Why Short Circuit & Overload Segregation for Electronic trip unit based Circuit Breakers

[abrahamJP]

Hi,

This may be a hypothetical question,But I am glad if somebody can share their insight on it.

If for Electronic trip unit type ACB acts and trip in same manner for Short Circuit and Overload the why we have to segragate on basis of Overload and short cirucit.

For eg: a 2500A breaker set to trip for 8KA at 5 seconds(breaker (Slanding line- L region )adjusted to this point) and 20ka at [0.3 seconds to 2seconds range] (breaker S region-Vertical Line),If I am getting tripped in both cases -can I say short circuit trip or overload trip .

Thanks in advance

Abraham

 

[waross]

On a 2500 Amp breaker:
20 kA is probably a short circuit. However the instantaneous trip may be set as high as 25 kA, in which case a 20 kA trip would be an overload.
An inverse time trip in the region of about 2.75 kA to the setting of the instantaneous trip is an overload trip.
Generally an overload trip follows an inverse time curve.
Often a short circuit trip may be referred to as an instantaneous trip.
Before the advent of electronic trips, a moulded case circuit breaker would have a thermal trip that tripped on an inverse time curve.
The thermal trip operated for overloads starting at about 110% to 120% of the rating of the breaker.
The breaker would also have a magnetic trip, often called the instantaneous trip. This would often be adjustable from 500% to 1000% of the rating of the breaker.
If the thermal element operated it was an overload.
If the magnetic trip operated it was a short circuit.
8 kA on a breaker rated at 2500 Amps is probably an overload. The lower limit for the instantaneous trip is generally 500% or 12.5 kA on a 2500 Amp breaker.
From the point of view of the breaker:
Anything below the setting of the instantaneous trip is an overload, anything above the setting of the instantaneous trip is a short circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

It seems unlikely both elements would trip at the same time. The circuit should not have >8kA flowing for 5 seconds if the breaker trips in less than 2 seconds on the 20kA short circuit trip.

 

[jraef]

It's only one sensing element, but the firmware inside of the trip unit will differentiate the trip cause based on rate of rise.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Mbrooke]

Look at it like this: even molded case breakers have separate short circuit and overload elements.

 

[FreddyNurk]

Conceptually it has nothing to do with the implementation in the circuit breaker (although past history tended to dictate that; i.e. thermal / magnetic) and everything to do with what settings you use in order to detect a fault condition. Functionally, detection is separate from tripping (or opening, although in a lot of contexts that can also be a separate function...), detection can be considered to be establishing that there is a fault condition, tripping can be considered to be the circuit breaker contacts separating.

Short circuit and overload are two different conditions, both can cause damage to downstream equipment and cable, and they both need different settings in order to balance between protection of plant and nuisance tripping. Ideally the circuit breaker would have the means to indicate what actually tripped, but for a lot of LV plant this isn't available.

If you consider what is often implemented in MV and above, where the tripping is separated from the detection (i.e. protection relay operating a circuit breaker trip coil), then the breaker has tripped, and you need to look at the relay to consider what actually caused the trip. It isn't really any different for LV ACBs, except the protection elements are part of the overall breaker assembly.


EDMS Australia

 

[waross]

Yes I agree with you completely jraef. However, our terminology and difference between overload tripping and short circuit tripping is based on the old thermal-magnetic trip breakers.
Yes the functions are now implemented in firmware and only one current element is needed, but I felt that reference to the old school breakers may be valuable in explaining the concept of overload versus short circuit. The hardware seems to have changed faster than the terminology. Hey, that's life. You still hear people refer to 110 Volts and 110 Volts was phased out as a standard voltage in North America over 50 years ago.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

No argument Bill. In my defense though, the OP specifically referenced ELECTRONIC trips, then we (the group) went off describing separate trip sensors, which in the new paradigm only exist as vestigial concepts for the algorithms. I felt compelled to point that out.

As an aside:
I fear that some day soon, we engineers too will only exist as vestigial algorithmic concepts... One of the software packages I use every day for my job has just been "improved" to the point where I now no longer have any leeway on deciding how some things are configured. It's all automatic, untouched by human hands, and now minds. [flush]


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

You need no defense, Ray. I was suggesting that the terminology was still based on the old thermal magnetic breakers. Your clarification was appropriate and timely. Thanks.
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I didn't describe different sensors, I just used the word elements. Electronic breakers typically simulate the trip elements that used to be in mechanical breakers, so to me an element is just a setting or group of settings for a specific type of trip.

Same could be said about all the ANSI protection numbers. You could call the settings for each number an element if you wanted.