Breaker Coordination - TCC included

[designer82]

I like the way these breakers coordinate on the long time & short time. Question is on the instantaneous region... They all overlap and probably all trip on a maximum fault.

Question is... is it best to disable the instantaneous trip of the upstream breakers or keep it on and let them all trip on worst fault situation? What is best practice?


Picture for reference:

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Thank You

 

[jim dungar]

What is your available fault current?

 

[xptpcrewx]

I like the way these breakers coordinate on the long time & short time. Question is on the instantaneous region... They all overlap and probably all trip on a maximum fault.

Question is... is it best to disable the instantaneous trip of the upstream breakers or keep it on and let them all trip on worst fault situation? What is best practice?


Picture for reference:

inst

Image inst hosted in ImgBB

ibb.co

Thank You

Even though the instantaneous region shows all the TCC’s overlapping, the manufacturer of these breakers will most likely have published data stating selective coordination is still met for certain tested combinations.

I don’t think it’s a good idea to disable the instantaneous as this will have a negative impact on arc-flash incident energy exposure, but that will ultimately depend on the minimum arcing current.

 

[paulengr]

First off it’s possible for simultaneous trips. But generally speaking they still coordinate or miscoordinate. Dynamic impedance and other factors play a part. Also contrary to popular belief all breakers do current limit to some degree even if they aren’t rated as current limiting. But the big problem is you cannot predict what will actually happen. The only way to know for sure is to test the series combination in a high current lab.

All manufacturers test and publish series combinations that work. Obviously they only do it with their own breakers so you won’t find a chart listing series combinations of Schneider and Eaton but there are some fuse/breaker charts and tables.

Whether it’s a good idea or not depends on the equipment. Panel boards, MCCs, and similar equipment only have 3 cycle ratings. They use UL 489 breakers. In these breakers instantaneous can be different (5, 10, or 20 times long term) but cannot be turned off. At best you might find some electronic ones supporting ZSI. Coordination is via series combination ratings (testing). In this equipment obviously disabling instantaneous is a bad idea.

Many switchboards, even ones supporting insulated case breakers, are also 3 cycle rated and so are most ATSs. But switchgear is 30 cycle rated as are some ATSs and switchboards. This equipment allows using UL 1066 aka ANSI breakers which are fully adjustable including allowing you to disable instantaneous. In fact that is pretty much the normal expected thing since the vast majority of these devices are at the distribution level and usually you can’t possibly be expected to buy everything from one manufacturer.

One caution to look out for: microprocessor based trip units like any “computer” may have a significant “boot time” and the self-powered ones don’t even wake up until current exceeds 10-25% of the long term rating. Those using external power like 125 VDC don’t have an issue except if it’s a CTD instead of batteries. So when closing into a fault can become a serious safety issue. As a result they often have a hardware based instantaneous trip that again you cannot disable or often even adjust. Just be aware that it is probably there and there won’t be any surprises. The old induction disc relays were immune to this issue but they were terrible in every other way.

 

[David Castor]

They don't really fully coordinate on short time (or long-time for that matter). There needs to be "daylight" between the curves for full coordination. As shown, the instantaneous elements will not coordinate, but this depends to some extent on the maximum fault currents at each breaker. Turning off the instantaneous is generally not possible on molded case breakers, only on low voltage power circuit breakers. Turning it off can definitely improve coordination but it would need to be determined if the downstream equipment was rated for the longer fault duration. Also, without instantaneous, arc-flash energy downstream would probably be higher. It's always a compromise.

 

[paulengr]

They don't really fully coordinate on short time (or long-time for that matter). There needs to be "daylight" between the curves for full coordination. As shown, the instantaneous elements will not coordinate, but this depends to some extent on the maximum fault currents at each breaker. Turning off the instantaneous is generally not possible on molded case breakers, only on low voltage power circuit breakers. Turning it off can definitely improve coordination but it would need to be determined if the downstream equipment was rated for the longer fault duration. Also, without instantaneous, arc-flash energy downstream would probably be higher. It's always a compromise.

Don’t forget that we don’t care what happens above the available short circuit current. If that’s above the instantaneous trips then tested combinations is your only realistic solution but if you are below that point then TCCs are adequate.

 

[topgone]

They don't really fully coordinate on short time (or long-time for that matter). There needs to be "daylight" between the curves for full coordination. As shown, the instantaneous elements will not coordinate, but this depends to some extent on the maximum fault currents at each breaker. Turning off the instantaneous is generally not possible on molded case breakers, only on low voltage power circuit breakers. Turning it off can definitely improve coordination but it would need to be determined if the downstream equipment was rated for the longer fault duration. Also, without instantaneous, arc-flash energy downstream would probably be higher. It's always a compromise.

You can only do so little with that almost vertical TCCs! If those were breakers with electronic trip units, you can see the "daylight" you are hoping for there! Time grading is not possible there so you might as well change those breakers.

 

[David Castor]

In most cases, you probably just live with the lack of coordination. NEC doesn't require selectivity except for special cases.

 

[designer82]

Can you take a look at this and see if you can figure out what is going on. I cannot get the downstream breaker "DP2_MCB" to trip first.
The snapshot will provide more info. I am thinking it has to do with the instantaneous trip characteristic of these breakers.
But the weird thing is, if I swap out the make/model of "DP2_MCB" & "CB-DP2", it still has the same problem. So strange

Any insight is greatly appreciated

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[jim dungar]

I see fault current of roughly 13kA, this is in the part of the curve where the breakers overlap.

 

[topgone]

IMO, you don't have to coordinate DP2_MCB against CB_DP2, they are in series and protecting the same load! You need to only coordinate DP1_MCB with either DP2_MCB or CB_DP2.
If you investigate both TCCs of DP2_MCB and CB_DP2, they follow the same curve at faults under about 27kA! And a fault of 13.8kA is well below the point (27kA)! It will be a racing game between that two downstream breakers.
See that DP1_MCB trips later at 13 seconds at 13.8kA than either of the downstream breakers DP2_MCB and CB_DP2 which trips at around 5 seconds--> which is good!

 

[xptpcrewx]

Can you take a look at this and see if you can figure out what is going on. I cannot get the downstream breaker "DP2_MCB" to trip first.
The snapshot will provide more info. I am thinking it has to do with the instantaneous trip characteristic of these breakers.
But the weird thing is, if I swap out the make/model of "DP2_MCB" & "CB-DP2", it still has the same problem. So strange

Any insight is greatly appreciated

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ibb.co

Your TCC and SLD are kind of a mess. I don’t see CB P32 in the SLD but its being displayed on the TCC. Also, the devices and settings are not properly displayed so we don’t know what devices you’re dealing with or their limitations. What type of facility is this? Is selectivity even a requirement?

 

[designer82]

Would you rather have them beautifuly coordinate like this but with instantaneous all disabled:

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Or...

Ugly like this but with instantaneous all enabled:

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[xptpcrewx]

Would you rather have them beautifuly coordinate like this but with instantaneous all disabled:

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Or...

Ugly like this but with instantaneous all enabled:

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The ugly one.

 

[pv_n00b]

When it comes to coordination your friends are large differences in the rating of the OCPDs in series in the circuit and long distances so you get some nice reduction in the available fault current at different OCPDs. You have no friends in this circuit. You can throw money at it, ZSI, arc fault maintenance switches, etc. and that is really all you can do. Assuming this is not a hospital or somewhere else where selective coordination is mandated I would call this the best possible coordination and move on.

 

[topgone]

When it comes to coordination your friends are large differences in the rating of the OCPDs in series in the circuit and long distances so you get some nice reduction in the available fault current at different OCPDs. You have no friends in this circuit. You can throw money at it, ZSI, arc fault maintenance switches, etc. and that is really all you can do. Assuming this is not a hospital or somewhere else where selective coordination is mandated I would call this the best possible coordination and move on.

The TCCs provided tells you you can't coordinate at the instantaneous region.

 

[WA_Sparky]

Correct me if im wrong but when i was doing selective coordination studies i only had to coordinate above the 0.1 second line. The only exception being if it was for an emergency power or life safety I had to coordinate to 0.01 seconds.

In general, like others posted, you need some daylight between curves. If you're evaluating breakers that are between equipment, as in picture "3" CB-DP2 AND DP2_MCB, those do not need to coordinate so you can have those curves overlap since one tripping over the other wont effect other equipment.

One way i was able to get around this kinda issue is get an electronic trip of a breaker larger than you need and dial it back. In this example using a 225A frame breaker for MCB-2E wouldnt couldn't coordinate properly, so i had to use a 400A and dial it back to 225A. If the larger breaker doesnt fit in the panel, change the panel to an MLO and use an enclosed circuit breaker to contain the 400A frame.

 

[don_resqcapt19]

Coordination, Selective (Selective Coordination).
Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents. (CMP-10)

I see that as requiring the coordination to time zero. The breaker manufacturers have tables for the times below the bottom of the time trip curve.

 

[paulengr]

You CAN coordinate in the instantaneous region but you cannot predict it based on TCC, which should be obvious.

What you do is find out the series ratings, if they exist. The only way to verify which device trips first is by actually testing them. So manufacturers pay for testing with their own breakers and publish charts. But obviously GE isn’t going to test their breakers against say Schneider and vice versa. So you can only use this information with the same manufacturer and only if it exists.