Short Circuit, Coordination, and Arc Flash Studies

[buddykin]

I am new to power studies and have a few questions that I'm hoping someone can help with. I understand there are many types of breakers but the two I deal with the most are thermal-magnetic, and electronic trip. I'm getting confused with the settings and options.

1.) Thermal magnetic breakers have an instantaneous portion (magnetic) and a long time portion (thermal).

- Are there any thermal magnetic breakers that are adjustable? I think I saw one thermal mag had a dial for the instantaneous setting.
- Can you adjust the long time (thermal) trip?
- I know there are 'plugs' you can put in breakers, is this how you adjust the thermal portion?


2.) Electronic trip breakers have an instantaneous, short time delay, short time pickup, long time delay, long time pickup, and other settings like ground fault and I2T.

- Why does SKM call electronic trip breakers 'static trip'? Static trip to me sounds like it's static (not moving).
- Would you ever put a 'plug' in an electronic trip breaker?
- Say you have a 1000A frame electronic trip breaker and want the breaker set for a 900A load (trip at 900A for an overload). Would you put a 900A plug in the breaker? Or would you just adjust the long time pickup to be 900A?

Thanks in advance for any help with my understanding!

Trevor

 

[Gr8blu]

Thermomagnetic device:
1) The thermal part consists of a bimetallic thermal device WHICH IS NOT ADJUSTABLE and which actuates the opening of a circuit breaker with a delay depending on the overcurrent value. This trip unit is intended to protect against overloads.
2) The magnetic part consists of an electromagnetic device, such as a solenoid with either a fixed (fixed instantaneous trip) or adjustable (adjustable instantaneous trip) threshold, which actuates the instantaneous trip of the circuit breaker on a pre-determined overcurrent value (multiple of the In) with a constant trip time (about some tens of milliseconds). This trip unit is intended for protection against short circuit.

Electronic trip units:
The electronic trip units use a microprocessor to process the current signal and operate the circuit breaker opening in case of fault. Due to the signal processing capability, electronic units offer more tripping criteria (some of which are not feasible with electromechanical releases).

My (admittedly simplistic) understanding of "plug" units is that it allows the same frame (X by Y x Z housing) to handle multiple current conditions. In effect, it gives the manufacturer some standardization in terms of geometry. The plug acts as to further limit the allowable current rating(s) based, on the plug value. Typical plugs are rated at 80 and 60 percent of the full breaker rating - which means the next physical size down typically has a max rating that is around 60 percent of the larger unit.

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