Current Limiting Fuses
Current Limiting Fuses
(OP)
Dumb question...
Can current limiting fuses be used to derate (kA wise) downstream devices if the fuses are on the primary of a power xfmr and the devices (line of feeders) are on the secondary?
Can current limiting fuses be used to derate (kA wise) downstream devices if the fuses are on the primary of a power xfmr and the devices (line of feeders) are on the secondary?






RE: Current Limiting Fuses
If a fuse takes more than 1/2 cycle to operate, it is not current-limiting.
And if a tested combination existed, this would mean that the fuse would blow for nearly any fault on the secondary side, which is probably not what you want. Keep in mind: a CLF has to melt to limit current.
RE: Current Limiting Fuses
Don
RE: Current Limiting Fuses
RE: Current Limiting Fuses
In my opinion definitely not through a transformer.
RE: Current Limiting Fuses
Series ratings have their use, and there are a bunch of misconceptions about differences between fully rated and series rated systems floating around, but series ratings of circuit breakers is something that requires extensive testing and can not be calculated.
The theoretical basis for the "Engineering Supervision" provision added to the 2005 NEC is that if the downstream breaker takes over 1/2 cycle to do ANYTHING, absolutely no mechanical movement, no change in electrical characteristics, it is possible to evaluate a series combination between that breaker and a CL fuse. Any thing that changes the circuit impedance between fault inception and fuse clearing means the operation of the CL fuse must be tested and can not be determined from fuse curves. The problem is knowing if the old breaker remains completely quiescent during the first 1/2 cycle of the fault. Very important information there is no way of knowing without extensive testing. Hence the provision is only going to be used by those who don't know enough about the subject to be thoroughly scared off.
RE: Current Limiting Fuses
Also, in most cases, a transformer primary fuse that is sized to handle the transformer inrush and full load current just isn't going to be able to be current limiting for low side faults.
I suppose if the downstream breaker was a LVPCB that had only a short-time trip, the 1/2 cycle delay might apply. But a lot of MCCBs will at least start to trip within 1/2 cycle. In fact, in the newer "blow apart" breakers such as C-H Series C, the contacts can actually begin to open before the breaker even trips.
RE: Current Limiting Fuses
why don't you use fuse on transformer secondary upstearm of the protected devices to limit prospective fault current instead of relaying on transformer primary fuse for doing this? I find it being a poor practice in first place.
All you have is to install fuse on the secondary that is selectively co-ordinated with primary fuse and is current limiting enough to get expected performance.
RE: Current Limiting Fuses
As might be expected this code change was proposed and lobbied for by the fuse industry for at least 3 code cycles before it was passed. It was strongly opposed by the breaker people for the reasons stated in the previous posts.
Don
RE: Current Limiting Fuses
RE: Current Limiting Fuses
RE: Current Limiting Fuses
I agree a fuse sacrifices it's life protecting downstream breaker:). So, it has to be replaced by new calibrated part before the equipment is back to service. Yes, you have to have spare fuses to avoid lengthy downtime. How often breakers are tested to insure they didn't loose calibration and are evaluated for suitability after they trip? I want to say that there are "pros" and "cons" in using either breaker or fuse.
Under co-ordination I understand that downstream fuse melts and clears the fault before upstream fuse starts melting. Normaly, fuse manufacturers provide with fuse selectivity ratios for co-ordination which should be followed up to avoid nuisance trip of upstream fuse.
The other advantage of using a fuse is that it performs current limiting action and provides high interrupting rating for reasonable cost so there is less chance motor starter is damaged, or damage is limited in case of short circuit fault. Nevertheless, high-amp rated fuses (2000A and up) are not much more advantegous in terms of current limitation.
I guess I wanted to highlight poor design in system where one has to rely on transformer primary fuse only to clear faults on secondary feeders.
RE: Current Limiting Fuses
My concern was lack of coordination, not the need to replace the fuse. Single-phasing is also another issue.
I agree that lack of secondary protection is a concern - I see this all the time. But, unfortunately, the notion that a primary current-limiting fuse will provide limitation of fault current on the secondary side of the transformer is a very common misconception.
I also agree that large fuses on the secondary side are not of much use for limiting downstream current or reducing arc-flash.
RE: Current Limiting Fuses
Mykh I was not considering doing this. I just witnessed an installation that had a transformer with the main secondary disconnect bing a contactor with no other secondary protection.
Just glanced at it an wondered if it was possible...
RE: Current Limiting Fuses
Any comments?
RE: Current Limiting Fuses
The change in NEC is primarily for instances where a series rating is desired to boost up the rating of existing circuit breakers particularly obsolete ones. This way somebody does not have to replace everything which makes upgrades more affordable.
This also encourages class J fuse refits of existing feeders which reduces arc flash hazard.
RE: Current Limiting Fuses
RE: Current Limiting Fuses