What is causing downstream breaker to not trip first

[Designer_82]

Take a look at the attached snapshot for more info. I cannot get "DP2_MCB" to trip before "CB-DP2"
when running a fault injection simulation in ETAP. I think the issue is in the instantaneous trip characteristics of the breakers. However, even if I swap out the make/model of "DP2_MCB" with "CB-DP2" and vice versa, it still has the same problem!

If anyone can figure this out, you are a god

https://ibb.co/j8C1GZD

 

[dpc]

Instantaneous elements in series generally will not coordinate. Anytime the TCCs overlap like this, that's a value of fault current where the devices may miscoordinate. That's the benefit of the TCCs. In many cases, it is simply not possible to fully coordinate between two molded case breakers in series. You can Google "series coordination" of MCCBs for some possible exceptions based on actual testing. But these will be limited to a maximum fault current value. If you absolutely MUST be fully coordinated, fuses will be better than breakers. Your friendly local fuse rep will be more than happy to explain.

 

[cranky108]

Yes, sadly it is difficult to find a main breaker without an instantaneous trip.
Likely it is not a problem in most homes, but in some conditions it is a big problem.

 

[RRaghunath]

MCB/MCCB manufacturers do publish coordination charts.
But depending on the ratings, the coordination is (generally speaking) available only up to a SC current level and not total coordination.

 

[Designer_82]

So I could just disable all the instantaneous functions and have them coordinate like this:

https://ibb.co/61RTXD9

Or...

Ugly like this but with instantaneous all enabled:

https://ibb.co/hFNrmfZ

ETAP still shows these actually coordinating even with overlapping instantaneous regions because there is still a few millisecond delay but it wouldn't be guaranteed.

 

[RRaghunath]

MCCBs below certain rating do not come with feature to disable Instt element. Just verify/confirm.
Many MCCBs have current limiting characteristic these days.
ETAP and other Electrical Engineering software are not capable of simulating the operating characteristic of MCCB in-built protections precisely.
Manufacturers provide coordination tool for their products - MCCBs MCBs, fuses etc.. Better to use that software tool or the published charts.

 

[cranky108]

Or just use a set of fuses for the main.

 

[Parchie]

@Designer_82.
You don't have to coordinate those two downstream breakers; they are serving the same load. And either of the two downstream breaker that wins the race to trip first, you lost power to the same load! You should stop chasing hosts now!

 

[dpc]

For UL-listed MCCBs the instantaneous function cannot be disabled. You might be able to increase the pickup settings to a level above your available fault current - this would effectively turn off the instantaneous function, but this is not always possible. Low voltage power circuit breakers can be equipped with trip units without instantaneous.

No instantaneous trip will also often mean higher incident energy (arc-flash) levels downstream. It's a trade-off.