Current Limiting Fuses

Don't know about the rest of the world, but here in the US you could contact any of the major fuse manufacturers such as Bussman, Littlefuse or Ferraz.

See#2 for fuses.

600VAC - Canadian eh(?)

I assume what you are trying to do is limit the peak let through current and I2t rating of the CB for co-ordinated protection.

Your rise time seems a little fast. Have you included the transmission line impedance from the generator to your breaker? In my experience literally a few feet of bus or cable will bring that fault capacity down. Even some modest air core reactors will severely limit that current. (don't use iron, they saturate). If you opt for a little reactance with air core reactors be sure they are mounted with non magnetic materials (stainless steel), well braced, and good ventilation. I have blistered the paint off the metal using cold rolled steel.

We are often caught with the "standard" 65kA breaker ratings, but when you try to current limit with fuses they want an approved assembly ...so you end up buying an HRC MCB or vacuum breaker with the 100kA rating.

You may even have a problem getting fuse clips that are rated for the fault level.

SCR or semiconductor fuses tend to be some of your fastest fuses with tight current limiting peak let through ratings and limited I2t.

I have good experience with reactors. Look into that closer. Talk to Trench.

Not Canadian, eh ! But it is "drilling" where we take 600 VAC 60 Hz and rectify it via a SCR switchgear to DC, and then out to the drilling rig for the DC drilling motors scattered here and there. Power factors can be 0.2 to 0.9. This is how it was done 25 years ago (the age of my gear). Today we use VFDs since the AC Motor people have figured out how to get some torque going. Back to my project: We are upgrading the 'engine room' with new gen sets that feed the SCR gear. The SCR gear cannot be changed out or feeder CBs upgraded now. 1.5 years from now yes, but not now. Too complex to explain why. Just trust me. Top level consultants have studied the entire system impedance, and I'm going to use special wind high reactance drilling generators to limit the short circuit amps even more. 1500 kW, 0.6, 2500 kVA generators with 20% sub-transient reactance. But I am still exceeding the interrupt rating of the breakers during a short circuit so I'm faced with installing smaller generators (I don't want to do this so I can maintain large units now knowing I can upgrade the SCR gear next year when we get that part of the project approved). So, in the mean time, I want to install a failsafe device to limit my short circuit amps before I reach the current peak in 4.2 milliseconds.

Maybe this helps explain more. Thanks for looking!

There are still lots of the DC drives out there.

Old Reliance (US) equipment used to have an MCB disconnect ahead of the fuses on the bridge. The fuses had a very fast current limiting that would clear long before the breaker reached its limits. It was considered an approved assembly (for the US).

However, it did not meet CSA requirements. So Reliance Canada (largely 600VAC) opted for Tmag Breakers fully rated, with the fuses on the bridge considered as SCR protection only, and the MCB as the branch circuit protection and disconnect. This was usually on a buss lineup with the buss fed directly from a close coupled transformer. Primary protection of transformer provided protection of secondary bus.

What you don't mention is the size (amps) of the drives involved. Under 800 amps it is usually just as cheap to replace the MCB as it is to provide a fused current limiting disconnect ahead of the mess. Siemens breakers in the CLD6 frame provided 100kA, larger frames were 65kA, but that was from 1999, and I believe they came out with higher ratings after that. The Westinghouse breakers were available with an "add on" fused current limiting block that attached to the load side terminals and was an "approved assembly".

Replacing the MCB with a fused disconnect is also valid.

I believe in one installation they broke the buss and inserted one set of fast acting current limiting fuses protecting the entire lineup.

Unless you are using Lo-Z buss, I find it surprising the 85KA is not greatly diminished. Usually it only took 10 to 20 ft of cable or standard buss to reduce the fault current.

You mention gen sets. Usually under those conditions the prime mover can't supply the fault energy. If this is only supplying DC drives, the majority are non-regen and cannot feedback into the fault. Personally, I would double check all the assumptions and the model the engineers used. I have seen too many take the "conservative" quick route and not include all the factors. (Been there, done that, both sides of drive manufacture and consultant).

I like Gw's response.

Another thing which could help limit the fault current is to "crowbar" the excitation when the dt/dt limit you set is exceeded. Off Highway Vehicles have no breakers or fuses and rely on excitation control to protect the semiconductors. Considering the trucks use 2Mva power plants it is rather effective, failed semis are rare.

One way is to use Semiconductor Fuse links instead of HRC Fuse links. Visit Bussman web sit to download information.

PetroPower

just teasing, but your avatar looks like you have siphoned one too many tanks of gas.

Thanks for the compliment to my pic.

This is a picture of me testing the current limiting fuses !!

When I Googled these I saw an article where utilities actually use these (??? true) to limit skin burn risks for linemen. < 0.1 second interrupt = curable burns. > 0.1 second = permanent burns.

Look at this dude crawling on a HV line:

http://www.youtube.com/watch?v=HcalasGr_uk&eurl=file%3A%2F%2FC%3A%5CDocuments%20and%20Settings%5Cgmccoy%5CDesktop%5CEnergizers%5CWorking%20on%20live%20power%20cables%2Ehtm

The WCB regs are changing.

We used to be able to work on the 600VAC and less in civvies, now we have to be in FR2 rated coveralls and no synthetic underwear. When equipment blows up the flash can be at several thousand degrees and give a flash burn. When fault capacities are high then they want face shields, balaclavas, insulating gloves, neck protection etc. You end up looking like a spaceman.

The place where I used to work requires us to wear all those protective gears (FR2 rated clothing, face shield, and insulating gloves) just to open up a door on the supply panel. If you have to actually work on a buss, you need to wear 40 cal suit, along with all you FR2 rated proective gear. Their site the reason is from OSHA that we need arc flash protection whenever we are exposed to live voltage. So, they (the company) said we are making it even safety for you if we limited the exposed voltage to be 50 volts or higher, you MUST wear the protective gear. In addition, the companu stated that IF you are an electrician, then you must ALWAYS wear your FR2 protective wear AT ALL TIME, even if you are sitting next to someone else who is not an electrician, in a break room, without a FR2 protective gear, having their lunch. Guess the company feels that you are only worth protecting if you are an electrician.

MidniteFighter

Hmmmm. My company, just for me, wants me stripped down and in wet shorts standing in a tub of 1.2 specific gravity salt water with aluminum foil wrapped around my head and chain mail shark proof gloves. What are they trying to tell me?

Seriously, we just had a 'tagger' crawl over a 4 m fence with razor wire on top to tag (spray paint) a 110 kV transformer with his gang crap and when the spray paint mist filled the air a bolt shot to his hand blowing a hole in his hip to the transformer ground. The news showed his full pair of baggy blue jean pants which was a charcoal belt and about 20 cm below that of charcoal cloth . . . . nothing else, like burned short shorts. The legs were cheese grated thorough the fence wire. When he heals up I wonder if they'll make him go wash it off ?

We are having big problems with copper thieves.

It is sometimes self limiting.

One bright bloke climbed razor wire, an 8 ft concrete wall, and proceeded to climb on top of an energized transformer. The cutters were found neatly set on top of the transformer, but his head touch the lines when he climbed up. Blew him back about 10 ft. He was found the next morning still smoldering. Needless to say he did not survive. Probably killed him instantly.

You mean he actually survived??

I hit up my power engineer friend with the idea of foreign travel, but he was not too interested. I think what you need is a young single buck, with no dependants, but with 20 years experience in the industry. Regretfully, such people are few and hard to find.

Bill

Actually, he died a few days later. Of course, the utility is being sued now since he was able to scale the wall, read the warning signs (English and Spanish), scale the razor wire and still get killed . . . so obviously they didn't do enough or he would still be alive.

******* The other deal ********

Solved now; thanks anyway (actually not really solved as the 2 guys we hired are asking kind-of silly questions. But, that's life.