Fusing for Soft Starter Problems 1

[mjm4273]

Hello all! This is my first post after coming here for information for quite some time. I have a problem regarding the installation of a Siemens 3RW30 soft starter in a control panel. After extensively reading the manual and associated technical information, I have determined that I need to use semiconductor fuses and they do provide a model number for a recommended fuse, 3NE1813-0. The catalog and iMall are devoid of information regarding all the necessary accessories for properly installing this fuse, including fuse holders and touch safe covers for installed fuses. The recommended fuse is a gS type, which is some kind of IEC designation for a semiconductor fuse. I cannot, however, figure out A: what this designation means and B: what the difference is between a gR class fuse and a gS fuse as everything I can find shows them to be equivalent. The goal is to use one fuse to protect both the cable and the power semiconductors in the soft starter. The reason I ask about gS vs gR is that I can assemble a complete fuse/fuse holder/touch safe fuse cover assembly without too much effort. If anyone has any information that can help me understand these IEC fuse designations and determine the best choice. I have an email out to my Siemens distributor, but they're just as confused. [pre][/pre]

 

[ScottyUK]

The most common style of European semiconductor fuse is arguably the IEC 269 series. There are a lot of variants depending on the required fuse characteristics.

The 'g' indicates that it is a full range fuse, unlike those used in motor combination starters which have an 'a' prefix. The other letters give a clue as to the intended duty: 'G' - general purpose; 'L' - lighting; 'M' - motor; 'S' - semiconductor and so on. I think 'R' may come from rectifier but I'm not certain.

Mersen - who now own Ferraz - publish this rather helpful guide which does a better job of explaining than I am here.

 

[mjm4273]

Scotty,

Thank you for responding, especially with a European perspective! I'm in America and my customer is a dedicated europhile. Everything in this project is Siemens, so basically I need an IEC crash course to understand what these technical documents are describing. So do yo think a type gR fuse would be an acceptable substitute for a type gS fuse? I took a look at the document you linked to and they also put gS and gR in the same category. I think my application is known as Type 2 Coordination. The gR fuse I found to use is a DIAZED style. Is that style common where you're at? Is there anything special I should know about this set up? Could you refer me to any tripping curves that I can compare for the different fuse types?


Thank you,
Matt

 

[mjm4273]

Also, I found out that my soft starter uses thyristors, which Mersen lumps in with rectifiers.

 

[jraef]

I would eschew the idea of using IEC fuses here. If one blows, you will be down for a lot longer waiting for replacements, hardly anyone stocks them here. Siemens publishes a spreadsheet that shows you which fuses were used at UL to attain their SCCR here in the U.S., those will be North American fuses attainable anywhere here. If your Siemens distributor couldn't figure that out, you need s better one. You can google "Siemens 3RW SCCR" and get directed to that list.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

I would follow jraef's advice - find a North American equivalent.

The Diazeds and Neozeds are available in the UK but I wouldn't go as far as saying they're 'common'. The most common style in the UK is the BS 88 series, which are a British design dating back many years. Both BS 88 and IEC 269 types are very capable of doing their job and there's little to separate them performance-wise although I think the IEC 269 types may have a higher breaking capacity for ultra-high fault level applications. There are plenty North American fuselinks offering similar performance which will be available locally - if you use imported fuses the maintenance crew will curse you. I've been in the reverse position with North American fuselinks over here and we ended up importing them until we could modify the design to accept locally-available fuselinks.

Type 2 coordination essentially says that the equipment won't damaged by a through-fault because the energy let-through of the fuselink is less than the damage threshold of the equipment, and that the equipment can be put back in service after the fault has been corrected. Type 1 coordination allows the equipment to sustain damage before the fault is cleared.

For curves I would look at the websites of some of the big manufacturers: Bussmann, Mersen or Siba for example. For the Diazed / Neozed types then Siemens themselves are a manufacturer which might explain their eagerness for you to use their products. ;-)

 

[mjm4273]

Thanks for the input guys! I'm stuck between to fuse options. Siemens is not very forthcoming on alternatives that aren't their own. That being said, I have found that Class J fuses are the closest local approximation to semiconductor fuses that are branch rated. I've narrowed it down to these two options and I'd like an opinion on which I should go with:

http://www.cooperindustries.com/con...asheets-a/BUS_Ele_DS_2147_FCF_RN_CUBEFuse.pdf

http://www.cooperindustries.com/con.../product-datasheets-a/Bus_Ele_DS_1048_DFJ.pdf

 

[jraef]

You are going overboard. Had you done what I suggedted, downloaded the SCCR spreadsheet for the 3RW30 product, you would see that it is listed with any RK1, RK 5 or Class J fuse.

http://www.industry.usa.siemens.com...ents/Solid_State_Soft_Starters_with_Fuses.xls

Select based on what your available fault current is, but the Class J will be the most expensive. Bussman and the other fuse people want you to believe that using a fast acting fuse like a Class J will protect the SCRs better. While that may be true in theory, the reality is, protect from what? If you have a short circuit on the output side of the soft starter, it's highly unlikely that the fuse will clear fast enough to protect all of the SCRs. Then if you read the manual, it will tell you that if one SCR shorts, the others were stressed and you really should replace them all. So not only do you end up replacing all of the SCRs anyway, but you also end up replacing the very expensive fuses too. That makes the fuse people happy, but did nothing for you.

I have, in a long career with soft starters, NEVER ONCE seen a high speed fuse act fast enough to save an SCR anyway. Why? Because usually the fault situation IS the shorted SCR! Most soft starters include internal electronic short circuit protection that is faster than the fuses at picking up a simple short circuit on the load side anyway. But once in a while that short causes such a high di/dt on the load side that the SCR self-commutates at the wrong time and shorts, which THEN clears the fuses. In other words the cow leaves the barn, then the expensive door you bought to protect it closes behind it after it is gone.

I just use fuses that provide the required SCCR for the unit and live with the consequences. It takes quite a nasty little event to short an SCR, so I've found that's usually the least of your problems, so finding and replacing a fuse easily after you find and fix the cause is going to be more important. Using a fancy expensive fuse that nobody has in stock just becomes an added headache at a bad time.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

If you need fuses to achieve the SCCR, then just stick to the common time delay J type, such as Mersen AJT or the Bussmann JDL. Otherwise just use a standard circuit breaker.

If the fuses you require are 60A or smaller then you can get finger safe fuse holders for them. Mersen calls them UltraSafe. US3J3 for up to 30A or US6J3 for 31A-60A.

I've never seen that Cubefuse used in an industrial application and high speed J fuses are typically not locally stocked and have to be sourced from the fuse manufacturer.

 

[mjm4273]

Jraef, thanks for the great information. I tried to find the document you linked to via support.industry.siemens.com using their normally great search and I was unable to find it. How did you look it up? All I could find for my model number was the same references I've been working from. My usual brand is Schneider, so I'm used to working with their site. My local distributor isn't the greatest with cross-referencing parts, but I'm stuck with them. They did confirm that Class J is the way to go, in accordance with the document you linked to. The company I used to work for switched to using cubefuses a while ago because they save a lot of panel space and they meet 200kA SCCR rating easily. Bussmann can get them out quickly, but they can even be ordered next day from Grainger. Near me, they even have them in stock ready for pickup. We always supplied customers with a set of replacement fuses as part of the product. The panel in question is spec'ed out at 100kA SCCR, so we should be good. Also, really good info on soft starter protection jraef, thanks. The goofy thing about the 3RW30 product is that it doesn't include thyristor protection. I'd have to go to the 3RW40 for that. If I did, I'd have a majorly oversized soft starter with an overload. The overload wouldn't trip because the motor FLA is too low for it.

 

[LionelHutz]

Well, if you can easily get them then you could use the CubeFuse. I personally never use or recommend fast acting fuses on a motor starter application. You often have to over-size them to the point they don't meet the electrical code to eliminate nuisance fuse openings.