Fuse or Circuit Breaker

[Mick Gerdes 0]

I know this is a bit off track for this site but any advice would be helpfull. I have a 312amp motor on a centrifical compressor. It is controlled by a smc softstarter. It has a very high enercia load and takes quite a long time to start. It pulls 2860amps for 2sec ,then 1500amps for 10 sec, then 2600amps for 1 sec when the starter applies full voltage. The supply fuse is a TTM500 HRC. On a time vers current graph it shows that the fuse can only hold approx 3000amps for 5sec. What i thinks is happening is that over a period of time these fuses are degrading and failing. The 3 fuses have blown one after the other ie a ,b,c over approx 1.5 years use. This has happened only in the last 2 months. Even if the fuse can handle the current can they fail like this? Going to a larger fuse will give another 5 sec or so(for a 630amp fuse) however the cable,main isolator(500amp) and main feed are not large enough. Would a better option be to install a circuit breaker with adjustements to get passed this high starting currents?

[BobLfoot 302]

Mick - I am neither a utility electrical engineer or a lawyer by trade but on the upside here in the states your readings and data would be considered an engineering study under the NEC and would allow me to oversize the fuses to accommodate the known starting performance curve. Personally I like fuses in applications like this because they limit the I^2T values and the arc flash hazard better than breakers in most cases. Others with more Power Electrical experience will probably have other input and opinions we'll see.

[jeffhansen 0]

This doesn't really help you Mick, but I've never had much luck with soft starters when coupled to big motors- especially compressors. It's always a fine balancing act to keep the ramp time fast enough to not trip the supply protection, and slow enough to keep within the current capability of the starter. On something with a high inertial load, I'd be oversizing the starter to keep the ramp time down and the current up, thought this may exceed your supply capability from what you are saying. Is the fuse you are using a motor starting rated fuse? In my experience fuses do degrade over time (as do MCCB's) when subjected to high starting currents. There are some very good electronic trip MCCB's on the market that would suit the job. Talk to your local NHP/Schneider/Moeller/ABB rep - any of them would be able to supply a MCCB to do the job. Is 500amp the largest fuses you can fit to the cable as per AS3008? Remembering that when using fuses you must automatically derate to 0.9 so you should be able to use a bigger circuit breaker than you could a fuse. The benefit of using an adjustable circuit breaker is that you can dial up the exact current carrying capacity of the cable, not just pick the next smallest fuse size.

[Leitmotif 0]

-------------------------------------------------------------------------------------------------- I do not recall you saying how often you start the unit. No doubt replacing these fuses every 1.5 years is not exactly cheap. Based on my quite rusty experience with large distribution, breakers and calibrating them I would Get the instruments and get a good strip chart recording or data logger of all parameters volt amp on each phase and motor RPM if you can get that far THENI would call the friendly folks at Littlefuse or whomever and get their thoughts and recommendations for fuses vs circtuit breaker I would also call a switchboard mfr local to you and get their input. My gut instinct on this is to go with circuit breaker and if you wish fuses for very high or very fast protection. I would get a spare set of contacts for it and maybe a spare set of current sensor trip devices. The problem with a circuit breaker is now you should have to calibrate the trip settings at some interval and a power supply for this is not cheap. Make sure you have a contractor in area who has the unit ie a Multi Amp tester. Dan Bentler

[paulengr 44]

First, based on your numbers, it sounds like you are starting the compressor under load. Or not waiting the 10-30 minutes that it takes for all the oil to drain when you do have to restart after say a power failure. When you start up, it is extremely important to leave the inlet damper closed and/or the turn screw wide open (if you have one), and to wait for the oil to drain back into the sump. If you don't do this, you will have some very nasty startup loads and you will almost always trip whatever you have for protection. On shutdown, normal practice is to always unload the compressor first. Of course this requires trained people who can follow directions. If they can't follow air compressor manufacturer's directions, then let them suffer with further downtime. It sounds like there's something extremely wrong with your soft starter. There should NOT be a jump in current when the contactor pulls in. Usually you only engage the contactor after 20-30 seconds...after the motor is definitely fully running. It sounds like you might have something wrong (blown SCR? Miswiring?). Instead of timed startup, why don't you use current limiting mode in the soft start? That will extend out the startup time to whatever is necessary to maintain a reasonable starting current such as 150% of FLA (468 A). You would never see the starting currents at that point since you are well within the 175% range of a current limiting fuse. If you are tripping the overload, it's because you didn't set it correctly for the application (air compressors are definitely a class 30 trips). Unlike circuit breakers, you can't just arbitrarily set your fuses up higher and higher and higher. In fact it already sounds like you are running outside the limits of the motor and I'm surprised you aren't experiencing winding damage (which also probably corresponds to blown fuses since the soft start isn't programmed to do it's job of restricting startup current, and the overloads if they exist at all are similarly disabled). Get an AB rep in your shop because the SMC unit is obviously screwed up in terms of settings. And if the current settings are what the AB rep recommends, find another one or get AB on the phone. Something's not right here. You don't have to stick with the SMC-1 either. You can often upgrade to the newer ones while leaving the existing unit in place. I have very little experience with the SMC-(not 2, 3, flex, etc.). It might be due for an upgrade if the performance is as lousy as you describe. Fuses do NOT degrade over time as far as the active device at least with those made within the last 20 years. I have seen a few of them that might possibly have failed that were made in the 1970's because the end caps got loose but that's the only exception I can think of. The various "trip indicator" devices are known to failure, however. What usually gets them is if you have surges/spikes and incorrect settings. On the circuit breaker side, at the size you are buying though, you can easily get an electronic circuit breaker such as a GE Spectra breaker. If you get the fancier one, you can set up a very high instantaneous trip (and these babies are fast...1/4 cycle, same as a fuse), a short time trip, and a long time trip. And you can dial in all the curves to match exactly. From your coordination curve, it should be very easy to match both the protection needed for the cable and the vagaries of the motor. I suggest you start by drawing the curves and then matching up to the devices in this case especially. Also, all the circuit breaker and/or fuse companies can run the curves for you and give recommendations. Right now as a minimum, you need to get the motor safe operating area and the cable safe operating area. NEC's rules on fuses are absolute. There are no arguments when it comes to dual element fuses. I've had fuse manufacturers try to push me to rate them below NEC limits (175%) but never above. With circuit breakers you have that extra leeway because in the simple thermal-magnetic world, they really offer little to any protection because they aren't designed to deal with motor starting currents. With electronic trip units on CB's, they are much more reliable and can be set to dial in almost any setting (again, remembering the NEC specifications as well as the wiring and motor limits). In short, you can do things with them that you simply can't do with an "ordinary" circuit breaker and they work better than "ordinary" ones. They also tend to be smaller and less expensive than a large molded case thermal-magnetic unit or the old dash pot trip units on draw outs. I would use them everywhere if it wasn't for the fact that below a certain size, the pricing makes them unattractive. I've also found that I've had to pretty much ban Siemens motors. When it comes to starting currents, their energy efficient motors simply don't meet NEMA rules. You can't start them in many cases while simultaneously meeting NEC rules, and the manufacturer won't give me straight answers to straight questions. Finally, at the currents you are talking about, you would be better off with a medium voltage setup. Of course the conversion will probably cost you in excess of $50K so I'm not recommending retrofit.

[BobB 58]

The study of fuses is extensive and interesting. I2T is important but so is the type of fuse. If the fuses are European types they are probably made with copper and a eutectic. British type fuses (old GEC) were made with silver and a eutectic (usually solder). The British type GEC fuses did not deteriorate due to the silver content, IEC fuses did due to the copper content. Copper gets hot and oxidises causing a reduction of the csa of the copper conductor. Silver did not do this. In fact silver could go almost to the point of being gaseous and recover and maintain rating. Copper can not. Lawson fuses were more like the European types and I would not use them at all - they were cheap though. I do not know what the situation with the manufacture of fuses is these days. I know GEC did use copper with a silver centre for some years to compete with the European 'cheapies' that use copper. The silver section was the actual fuse and the copper was only a conductor to the fuse section. I cannot comment on soft starters at the level you refer to. I have never used them. However, the above info on fuses is accurate. In a past life I was a HRC fuse consultant to all and sundry in Ozz including steel mills etc. 25 years ago but the principles remain the same. GEC are gone - I do not know what happened but it was broken up and flogged off -mainly to the French. They had some interesting joint ventures going with GE in Europe etc etc. Soft starters are genarally as described and do not cause huge increases in current at certain levels. The increase is usually controlled and gradual. What brand are the soft starters? Probably cannot help but interested. By the way, it is also interesting looking at the trip time of a breaker compared to a HRC fuse. Fuses are so much faster on fault it ain't funny. Have a look at the fault current rise curves to the break of supply time. Breakers are quite often cycles away - fuses are on the same cycle rise let alone going as far as fall. Edited 2 Sep 2009 by BobB

[jeffhansen 0]

GEC became GEC Alsthom Industrial Products Division. Now known simply as IPD, and still have similar product lines.

[paulengr 44]

Modern dual element fuses use either a mechanical plunger-type design or a eutectic where the copper is just a conductor, not an integral part of the design in the time delay portion of the fuse. The plunger dynamically changes the surface area of the connection which creates a current limiting effect. The instantaneous trip portion consists of either a serpentine or notched copper bus bar surrounded by silica sand. If the instantaneous trip point is exceeded, arcs form simultaneously in dozens of spots along the copper bus bar. The silica/copper mixture melts together and forms a high resistance connection in very short order. The reaction is thermally limited so the higher the current, the faster the reaction. The instantaneous trip section is also current limiting. The only other fuse design still manufactured that I know of are the expulsion fuses. These types are strictly instantaneous trip types (with one exception) that use the explosive force of the fuse link rupturing to quench the arc. A variation on it uses a heavy spring and a plunger attached to the fuse element with a muffler mechanism on the end to allow a traditional expulsion fuse to be used inside of metal clad or other enclosed switch gear. The except in terms of these types of fuses is that S&C sells an "electronic fuse". This is simply a standard fuse link except that they have attached a protective relay and a capacitor to it so that the relay can trigger the fuse link by imposing the energy stored in the capacitor. This gives you the flexibility of a protective relay/circuit breaker but the speed and current handling capability of a fuse. I'm not aware of any other manufacturers with anything like it on the market right now and it's strictly a medium/high voltage device. As to circuit breakers in general...two innovations have been happening. First, there are now current limiting circuit breakers on the market. Second, the units have gotten a lot faster in general. You can actually buy molded case circuit breakers (such as GE Spectra) with electronic trip units that can trip in 1/4 cycle (same as fuses) and the trip settings don't degrade over time. They are physically smaller and somewhat simpler internally than the thermal-magnetic types so in certain current ranges, they will actually turn out to be cheaper than their thermal-magnetic counterparts and the impact on arc flash ratings is amazing (you can often take a hazard class 3 or 4 situation down to 0 or 1).