Current Limiting Fuses 1

[tulum]

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?

 

[dpc]

No, not unless it's a tested combination and I doubt that there are any. For a fuse to limit current it must clear the fault within the first 1/2 cycle. A primary fuse is almost never capable of that for a fault on the secondary.

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.

 

[resqcapt19]

A change in the 2005 code permits the use of "calculated" series rated systems for existing installations.

(A) Selected Under Engineering Supervision in Existing Installations The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.

Don

 

[davidbeach]

That was a very dangerous change in the code. If the manufacturer's can't create series ratings for new equipment by design, but must use tests to determine the suitability for series rating how the he!! is the poor engineer in the field to make that determination. No thanks. I would never touch that code provision, too much inherent liability.

 

[RalphChristie]

I am not familiar with the American codes, and although it does not say it in words, I would interpret the "series rated combination devices" as devices in series with each other and on the same voltage level.
In my opinion definitely not through a transformer.

 

[davidbeach]

Ralph, All listed combinations I have seen are at the same voltage level. The series ratings also become void when there are motors connected between the two devices and the total motor FLA exceeds 1% of the full interrupting rating of the downstream device.

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.

 

[dpc]

Thanks, resqcapt, I was not really aware of this 2005 change. But I agree with David B. that this provision does not seem to match the real world situations that I encounter. Just another example that it does not pay to apply logic to the NEC in many cases. It is what it is.

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.

 

[mykh]

tulum,

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.

 

[resqcapt19]

dpc,
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

 

[Dumbo2929]

I agree that this is dangerous for the PE. I often get asked about adding fuses to lower the available fault current to old switchgear. I have to remind people that the fuses should be UL listed by the manufacturer for series use. I also remind people that the interrupting rating of switchgear, breakers, etc. is based on the day it was installed. Old equipment may not perform as it once could (given maintenance conditions) and running on the hairy edge is not advised.

 

[dpc]

A fuse on the secondary side could provide short circuit protection for the secondary breaker if it is tested combination and properly sized. However, this would mean that the fuse would likely not coordinate with downstream feeder breakers for a high magnitude fault. The only way the fuse can "protect" the downstream breakers is to melt. Coordination generally is compromised in these situations.

 

[mykh]

dpc,

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.

 

[dpc]

mykh,

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.

 

[tulum]

Thanks for the answers.

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...

 

[waross]

I haven't seen a fused breaker for years but I wonder if they are still available, and what are the implications of their use. As I remember them they were a large molded case thermal circuit breaker with three internal current limiting fuses. The fuses had a small rod that protruded when the fuse blew, and tripped the breaker.
Any comments?

 

[mc5w]

The change in NEC for series rating for current limiting fuses really only applies to 600 volt and less North American fuses. Every North American fuse manufacturer has published curves for current limiting fuses to determine apparent downstream available short circuit current versus upstream available current. For the 3 major makes of class J fuses the current limitation curves agree with say SquareD's published series ratings for a class J fuse ahead of a circuit breaker. All you have to do is add in motor contribution ( <= locked rotor currents ) to the let through current from the curve to get a reasonable series rating.

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.

 

[Electic]

Probably not answering the question, but to limit available fault current, consider an air core reactor. Instant response time, unlike even 'current limiting' fuses.

 

[davidbeach]

Passive current limiters, such as air core reactors, are a safe and effective means of solving the problem. As far as I can tell, anybody who tries to use the "Engineering Supervision" provisions of the 2005 NEC doesn't understand the problem sufficiently to come up with a workable answer. If what the code is giving permission for were actually possible, the manufacturers, who know more about the breakers than any engineer in the field, could publish a list of breakers which could be series rated using the current limiting fuse curves. Such a list does not exist to my knowledge.