57X/R fuse problem 1

[SELEC]

I have a prolem here. The switchgear company has purchased all the fuses for their use based on the co-ordinaation study they did inside their switchgear, but the 57X curve is way above our upstream overcurrent protection curves. Even above the transfirmer curve. This means, this fuse is useless for the overload protection. Can we allow this? It is a tough decision to make. Who has a better solurtion for this problem?

 

[mpparent]

Can you give us a bit more info?

Mike

 

[SELEC]

Mike,

Thank you for the quick response.The problem is, 57X fuse is used for the backup protection of SR469( Overlaod and short circuit). The switchgear company did their co-orination study and decided to use the 57R, which i think can only provide backup protection for short circuit fault.The overload portion of the fuse is above the upstream SR750( switchgear main CB) and even the SEL-587( transformer relay). This means, if SR469 fails, the motor overlaod would trip amin CB first. Apparently it is not a good design, but my syperviser told me it is an acceptable practice in North America. I would shift the fuse further down to achieve a better co-ordination.

 

[davidbeach]

What is a 57X? It's not something I recognize, and I know you wouldn't fuse a 57, 'cause that would defeat the whole purpose of a 57. Why is a bad decision by the gear manufacturer your problem?

A whole lot more information would be useful, and this site isn't really supposed to be where you turn for every question that comes up in your work. It seems that you need to find a good reference on doing coordination work and do some studying. A couple of good references are the IEEE Buff and Brown books. After finishing those, the Red and Grey books would be a next good choice. You'll get a whole lot more out of those books than from snippets of material here.

I'm sure that others can also recommend other good references.

 

[SELEC]

It is ferraz Shawmut Fuse 57X (A051B).

 

[davidbeach]

Well, I missed the 20:37 response, must have been posted while I was typing. I'm having a hard time picturing your one-line in my mind, but am I correct that you have breakers controlled by relays and you have fuses? What kind of gear is there that mixes both fuses and breakers, and why would you want fuses anyhow? That question comes from a relay guy, so I may be a bit biased, but seems to me that either all fuses or all breakers would be better than mixing the two together. The mix seems like asking for trouble.

That aside, I'm curious about that fuse, which I can't find at

http://www.ferrazshawmut.com/products/mediumvolt.html.

Do you have a link within the Ferraz Shawmut site to that particular fuse?

I did find that 900E fuse of the other day, which I see only goes up to 5.5kV, but even at 4160V, a line to ground fault with enough resistance to limit the fault current to a mere 2200 amps would take at least 1000 seconds to get to the minimum melt curve. Lets say 1 ohm fault resistance, 2400V and 1000 seconds and say enough voltage drop to limit the current to 2000A; you would dissipate 4MW in the fault resistance and at 1000 seconds, provided it hasn't all burnt down yet, you would be at 4 gigajoules of energy dissipated. Ouch. On the other hand, a relay could detect that ground fault at 10s or 100s of Amps and have it cleared in a few cycles. But to each their own.

Don't get me wrong, fuses have their uses, but not nearly as many uses as the fuse manufacturers would like everybody to believe.

 

[RalphChristie]

I would also like to see more information, but at first glance I just want to state:
Do not confuse overload protection with overcurrent protection.

 

[vic3fan]

RalphChristie is right, fuses are for short-circuit protection only. Overload protection is provided by thermal relays, which are based on a different operational principle. Using the same fuses for both protection types is certainly risky because the overload current is often very little higher than the rated current. Dimensioning the fuse for overload protection means choosing a fuse very close to the rated current the equipment operates at. This will cause fuse blows very, very often, especially for motors that take a higher current when starting. To have a smooth operation of the equipment, the segregation of overcurrent (short-circuit) protection and overload protection is required.
If the fuses were dimensioned based on certain hypothesis, then they should be used for that situation. If the conditions change (i.e. the characteristics are far above the protected equipment needs) then do not use these fuses: order more appropriate ones. The existing fuses should be kept for other cases, where the load demands a overcurrent protection close to their characteristics. Check with the manufacturer, maybe they have a return policy that allow you to recover some funds. If not, that's it, you have to keep them or you can try some other tricks like selling them via E-Bay or advertising them on your website.
Keep in mind that the fact the fuses were already chosen based on certain criteria does not eliminate your responsibility to assure a proper protection to the electrical equipment, even if you have to spend other money for that.

 

[SELEC]

Thank you for you guys. Now I understand more the common practice here in North America. Fuse is good for short circuit protection but can provide the backup protection for overload relay.Our Motor circuit overload is protected by SR469. The main overlaod can be protected by SR750. I believe it should be an acceptable practice.