommended, but not necessary to use the same brands, and to change out all three fuses.
Just got this from a fuse manufacturer.

"It is recommended to use the same brand, but not necessary. It is also not necessary to replace all three fuses when one opens. Again, it is recommended, but not necessary. When you have three fuses from different manufactures, the fuses may not operate exactly the same. UL does regulate how the fuse is going to act at certain points of the curve, but that is about two to three points. Beyond those points, it is all up to the manufacturer."

db
 
In the current limiting range fuses from different manufacturers may behave differently. Every manufacturer says their fuse "selectivity tables" are only good for the specific fuses listed. Intermix manufacturers and your coordination may be compromised.

When UL tests equipment as being protected by fuses, they usually use a generic 'Umbrella fuse'. Per UL all normal fuses must offer at least the same protection to conductors and equipment as this generic fuse.

Fuses with 'loaded-link' (decades ago this was a common design) and replaceable link elements should not be intermixed with different designs. The construction of 'loaded-link' fuses, meant that they could change their melting characteristics if they experienced an overload but did not open, therefore it was recommended that all the involved fuses be replaced at the same time.
 
weressl said:
I love these excuses. :mad: If you have a spare fuse, then why don't you have the right one to begin with?
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Because purchasing bought the cheapest fuse that they could find, and the salesperson told purchasing that their cheap fuse is just as good as the expensive one. The people that really understand what is needed often have little or no input into the purchasing of the item.
 
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