Breakers protecting generators, false protection?

[templdl]

Even though NEC is complied with does a standard breaker really provide protection for a generator when one considers the available output from the generator as compared to what it would take to trip a common breaker?
Does the generator have the capacity to provide a sufficient amount over current or a long enough time in order to trip a breaker thermally? And, can it provided enough fault current to trip a breaker should there be a bolted fault.
I trust that this may open up a good point for discussion.

 

[charlie b]

Re: Breakers protecting generators, false protection?

The setpoint of the generator breaker is based on the amount of fault current it can provide to the system to which it is connected. First you do the calculation, then you set the breaker. If the breaker is properly set, there is no reason to suspect that it would not do its duty. I do not understand the intent of the question.

Perhaps you could clarify your use of the phrase "standard breaker"?

 

[templdl]

Re: Breakers protecting generators, false protection?

The intent of the question is if "generator protection" is really understood. What is commonly misunderstood is what does it take to trip a breaker.
Simply for example, a breaker that is rated 2000a has a specific inverse time current trip current curve, I2t. Does the generator have the capabiltity of producing enough overcurrent, that is current abouve 2000a, for a long enough period of time in order to trip the breaker? I do have a file of trip curves on file but for simplification will use NEMA AB4 Guidelines for Inspection and Maintenance of Molded Case Circit Breakers Used in Commercial and Industrial Applications as a basis to illustrate my example. Table 5-3 is a table of values for inverse time trip tests at 300% of the rated continuous current of circuit breakers. breakers with a range of rated continuous current amperes 10 1601-2000a should trip in less than 800 seconds. That's over 13 minute! Can a generated with a rated continuous output of 2000a generate 6000a for up to 13 minutes without damage?? Also, just how much fault current will the generator be able to provide into a bolted fault? Breaker commonly have a instantaneous of magnetic pickup range of between 5 and 10x the rating of the breaker. In the case of a 2000a rated breaker that's between 10,000 and 20,000 amps.
This reflects the "Set point" of the commonly available inductrial breaker. Unless there are set points available that are more in line with the currents that would be damaging to a generator there may be not real protection.
However, there are some specially calibrated breakers that are available from some manufactures.

 

[steve66]

Re: Breakers protecting generators, false protection?

All the generators I've ever delt with for commercial applications have a factory set main breaker mounted right on the generator. Maybe its a cop out, but I assume they have considered your point in detail.

Steve

 

[templdl]

Re: Breakers protecting generators, false protection?

Yes, if the generator manufacturer supplies the breaker as part of the package one would trust that the breaker would provide protection.
The point I'm trying to make is that common breakers which are intended for branch circuit protection by themselves are difficult in not impossible to configure for generator protection.

 

[ron]

Re: Breakers protecting generators, false protection?

The thermal mag (or basic solid state) output breaker on a generator is selected with some consideration of the generators performance, but it is mostly to protect the downstream cable. Seperate relays and monitoring points are used to protect the generator. such as oil temp, water temp, over/under speed, internal generator winding temps, etc.

 

[templdl]

Re: Breakers protecting generators, false protection?

Ron, that's the point, that is using other monitoring points within the generator which can in turn trigger a shunt trip equiped breaker which trips the breaker to shed the load.
However, protecting down stream cable which has been sized to carry a given load and the generator size selected to power the load, the generator will with most certainty not be able to provide enough overcurrent to overload the cable anyway.

 

[ron]

Re: Breakers protecting generators, false protection?

templdl,
There are many aspects of the characteristics of a generator set mounted breaker that the generator will be able to perform to and some not. It comes as part of the package; long, short and instantaneous part of the trip unit on the breaker.
Every generator that I have been involved with has some type of "other" protection. Generally the set mounted control panel has many protective functions that I use.

 

[brian john]

Re: Breakers protecting generators, false protection?

TEMP:

Depends on the generator or multiple generators system)

(We have a site with 6-2 meg units). Also remember the 300% overload is for a set period this thermal unit also provides protection at 125% (though the time frame is quite long). Generators can supply more than their rated KVA/KW for small time frames, And a lot plays into this; type of prime move (diesel natural gas), what is happening with other loads at the time of the fault, motor loads on the system.

But having stated the above it has been my expierence that most small generators (100-500 KW0 are equipped with Circuit breakers that are oversized for the application. At that size if there is a fault the generators fall on their butts.

 

[templdl]

Re: Breakers protecting generators, false protection?

Brian,
You picked up upon my point. Without the considerations with regards to breakers settings that have the ability to respond to what the generator is capable of. Also, other monitoring and protecive features provided by a suitable generator control panel specific to that generator as Ron has pointed out is a another excellent way to provide generator protection.
The objective of my posting is to raise the awareness of the relationship of generators and breakers.