TCC Curve

Assume that there are two protective devices (PRIMARY and SECONDARY) connected in series and feeding MCC. PRIMARY respondes first before SECONDARY in the event of "fault" at the MCC. Is system coordination preserved though device coordination is not? Apart from NEC, are there any standards/codes which do not allow serries protective device installation due to lack of device coordination even though system coordination is preserved?

Thanks

Depends on the level of fault current available at the downstream device, but generally the answer would be no, especially if they are of the instantaneous type devices. Sounds like you need more help than you will be able to get here. Cooper makes a great device coordination book you could study, and there are many others as well.

Given enough time, you will discover that coordination is seldom black and white. Many times you end up giving on one end to get what you want at the other end.

It can be described as more of an art than a science, but the art must be grounded with sound principle.

If the two curves overlap anywhere then they are NOT coordinated. It is often impossible to completely avoid overlap. I go on the belief that the breaker operates within a band, but the center of that band is the design point. So if I get just two curves touching, or just crossing by less then 10% of the band width, then I consider it coordinated, or at least as good as it can be.

For arc flash analysis, if the two curve overlap use the one that gives the longest delay.

How does this fit with your later statement that they can coordinate if they touch or cross less than 10% of your "band"?

From my previous post:
"Depends on the level of fault current available at the downstream device, but generally the answer would be no"

If the fault duty of the downstream device is less than the crossing point, they may well be coordinated for now. This is a prime example of an increase in fault duty causing lack of coordination between 2 devices if the increase results in the fault duty of the downstream device now being greater than the crossing point. Have seen it happen.

True, so you either live with it, modify settings if you can make it work, or change equipment.

Might be as good as it can be, but I would consider it mis-coordinated. I go on the belief that when the maximum clearing time of the downstream device is not less than the minimum operating time of the upstream device, there exists the possibility of mis-coordination of the two.

There are a couple of points I would make to back up my previous statement.

We agree that breakers operate within a bandwidth of min to max times as a specific current level. The bandwidth tends to be of constant width. We would agree that the ideal operating point would be the center of the band. This would be the design point of the breaker trip controller. Now think of statistics. By nature of statistics, the greatest percentage of the breakers have to operate toward the center of the bandwidth with the lower percentage operating at the extremes. It would be statistically impossible to have equal probability of a large sample of breakers to operate at random within the bandwidth. Lets break the bandwidth into 10 segments. Now I don't know if the manufacturers guarantee 6 sigma or 3 sigma for the bandwidth so this is just a general assumption, and I am just speculating based upon manufacturing tolerances I have worked with in other process control areas. But the premise is true for most all electronic controllers.

Lets breakup the operating bandwidth into 10 equal segments, with segment one being the bottom fast portion of the curve and segment 10 being the upper slow portion of the curve. Lets say that between segments 2 (fast) AND 8 (slow) 80% of all tested breakers operated. From 1 to 2, and 8 to 9, you would expect 15% of the breakes to operate, and at the exact 1 and 10 segment blocks, you would expect less than 1% of the breakers to operate. This would approximate a 3 sigma deviation control scheme.

So if I have the downstream breaker curve with the 9 to 10 segment, overlapping with the upstream breaker in the 1 to 2 segment, so we have to calculate two 5% probabilities occurring at the same time. For sake of arguement lets just say that there is something like a 95% probability that in the real world they will coordinate. I don't recommend this, and when I coordinate I prefer to seperate the curves so they not only don't overlap, but have a seperation equal to the bandwidth.

My point is that a vast majority of the relays are NOT infinitely adjustable. So you are left with step functions. If I had to choose between a 5% overlap, that has a 95% chance of being coordinated, versus, taking the upstream breaker to the next step to guarantee 100% coordination, I first look at what that step does to IE. If it is only a minor change, saying 6.2 to 7.8 cals, then I probably implement the higher curve. But if it goes from 6.2 to 11 cals, then it shifts into a much higher PPE category and I would have to decide which case was the better compromise. I would probably take the overlap and the lower IE.

The way I coordinate breakers today is very different than the way I did 10 years ago. Back then, you'd push the equipment damage curve to assure coordination. Now, I still try to coordinate, but I find I am using the fastest curves that still assure proper operation without nuisance tripping - but my focus is to bring down the IE numbers. In the olden days we would assume high fault currents everytime we made a judgement decision. The choice today is whichever causes the higher IE, typically the lower fault value.

How many times have you come across breakers with the Instantaneous set at 10x. I see it all the time. After analysis and using the least fault current assumptions, I find that the breaker would never even see 10x as that much fault current isn't even available. I like to MAKE SURE I operate the breaker in the instantaneous area whenever I can. After calculating the Inst fault, I take 80% of that and use that value to select my Int multiplier on the relay. I'm using a lot more 6's and 7's for multipliers now, and even some 4's at time. It takes more work, looking up motor inrush current that is downstream, but it provides lower IE and probably helps save the equipment. I look at some older coordinations studies and see that yes they are well cordinated, but do I really need to hold a 20K amp fault on my MCC for 3 secs? I reach the point that I say its probably better to suffer the interruption than to risk damage to the equipment, and have to day to day advantage of lower IE. Of course I am talking manufacturing and not life safety.

This is just the way I do it, others may have contrary opinions and if you can find a problem with this approach I am always willing to learn. You just don't get much opportunity to discuss pro's and con's of different coordiantion theories other than on forums like this.