Circuit breakers selectivity

[liviu2004]

Hy,
I am involved in a project which requires plan approval for electric diagrams. I am using cascade low voltage distribution which contains Merlin Gerin NS250N, C60N-C40 and C60N-C10.
I am received the following comment: "outstanding point selectivity c32- c10 selectivity is till 240A.
and for c40 - c10 selectivity is till 300A
so if generator capability is bigger than their is no selectivity".
After that I have checked the tripping curves for the breakers and everything was ok. The following remark was: "you maybe correct on that subject but I don't think that what we are discussing is visible in this diagram.Often it is seen as a horizontal line at the lowest part of the drawings and in this case not drawn. For that you should also use the book and checking the I2t diagrams. We don't know how good the diagrams are but we know that the books are correct and this is confirmed by Merlin gerin. So please don't solely rely on this software."
Now I don't understand anything. What I2t diagrama are? I didn't find any info about Compact NS i2t diagrams. Any clue? Thanks

 

[davidbeach]

liviu2004, where are you, what code is requiring selectivity, and what is the application? At the sizes you seem to be implying - 10A, 40A, 250A - you will not get selectivity with circuit breakers for any fault current above minimum instantaneous value of the 40A breaker. You would have to severely limit the fault current to achieve this. Eliminating the 40A breaker in the middle will improve the situation some what.

If you can't limit the fault current sufficiently for this to work, and if you absolutely have to have selectivity, your only viable option is current limiting fuses.

 

[alehman]

Unless this is part of a life safety system, I don't understand why there should be any concern for coordination on the part of the authority. This may be a case of an overzealous reviewer thinking his responsibility is to review for design practice rather than code compliance (which seems to happen sometimes in my experience). Unless you know for sure this is a code requirement, I would ask the reviewer to cite the applicable code.

As for I^2T, this represents the maximum fault that the overcurrent device will pass without risk of tripping (or melting in the case of fuses). Some fuse manufacturers and (very few) circuit breaker manufacturers publish this information.

I agree with David's comments. Fuses are certainly another option to explore, but they are not a guaranteed fix either. Fuse manufacturers typically provide selectivity ratios which they say will assure coordination at any fault level. Some circuit breaker manufacturers dispute those claims. This is a controversial topic.

 

[liviu2004]

To be more explicit: I am young designer for electric installation for tugs (asd, stan tugs) with lenght below 50 meters.
The main breaker is the generator breaker (NS250N with STR22GE). The 40A automatic fuse is a group fuse neccesary because of limited space in the main switchboard (it is a small ship). The 10A is consumer fuse.
I am using Merlin Gerin software to determine selectivity in a I/t diagram. All seems ok in the courves, but Lloyd's Register representative ...
I2t diagrams I have for 63 to 0.5 breakers but I cannot find for main breaker. Replacing automatic fuse is out of question (only think what is happening if in all the ship I have simple fuses.
The situation must be very clear because in case of short circuit the main breaker must not tripp before small fuses.

 

[davidbeach]

Well, shipboard installations are way beyond my experience, so I don't know anything about the expected requirements. On the other hand, I do know a bit about coordination, and it can be tough. The 10 and 40 being fuses rather than circuit breakers improves the likelihood of success, but does not guarantee it. 250A as the main breaker from a generator suggests your maximum fault currents may be fairly low, which can also help coordination. I[sup]2[/sup]T curves are for current limiting devices, fuse or circuit breaker, but I've never seen one for a non-current limiting device. Good Luck.

 

[liviu2004]

To make it more difficult :generator breaker have nominal 250A setting but STR22GE settings are:
Io=0.63*In
Ir=0.98*Io=154.4A
Im(Isd)=3*Ir
Iinst=11*In
So real setting of the breaker is 154.4A

 

[davidbeach]

Good Luck, there's not a whole lot that can be done.