True or False Current limiting?
True or False Current limiting?
(OP)
I thought if you wanted to decrease the available fault current on a system, you could place the appropriate current limiting fuse ahead of it to limit the let-thru (this is what I have been doing).
Recently I was told that you can not do this without using a manufacturers published series rating chart.
True or False?
Regards,
TULUM






RE: True or False Current limiting?
The interaction between an upstream fuse and a downstream circuit breaker during fault conditions is dynamic and complex. The old "up-over-down" method we used to use for applying current-limiting fuses has been shown to be unreliable. The circuit breaker itself has substantial impedance during faults and sometimes the actual fault is much less than expected and the fuse doesn't operate quickly enough to limit current, but the breaker is still over-dutied.
Also, keep in mind that for a fuse to be considered current-limiting, it must clear the fault within the first half-cycle of the fault. If the prospective fault current indicates a slower operation than this, the fuse isn't providing current-limiting, at least per UL's defintion.
RE: True or False Current limiting?
Don
RE: True or False Current limiting?
dpc
RE: True or False Current limiting?
RE: True or False Current limiting?
However, I have only applied current limiting fuses for the purpose of decreasing fault levels on static systems such as cables and bus. Dynamic systems, such as breakers are another story all together.
Regards,
TULUM
RE: True or False Current limiting?
One thing to watch for, the current limiting is based on the fault current, at the fault, not the prospective fault current at the fuse location. So if you have a fuse applied at a location where there might be 60kA available, and the fuse is current limiting for currents above 50kA, then if you have a circuit the due to impedance limitations will only draw 48kA, there will be no current limiting by the fuse. The fuse may well still blow, but not fast enough to count as current limiting.
RE: True or False Current limiting?
RE: True or False Current limiting?
I concur with David. I'm a PE, but I wouldn't want to approve such an arrangement without a lot of careful analysis.
RE: True or False Current limiting?
Since the vast majority of short circuits are at the far end of the branch circuit the breaker will open the fault before the fuse does. For those few shorts that are close to the panelboard you would want the panelboard to be shut of by the feeder overcurrent device anyways.
The better circuit breakers such as SquareD and General Electric have a 1/2 to 1 cycle clear time for any short that is 1000% of the overload rating of the circuit breaker. This works well with 200 amp class J fuses. A 2 or more cycle clear time is going to have coordination problems.
There are fewer series ratings for 400 amp class J fuses but it turns out that 200 amp equipment is 1/3 the price of 400 amp. A 400 amp 4 wire wye feeder that is derated because of harmonic in the neutral is really a pair of 200 amp lines in parallel. Makes more sense to wire as a pair of 200 amp lines feeding 2 panelboards.
For shorts that are above the interrupting rating of the breaker the class J fuse of correct size will HELP the circuit breaker open the fault. The key is that the let through of the fuse has to be less than the let through of a 1/2 to 1 cycle clear time breaker at maximum rated intrrupting level.
For any circuit breaker that is still being made you should be able to get a UL listed series rating from the manufacturer. If your breakers are no longer made you have a Spare Parts Conspiracy problem anyways.
RE: True or False Current limiting?
UL listed fuse/breaker series ratings have been permitted by the NEC for many years. I don't have a problem with using those listed combinations. The new 2005 provision allows a PE to evaluate and use UNLISTED and untested series combinations. That can be dangerous. I wouldn't do it unless I was very confident of the characteristics of the devices involved.
RE: True or False Current limiting?
The only breakers I know of that might be worth considering for "evaluation" are old power breakers. There is really no expectation that a molded case breaker with a magnetic trip element won't start to do something in that critical first 1/2 cycle. At that point all bets are off.
I don't have ready access to the IEEE Blue Book at the moment, but there is a discussion in it that became the basis of the 2005 NEC change. A I remember reading the material in question, it seemed to be more cautions than guidance.
Why chance it, if the existing installation suddenly finds itself subjected to more fault current than it is capable of interrupting/withstanding, either upgrade the system or reduce the available fault current back below the original equipment ratings. A 1:1 transformer can do wonders in limiting fault current.
RE: True or False Current limiting?
I'll dig out my Blue Book when I return to the office.
RE: True or False Current limiting?
RE: True or False Current limiting?
RE: True or False Current limiting?
The problem is that these "let-through" curves are just not reliable when you start looking at real-world situations.
Similar curves are published for low voltage fuses. But when the fuses are tested in series with a circuit breaker, the results don't always agree with these curves. The problems occur when the fault current isn't high enough to allow the fuse to operate in its current-limiting range. Sometimes the downstream breaker has enough impedance to limit SC current to a value where the fuse is no longer current-limiting and the breaker is subjected to first 1/2 cycle currents that exceed its rating, even though everything looks fine on the let-through curve.
In the old days, we called this the "up-over-down" method. But it was dis-credited back in the 1980s. That's why UL and NEC require that the fuse and breaker be a "tested combination".
But in reality, if you look at the breaker manufacturer's listing of tested combinations, most US-made current-limiting fuses seem to be on the list.
The problems have occurred when trying to retrofit a fuse upstream of some 50-year old molded-case circuit breaker made by I-T-E or some other defunct company. Maybe this is what the 2005 NEC revision is trying to address, but as a consultant, I'd be cautious about taking on that responsibility.
RE: True or False Current limiting?
I would NOT use an untested combination for over 600 volts because the definition of current limiting is quite different at over 600 volts. Under 600 volts current limiting for fuses is about a 1/4 cycle clear time. For over 600 volts a 2 cycle clear time is "current limiting" compared to a 4 cycle clear time.
Series combinations under 600 volts and less than or equal to 200 amps for class J fuses are well understood and test for the vast majority of circuit breakers that are still being made. If say you have an obsolete SquareD or manufacturer you can usually have their PEs help you with a combination that works.
For series combinations over 200 amps at under 600 volts things are a bit more iffy particularly for class L fuses. For class L fuses the downstream circuit breaker needs a lot more than a 10,000 amp or 18,000 amp interrupting.
There is also an NEC restriction as to how many motors you can have connected downstream of a current limiting fuse. Motor contribution to short circuits has to also be considered.
RE: True or False Current limiting?
Regards,
TULUM