Why do we still use fuses? Question, not a statement!

Status
Not open for further replies.
Brkr. Coordination

Brkr. Coordination

Even with electronic trip circuit breakers you would still have to look at coordination plots to makes sure there is no overlap just because it is electronic trip does not mean it will coordinate, mains to branches will almost still overlap in the instantaneous region of the curve, and yes you could dial up the instantaneous but what if your available fault current is say 30KA in order to coordinate you turn up your breaker settings high enough where you are beyond the available SC then you have unintentionally disabled the instantaneous of the device and if it is a UL489 breaker you might now take more than 3 cycles to clear and be putting the protection of the equipment and the breaker in jeopardy.
 
Mike01 said:
... you have unintentionally disabled the instantaneous of the device and if it is a UL489 breaker you might now take more than 3 cycles to clear and be putting the protection of the equipment and the breaker in jeopardy.
Click to expand...
If it is a UL489 breaker it will always have an instantaneous protective override function even when the adjustable Instantaneous function has been turned off. This protective function is part of the UL489 standard. Historically manufacturers have never 'reported' or shown this value as a separate part of their time-current curves, although some are now doing so.
 
Talking about the molded case circuit breakers vs fuses. A small molded case circuit breaker will actually race a current limiting fuse if they are in series with a fault. The contacts of small molded case circuit breakers use the magnetic forces of the fault to blow apart the contacts. This is why a series combination must be tested. The only way to engineer a series combination fuse with any circuit breaker is to be assured that the circuit breaker will remain passive in the first ? cycle of the fault. :)
 
Prene13 said:
Fuses will trip alot sooner then breakers (1/4 to 1/2 second sooner).
Click to expand...

I thought fuses opened in the first 1/4 to 1/2 cycle of trip current.


charlie said:
The only way to engineer a series combination fuse with any circuit breaker is to be assured that the circuit breaker will remain passive in the first ? cycle of the fault. :)
Click to expand...

That is a fast breaker!

boboelectric said:
When a breaker trips,I'll try one restart,then troubleshoot.
Click to expand...

Navy troubleshooting procedure allows one free breaker reset or fuse replacement. I took this policy home with me and I used to automatically reset breakers. I'm not so quick to do so anymore. Use sound judgment when resetting breakers.
 
boboelectric said:
When a breaker trips,I'll try one restart,then troubleshoot.
Click to expand...

NFPA 70E 2004 130.6 (K)
Reclosing Circuits After Protective Device Operation.
After a circuit is deenergized by a circuit protective device, the circuit SHALL NOT be manually reenergized until it has been determined that the equipment and circuit can be safely energized.
 
jeremysterling said:
I thought fuses opened in the first 1/4 to 1/2 cycle of trip current.
Click to expand...
Only if there current is high enough for them to enter their current limiting range.

Quote:
Originally Posted by charlie
The only way to engineer a series combination fuse with any circuit breaker is to be assured that the circuit breaker will remain passive in the first ? cycle of the fault. :smile:

That is a fast breaker!
Click to expand...
Actually many, if not most, molded case breakers built since the early 60's may begin to open this fast, if the current is high enough.
 
Many circuit breakers are a current limiting design as described, and many can be purchased with current limiting printed on the nameplate. The only time I use fuses is when the AIC needs to be >65kAIC, and even then I try to sue breakers. Some of the newer insulated case breakers can get me to 200kA without fuses.
If the application is critical, I prefer CB's. I think the selectivity requirement in 700 and 701 is no good because many people go directly to fuses because it is easier to coordinate, but sometimes a nightmare to reset when you don't have access to the correct replacement.
 
IMM_Doctor said:
NFPA 70E 2004 130.6 (K)
Reclosing Circuits After Protective Device Operation.
After a circuit is deenergized by a circuit protective device, the circuit SHALL NOT be manually reenergized until it has been determined that the equipment and circuit can be safely energized.
Click to expand...

That comes from OSHA 1910.334, here is the rest of it.

(b)(2) "Reclosing circuits after protective device operation." After a circuit is deenergized
by a circuit protective device, the circuit protective device, the circuit may not
be manually reenergized until it has been determined that the equipment and
circuit can be safely energized. The repetitive manual reclosing of circuit breakers
or reenergizing circuits through replaced fuses is prohibited.

Note: When it can be determined from the design of the circuit and the
overcurrent devices involved that the automatic operation of a device was
caused by an overload rather than a fault condition, no examination of the circuit
or connected equipment is needed before the circuit is reenergized.
 
zog said:
Note: When it can be determined from the design of the circuit and the
overcurrent devices involved that the automatic operation of a device was
caused by an overload rather than a fault condition, no examination of the circuit
or connected equipment is needed before the circuit is reenergized.
Click to expand...


Are there Circuit Breakers that have seperate indication when opened that an overload vs. a short circuit caused the breaker to open?

Are there fuses that have different indication the root cause of being open? (Without cutting it open to see inside)?

Just curious.

Thanks
 
IMM_Doctor said:
Are there Circuit Breakers that have seperate indication when opened that an overload vs. a short circuit caused the breaker to open?
Click to expand...

Yes, all modern power circuit breakers have this function and all power circuit breakers can be retrofitted with modern trip units to provide this (and many more) functions. We do a dozen or so of these retrofits everyday. Play with this http://www.utilityrelay.com/ACProSimProg.htm

IMM_Doctor said:
Are there fuses that have different indication the root cause of being open? (Without cutting it open to see inside)Thanks
Click to expand...

Not that I know of.
 
zog said:
That comes from OSHA 1910.334, here is the rest of it.

(b)(2) "Reclosing circuits after protective device operation." After a circuit is deenergized
by a circuit protective device, the circuit protective device, the circuit may not
be manually reenergized until it has been determined that the equipment and
circuit can be safely energized. The repetitive manual reclosing of circuit breakers
or reenergizing circuits through replaced fuses is prohibited.

Note: When it can be determined from the design of the circuit and the
overcurrent devices involved that the automatic operation of a device was
caused by an overload rather than a fault condition, no examination of the circuit
or connected equipment is needed before the circuit is reenergized.
Click to expand...


ZOG,
Thanks for the link and info on breaker retrofits.

I noticed sometthing interesting. I hand typed the text direct from NFPA 70E that indicates that manual reclosure "SHALL NOT" happen until diagnostic methods prove the equipment/cicuit id safe. I do nto have the OSHA document, does it say "may not" or "shall not"?

IMM_Dr.
 
IMM_Doctor said:
ZOG,
Thanks for the link and info on breaker retrofits.
Click to expand...

Sure, let me know if you need any more info on those, we are the factory installer of that product.

IMM_Doctor said:
I noticed sometthing interesting. I hand typed the text direct from NFPA 70E that indicates that manual reclosure "SHALL NOT" happen until diagnostic methods prove the equipment/cicuit id safe. I do nto have the OSHA document, does it say "may not" or "shall not"?

IMM_Dr.
Click to expand...

That quote is cut and pasted form the OSHA doc.
 
fuses vs. breakers

fuses vs. breakers

Regardless of which type of OCP you use. if your replacing fuses or resetting breakers at a "regular" interval then there is something causing these conditions. If no fault then no OCP problems
 
Status
Not open for further replies.
Top