nuisance trip 1

[cokeguy]

We have a recurring problem in a couple of our A-B MCCs, where the main Cutler Hammer HFD3125L 125A breakers sometimes trip whenever a certain problem such as a short circuit or grounded line/motor trips one of the secondary MCP breakers. The loads are typically 3 HP standard 3-phase AC motors driven by combination starters with Cutler-Hammer HMCP015BOC 15A (mag.trip-only) breakers with corresponding A-B SMP-type thermal OL relays.

The question is, why would the individual breakers trip the main breaker if everything is correctly sized (apparently)? I´ve been told to switch to high interruptive capacity breakers to fix the problem, but in that case should I change the main breaker, the secondary ones, or all of them? (Ouch!!)

The rest of our MCCs are SquareD units with SquareD high interruptive capacity breakers and haven´t had those types of problems whatsoever, but I´m still not sure if the interruptive capacity rating of the breakers is the root cause of our problems. I would believe that even with standard interruptive capacity breakers, the secondary one should trip before the main one,

Any comments will be most welcome. By the way, the mag-trip point of the troublesome individual MCP breakers is set to its minimum, but it still trips the main breaker upon a shorted-motor type problem.

 

[dpc]

Basically, this is actually normal behavior for two molded-case circuit breakers (MCCB) in series when both see a fairly high current fault.

MCCBs are required to have instantaneous trip elements in order to protect themselves during faults. (In the case of the MCPs, that's all they have - no thermal element). This is a fundamental limitation of these devices. And one reason why they are fairly cheap.

In your case, the main breaker and MCP both "see" a short circuit downstream of the MCP. If the current is higher than the instantaneous pickup of both the Main breaker and the MCP, they both will trip.

You can **reduce** the risk of this happening by increasing the instantaneous setting on the main breaker (if adjustable) and/or installing a larger main breaker. 125 A sounds like a pretty small main breaker for an MCC. But if the fault current is high enough, it will still trip, so you are only reducing the probability, not eliminating it, unless you can get the instantaneous setting so high that it exceeds the available fault current. This is tough to do on a 480 V system.

If you really want to coordinate, you'll have to think about using fuses. They will coordinate if properly sized. But then you get all of the other problems that come with fuses.

 

[jraef]

A star for dpc, but I'll add this.
MCPs typically have much higher adjustment ranges for their magnetic trips, because the idea is for them to NOT trip on normal motor instantaneous magnetization inrush, which on new energy efficient motors can be as high as 2000% of the motor FLA. There are exceptions in the NEC to allow for setting MCP mag. trips that high. In reality they should be set as low as possible while avoiding nuisance tripping, but that does not always happen. Factories often just run the adjustment to the maximum allowable and ship the order.

On the other hand, Thermal Mag breakers, such as your 125A Main CB, often do NOT have adjustable magnetic trips, and even those that do sometimes do not have as high of a range as the MCPs. So in other words, this is likely a coordination problem. Someone should have made sure the fault is cleared at the lowest level possible by coordinating the breaker trip settings with each other. That is typically done at the initial installation.

By the way, 125A seems small as a main breaker, but if all your motors are 3HP and under it may be appropriate. Just thought I'd bring it up.

As to the hogwash "I've been told to switch to high interruptive capacity breakers to fix the problem", that is complete and utter nonsense. HIC breakers will only buy you the ability to survive a fault in an environment where the available fault current is higher than the ratings of those breakers, but that has NOTHING to do with the trip settings! Whoever told you that is has no clue, probably a salesman who wants an order for new expensive breakers. If I'm not mistaken, the A-B MCC starters with HMCP breakers in them are rated at 65,000 AIC (Amps Interrupting Capacity), which means they can withstand opening under a fault of up to 65,000A. Now IF you are connected to a system that has 85,000A available fault current then you might have an additional problem to your coordination issue, but the two concepts are not at all related.

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[cokeguy]

Actually, I was told by a fellow plant employee that installing HIC breakers solved a similar problem here in the plant, that´s why I was told to do it, but I agree with your assesment jraef, I don´t see any relation. Probably the HIC breaker that fixed the other problem was a different brand, had other trip curves, different amp rating, or had other subtle differences that solved the problem, not the AIC rating.

You have both mentioned 125A as a small main breaker, should I try a bigger one, even though theoretically and standards-wise a 125A will do, or install fuses on individual circuits?

 

[waross]

Changing the breaker size may cause code issues in regards to conductor sizing. If the breaker is providing either overload or over current protection for any conductors it may not be wise to change it.
If it is mounted in the MCC and it's purpose is to provide a safety disconnect and/or coordination, it may be safe and legal to change it.
respectfully

 

[jraef]

That's right. If you are feeding this MCC from another breaker and the conductors are only rated for 125A then increasing the this Main CB size will be pointless. However if there are larger conductors feeding it from a larger switchboard breaker, then the next consideration would be the MCC main bus bars. AB does not offer anything smaller than 400A horizontal main bus, so increasing the breaker size might be an option. The rule of thumb on MCC main CB sizing is total connected load + any known future load x 125% for unexpected future growth and/or the continuous current derating of the breaker (i.e. if it is not 100% rated).

I would agree with your assessment that it was probably a serendipitous event that changing to HIC breakers fixed their earlier problem because the breaker had different standard trip settings.

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Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[LionelHutz]

When your fellow employees changed other breakers in the plant to higher interrupting types they likely changed from a thermal magnetic to an electronic trip breaker. Electronic trip breakers seem to become more common as the interrupt rating increases. Electronic trip breakers usually respond much quicker than the old thermal-magnetic breakers. Some even respond quicker than fuses. The quicker reponse time is likely what kept the main from tripping.