Circuit breaker discrepancy in 400 Hz system

[Gilf]

Hi,

I have taken over this project and I am fairly new at 400 Hz systems.

I have the below detail of a 400 Hz service bay power supply. The supply box has an 100 A breaker in it (see image) and it feeds a 60A 400 Hz cord & plug assembly. Up to this point, it's all ok. However the breaker that feeds the power supply box is rated 80A in the design (also a 400Hz panel). To my knowledge this is code violation of selective coordination. I think that the 80A breaker is sized to load and the 100A in the detail was not thought about.

Does selective coordination apply differently to 400 Hz system. Or is this simply incorrect?

 

[winnie]

What code rules apply? NEC, or something else?

There is not a general requirement for selective coordination. Some specific situations require selective coordination.

So the first thing you need to determine: Is selective coordination required in this situation. This has nothing to do with the system being 400Hz.

If selective coordination is not required, then this is simply a design issue.

One might reasonably choose a non-coordinated breaker simply as a convenient switch; sometimes standard breakers are cheaper than a similarly rated (but not common) switch.

-Jon

 

[junkhound]

Aircraft ground cart interface for interface with Boeing/Airbus/Ilyushin products at Esenboga?

Other?

NEC has no applicability as far AFAIK.

 

[Gilf]

This is actually an American project (designer is based in Ankara) and NEC rules apply.

 

[petersonra]

MCCB work somewhat differently at 400 Hz versus 60 Hz.

Effectively for certain Square D breakers a 100A CB ends up being rated for 92 Amps because the heating effect for the thermal side is different than at 60 Hz.

The IT rating is about 2.6 X. I.e. - if a 100A MCCB would magnetically trip at 500A at 60 Hz, it will take 1300 A to trip magnetically at 400 Hz.

One other thing. The interrupting rating at 400 Hz is about 1/10 what it is at 60 Hz.

Check with the manufacturer for the exact numbers for the MCCB being used.

 

[Tony S]

I had to get Schneider involved for a 400Hz protection system. Just about everything I’d selected was wrong.

Anything with RCD’s (EFCI’s) and you’re in trouble.

 

[steve66]

I agree with the previous post - the breaker curves may shift due to the 400 hz.

But I also see nothing wrong with supplying a 100A breaker with a 80A breaker.

The 100A may just be there as a disconnect.

Selective coordination would only be required for emergency systems. And that wouldn't apply to two breaker in series that serve the same load anyway.

 

[just the cowboy]

Is it controlling a motor?

Is it controlling a motor?

My question is you have a 100 amp breaker feeding a 60 load.
Most time when this is done it is for motor starting and the motors OL's are the feeder protection and breaker is there for fault protection.

Unless it is for a motor nothing is protecting the cord that is hooking up to it.

If it is for a motor the 80 amp breaker feeding it may not hold.

 

[junkhound]

For folks unfamiliar with 400 Hz (or 800Hz), many types of 60Hz breakers with a single internal contact may not open at all, simply arc till fried !.

Most higher current 400 Hz breakers or relays have double contacts - e.g similar to motor start contactors.

 

[GoldDigger]

It it can be almost as hard to extinguish a 400Hz arc as a DC arc (depending on voltage and distance).

 

[Gilf]

Thanks everyone!

This forum has so much knowledge

 

[Gilf]

Still stuck on this!

Still stuck on this!

Great, it seem i cannot convince 'the other party' on this.

So a summary of the situation:

I have an MDP of the building. Then i have a frequency converter for 50 Hz to 400 Hz, then the 400 Hz panel. All fine up to this point.

The 400 Hz panel feeds a bunch of 60A rated mobile support equipment with 80A breakers in the panel. (There are total of 5 circuits with 60A loads that have 80A circuit breakers in the panels. Each support equipment is one circuit.)

The issue is there is the stupid box that i have attached the image of above with the 100 A breaker in it for the 60 A rated load to plug in. So the 100A breaker box is fixed so that the mobile equipment can connect in.

Question 1: am i in code violance with NEC because I am feeding a 60A load with a 80A breaker at the panel but I have a 100A breaker between? To my knowledge I could have a larger breaker on the same line if i have a single load. This is what we call series rated breakers right? If I am violating the NEC can someone point me in the right code direction?

Question 2: I've been told that article 240 of NEC indicate that series rated breakershave to be manufactured by the same manufacturer and tested together as a combination. I could not find the exact reference in the code. Once again can someone point me towards the right section?

Question 3: This project is turning me insane and making me feel so small and knowledgeless (is there such word?). Is it time I consider a career change?

 

[jim dungar]

The issue is there is the stupid box that i have attached the image of above with the 100 A breaker in it for the 60 A rated load to plug in. So the 100A breaker box is fixed so that the mobile equipment can connect in.

Question 1: There is no NEC violation. The NEC primarily cares if the conductors are adequately protected (240.4). This is usually done by the breaker at the source of the circuit.
Question 2: The series-combination listed by the NEC (240.86) has to do only with fault currents that exceed the interrupting rating of one of the breakers.
Question 3: This is why there are forums like this one. In the old days we had mentors for support.

The degree that a downstream device trip before an upstream device is called coordination. The definition for Coordination, (Selective), can be found in Article 100. Very few locations in the NEC actually require selective coordination, and those that do usually involve an alternate source of power.

 

[winnie]

This probably _not_ a 'series rated' situation. Series rating is related the available fault current that the breaker can tolerate, which is probably not relevant here. Simply putting two breakers in series does not mean they need a series rating. If the available short circuit current is greater than the rating of one of the breakers, then you need to consider looking at series ratings.

Question 2 is irrelevant to your situation. Series rated devices must be tested together (not necessarily from the same manufacturer, but tested as a combination), but since series rating is not the issue here, this isn't relevant.

Question 3: time to learn and grow, not change careers We all have to keep doing that or we have to learn to push up daisies.

You are doing the right things here: some issue with the system is making you go 'huh? I don't understand...' and you are chasing down what you don't understand.

You have to determine if selective coordination is required; if not than don't worry about a 100A breaker fed by an 80A breaker.

You have to determine if the breakers are properly applied; are they rated for use at 400 Hz? (Many are, but with adjustments.)

You have to determine if the 60A cord and plug are properly protected by that 80A breaker; there are many situations where the rating of a cord or plug is less than the handle rating of the supply breaker.

-Jon