Circuit Breaker Coordination

[steve66]

I am wondering what is the procedure for sizing circuit breakers for motor feeders. (Where the breaker feeds several motors?) Of particular importance, is how do you specify the short time delays and pickups? And how do you know what the practical lower limit of the instantenous setting is?

Are there any general rules, such as adding all the motor currents and X times the largest current for the instantenous?

I am trying to specify breakers to reduce arc flash without causing false tripping.

Steve

 

[jwelectric]

2005 NEC
430.62 Rating or Setting ? Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.
Where the same rating or setting of the branch-circuit short-circuit and ground-fault protective device is used on two or more of the branch circuits supplied by the feeder, one of the protective devices shall be considered the largest for the above calculations.

Exception No. 1: Where one or more instantaneous trip circuit breakers or motor short-circuit protectors are used for motor branch-circuit short-circuit and ground-fault protection as permitted in 430.52(C), the procedure provided above for determining the maximum rating of the feeder protective device shall apply with the following provision: For the purpose of the calculation, each instantaneous trip circuit breaker or motor short-circuit protector shall be assumed to have a rating not exceeding the maximum percentage of motor full-load current permitted by Table 430.52 for the type of feeder protective device employed.

Exception No. 2: Where the feeder overcurrent protective device also provides overcurrent protection for a motor control center, the provisions of 430.94 shall apply.

FPN: See Annex D, Example D8.

 

[coulter]

Re: Circuit Breaker Coordination

Steve -

You are looking at doing a coordination study.

...Of particular importance, is how do you specify the short time delays and pickups? And how do you know what the practical lower limit of the instantenous setting is?...Steve

You will need to get the trip curves for each CB/fuse. Most of these are available on the manufacturers' internet site.

...Are there any general rules, such as adding all the motor currents and X times the largest current for the instantenous?...Steve

Probably lots, but I can't think of a one that would give me confidence the system was coordinated and had minimum arcflash. Take a look at the IEEE red book, ch 5. It is a pretty good discussion on trip curves and coordination.

..I am trying to specify breakers to reduce arc flash without causing false tripping....Steve

From what you are saying, you want the coordination pulled right down tight - the curve each for upstream CB is as tight as you can get it to the downstream load without having nuisance trips.

There are programs like ETAP available for $thousands$. I've got one available and it's complicated and a bear to use for a small study. And I don't use it enough to be any better than slow with it. There are lots of other programs available - I have little experience with any others.

As an alternative, you could get the trip curves for all of the CBs/fuses/overloads and a sheet of Log/Log paper and lay out the motor starting curve, and overlay the trip curves for each upstream device.

Here is an example of one I recently did using 11x17 log/log paper, a straight edge, and colored pens. It was an 800A CB protecting a 1200A feeder and MCC. The question was, "Would the motors start without tripping the feeder CB?

I added up all of the hotel loads (maximum normal running loads including heat and lights) except the largest motor. Threw out any non-coincident loads. Layed that out as the base load. Added the motor starting curve for the largest motor to the base load. Picked out a trip curve (long time/short time settings) for the feeder CB that didn't cross the feeder load. The short time part of the curve is the one that goes around the motor starting bump in load. Just using a hand done sketch got me close enough to give an answer - didn't really need a program.

Here are some generalities from my instructor in a four day coordination class I took 20 years ago: I think they are all still good.

1. If two curves overlap (upstream breaker and downstream CB, they will both trip when you would rather they didn't. Definition of "Coordination" is daylight between the curves.

2. The instaneous on molded case CBs tend to overlap at high short circuit currents. Set the downstream instaneous as low as you can and the upstream as high as you can.

3. Don't believe its coordinated just because the computer program says it is. Get the mfg curves and make sure the program isn't lying to you. (yup - occasionally they lie and sometimes you just fat finger the data entry)

4. MCC faults start phase to ground (unless you are impedance grounded) and the feeder may not trip until it goes phase to phase. A well set ground fault can limit the damage to the phase to ground part.

5. Fuses don't coordinate tightly.

6. You can coordinate anything if you have enough money.


This kind of stuff is generally considered "product of engineering" and all the state law stuff about PE tickets apply - unless you work for the company, and you are not selling the service, and there is no public access and a few other things - but, no news here, from your profile, you already got that.

I'd recommend a good class on coordination. The one I took twenty years ago is still serving me well.

Good luck, you have a tough job.

carl

 

[steve66]

Thats all good info Coulter. I'm also wondering how you got your motor starting current, and do you know how long it will last? With that, one could do what you said - overlay the starting current on the circuit breaker curve - and if there is daylight between them, I could get a quick go/no-go check.

BTW: I don't think I ever put it in my profile, but I do have a PE. I've done some breaker coordination, but adding large motors that are starting is another issue. Some of these are 250HP.

Steve

 

[coulter]

... I'm also wondering how you got your motor starting current, and do you know how long it will last? ...Steve

My example was an easy one. I had the factory acceptance tests that showed the starting current/running current profile. Without that, first I'd try calling the equipment manufacturer. Most of them are pretty good about talking, especially since someone spent a lot of money buying their equipment. Second I'd call other companies with the same or similar equipment. Yeah, I know they are competitors, but at my level (really low), we talk to each other. Third, I'd make a wild a-- guess. For the guess, worst case is locked rotor for the duration of the overload. Check the mfg recomendation on the OVLD class. I like mag-only combination starters, so the branch CB doesn't trip on overload - ever.

And of course if its an existing installation - go measure it - and I knew you knew that :lol:

... but adding large motors that are starting is another issue. Some of these are 250HP. Steve

Yeah, across-the-line starting a 250hp on 480V is tough. Even standard stuff is drawing 1800A tapering to 300A for 10 to 30 seconds. That will definitely blink the lights unless you have a 2MVA plus xfmr - might even anyway.

There are always options, remember rule #6. AB makes a nice soft start. We are putting some in for two 400hp (480V) and one 600hp (480V) and its a new installation.

carl

 

[steve66]

Carl:

I had actually suggested turning the instantenous down on a 3000A breaker from to 2x from 4x. I know 2x is usually really small for an instantenous trip, but I didn't think the 250HP motor would pull 6000A. And a 4x setting of 12000 A really shoots the arc flash calculation up.

I think we probably are going to put soft starters on these 250HP motors, but I am trying to be more careful about any other suggestions I make regarding breaker settings.

Now I'm thinking that the short time settings are more for coordination with downstream breakers than for motor starting. So maybe it would be really unusual to have a motor trip a breaker based on the short time settings.

Steve

 

[coulter]

Steve -

We're getting into an area where I know a lot more than I understand :roll: So consider this response as an "I think" as opposed to an "I know". And, I am pretty sure you already know all of this stuff.

...I didn't think the 250HP motor would pull 6000A. ...

I wouldn't either. But, I'd still check the Locked Rotor Code. Standard stuff, say Code G, you are looking at 6X (kva/hp) <=> 1900LRA

...turning the instantenous down on a 3000A breaker from to 2x from 4x. I know 2x is usually really small for an instantenous trip,

That sounds like a secondary main on a 2MVA xfmr. Yeah, 2X sounds really low, But, I'd go with your thinking - set it down all you can to just where it clears the downstream loads. As you said, that's gives you the minimum arcflash. Set the Short Time (pickup and time) down as well to where it also just clears.

..I think we probably are going to put soft starters on these 250HP motors...

I like them. But it generally has to do with small generation compared to the size of the motor. For example, the 600hp I mentioned earlier, is run from 3 - paralleled 410KW recip diesels with a 400KW hotel load. No way that 600hp would ever start without a soft start. I would expect that soft starts on your 250hp will cut back on the light blinks, but I would not expect them to be much of a factor in allowing lower CB settings that would significantly lower the arcflash. Then again ...

...I'm thinking that the short time settings are more for coordination with downstream breakers than for motor starting. ...

Yeah, I'd say that's true, I would word it slightly different, "The short time settings on a feeder allow you to close in on the required down stream loads - set your coordination tighter".

...it would be really unusual to have a motor trip a breaker based on the short time settings. ...

I'm going to translate you are speaking of a feeder CB to an MCC as opposed to an MCC bucket CB to a motor. Short time curves are generally vertical with a sharp knee to horizontal, so they cut off the long time curve. MCC feeders are usually pretty big compared to individual loads. So, unless you have the feeder short time set really low and the normal feeder loading is high, I would not expect the short time feeder to trip on motor starting.

Keep us informed on this one, it's interesting - and it's a lot of the same kind of stuff I fight (struggle with?)

 

[steve66]

Carl:

I have to take back what I said about it being rare for a starting motor to trip a breaker due to the short time settings. I was doing some simulations with SKM software yesterday, and found it was very easy to get the short time setting to cross the motor starting curve. Of course, it was even easier to get it to cross the motor branch circuit breaker curve (again, with the short time setting).

I would expect that soft starts on your 250hp will cut back on the light blinks, but I would not expect them to be much of a factor in allowing lower CB settings that would significantly lower the arcflash. Then again ...

I am curious why you don't think a soft starter would allow me to reduce the instantenous and short time settings? I would think that would push the motor starting curve to the left on a TCC curve. I would hate to suggest soft starts only to find out they don't have the effect I expected.

Although on the same issue, I found that even if the motor starting curve moves to the left, I can't reduce the main breaker settings without overlapping the branch breaker settings. So to get a lower arc flash, the breaker coordination would be sacrificed. As a result, the soft starters may be a no-go anyway.

Steve

 

[coulter]

...I am curious why you don't think a soft starter would allow me to reduce the instantenous and short time settings? ...

Ahh ... Communication problem - I suspect. As I mentioned earlier, I tend to go with a mag-only MCP in a listed combination starter in MCCs. Let the overloads take it out on locked-rotor, not the CB. The contactor won't like it, but it will be fine. (Assumption - you are using NEMA rated, not light-duty IEC contactors)

So, the CB I'd be talking (typing) about, with the Long-time and Short-time settings, is the Feeder to the MCC - not the branch to the motor. And since the feeder CB is generally a lot bigger than the individual bucket CBs, the soft-starts wouldn't tend to lower the required feeder instantaneous settings - of course that all depends on your hotel loads, other motor starting, simultaneous motor starting. And, if your operating scenerio allows both 250hp to start at the same time - that's another issue and could be a problem.

But if you are using a thermal-mag for the motor branch, then yeah you have to set the short-time out of the way of the motor-starting curve.

...Although on the same issue, I found that even if the motor starting curve moves to the left, I can't reduce the main breaker settings without overlapping the branch breaker settings. So to get a lower arc flash, the breaker coordination would be sacrificed....

Can't say I've ever been asked to sacrifice coordination to lower arcflash. Most industrial applications want the equipment to run, run hard, and run continuous (and generally they like it not to cause a lot of damage when it does burn up). Generally, one coordinates as tight as possible to keep the arcflash/equipment damage minimized, but it is what it is after the run, run hard... part is assured. And the company buys the appropriate PPE to mitigate.


carl

 

[coulter]

Steve -

After rereading all the posts, there is one thing I'm not clear on. Okay, there are many things - but one that is bugging me :? ) I even agreed with you when you said it, but now I'm not so sure.

This has to do with the arcflash calc being lower if the Instantaneous CB setting is lower. If that's right, then I may not understand all I know about arcflash calculation (and I certainly don't deny that's likely).

Here is my current thinking:

Arcflash energy release is porportional to (I^2)T, where I is the available Short Circuit Current and T is the duration. This is a simplifying assumption, but I think okay for this example (and then again, maybe the reason I'm goofed up).

Example:
2000kVA xfmr, 480V, 7.5%Z, FLA = 2400A, Available SSC = 32kA
Sec Main: 3200A Frame C-H Magnum DS with a Digitrip 1150i,
Secondary main Settings:
LT = 3000A (set for 125% FLA)
ST = 1.5X to 10X trip time = 100ms to 500ms
Inst = 2X to 10X Min trip time - 65ms

This is current mfg, relatively decent stuff, a typical for medium mfg plant. Data is right off the curves.

(I"m using the "~" symbol to mean "porprotional to")

Just looking at NFPA 70E, Annex D, Sample Calculation, one checks the energy at "Bolted Fault" (32kA this example) and at "Min Sustainable Arc" = 38%SSC (12kA about 4X (Ir) for thisw example)

Case A
For the following settings:
LT = 3000A
ST = 1.5X (4500A), T = 0.5sec
Inst = 2X (6000A) T = .065sec

arcflash energy at bolted fault ~ (32k^2) X .065 = 67E6
AFE at 38% ~ (12k^2) X .065 = 9E6 (still tripping on Inst)


Case B
For the following settings:
LT = 3000A
ST = 4X (12kA), T = 0.5sec
Inst = 5 (15kA) T = .065sec

AFE at bolted fault ~ (32k^2) X .065 = 67E6 (trips on Inst)
AFE at 38% ~ (12k^2) X ..5 = 72E6 (trips on ST)

I can get the ST incident energy greater than the Inst trip energy, but I have to work at it. And as you can see in Case C, it is pretty easy to mitigate.

Case C (decrease ST time 10%)
For the following settings:
LT = 3000A
ST = 5X (15kA), T = 0.45sec
Inst = 6 (18kA) T = .065sec

AFE at bolted fault ~ (32k^2) X .065 = 67E6 (trips on Inst)
AFE at 38% ~ (12k^2) X ..45 = 65E6 (trips on ST)


So, the issue is not the the Inst setting, but rather keeping the ST below the 38% min sustain or , If the ST is set up above the 38%, then keep the ST time down to no more than 7X the Inst trip time

Note:
For equal incident energy between bolted fault and 38% bolted fault:
T(inst trip)/Time(ST) = 1/(.38^2) <=> 7

The Question; What does the Inst setting have to do with the arcfault energy? Or, What am I missing?

i'm thinking the only way to reduce the arcflash energy is to get more impedance between the source and the fault. For example:

1. Get farther from the source.

2. Use a higher impedance source, trade a 2000kVA, 7.5%Z xfmr, for two each 1000kVA, 5%Z xfmrs, and don't run with the tie breaker closed.

I may well be goofed up, so I'm listening.

 

[steve66]

Carl:

(I've been doing all the work on SKM software, so I can't really post any equations. I have more of a graphical understanding of it from using the TCC curves.)

But, here is how I understand it:

1. Find the available bolted fault current.
2. Multiply by some multiplier for error (I usually evaluate the system at 110%, 85% and 75% of the bolted fault current).
3. Find the arcing current.
4. On the Time-Current-Coordination curve for the breaker, see where the arcing fault current lies.

Higher fault currents tend to lie in the instantenous part of the curve, giving a fast trip time and low arc flash energy (at least for the system I'm working on).

If the arcing current is lower, it may fall in the short delay time.

In my case, I can actually adjust the breaker so arcing current falls in either the short time or instantenous part of the breaker curve. (To be more specific: I can adjust it so it might trip on either, or I can adjust it so it will almost definately be in the instantenous part. For the first scenario, I assume the worst case will happen.)

Steve

 

[petersonra]

big motors

big motors

One thing you might consider with big motors is not to use across the line starting.

A VFD or soft starter dramatically lowers the starting current required.