The requirements of this standard cover molded-case circuit breakers (MCCB), circuit breaker and ground-fault circuit-interrupters, fused circuit breakers, and accessory high-fault protectors. These circuit breakers are specifically intended to provide service entrance, feeder, and branch circuit protection in accordance with the National Installation Codes in Annex B, Ref. No.1.
This standard also covers instantaneous-trip circuit breakers (circuit interrupters) specifically intended for use as part of a combination motor controller in accordance with the National Installation Codes in Annex B, Ref. No. 1.
1 ADJUSTABLE CIRCUIT BREAKER
A circuit breaker that has adjustable time/current tripping characteristics. These may include:
- Inverse-time (such as continuous current, long time, and/or short time);
2 ADJUSTABLE INSTANTANEOUS RELEASE (TRIP)
That part of an overcurrent trip element that can be adjusted to trip a circuit breaker instantaneously at various values of current within a predetermined range of currents.
3 CIRCUIT BREAKER
A device designed to open and close a circuit by nonautomatic means, and to open the circuit automatically on a predetermined overcurrent, without damage to itself when properly applied within its rating.
4 CIRCUIT BREAKERS WITH GROUND-FAULT PROTECTION FOR EQUIPMENT:
Circuit breakers that perform all normal circuit breaker functions and also trip when a fault current to ground exceeds a predetermined value.
5 CURRENT-LIMITING CIRCUIT BREAKER
One that does not employ a fusible element and, when operating within its current-limiting range, limits the let-through I2t (see definition 20 AMPERES SQUARED SECONDS) to a value less than the I2t of a 1/2-cycle wave of the symmetrical prospective current.
6 CURRENT-LIMITING RANGE
The RMS symmetrical prospective currents between the threshold current and the maximum interrupting rating current.
7 CURRENT SETTING (lr)
The RMS current an adjustable circuit breaker is set to carry continuously without tripping. It is normally expressed as a percentage of the rated current and is adjustable.
8 FIXED INSTANTANEOUS RELEASE (TRIP)
That part of an overcurrent release element which contains a nonadjustable means that is set to trip a circuit breaker instantaneously above a predetermined value of current.
An assembly consisting of all parts of a circuit breaker except an interchangeable trip unit.
10 FRAME SIZE
A term applied to a group of circuit breakers of similar physical configuration. Frame size is expressed in amperes and corresponds to the largest ampere rating available in the group. The same frame size designation may be applied to more than one group of circuit breakers.
11 GROUND-FAULT DELAY
An intentional time delay in the tripping function of a circuit breaker when a ground-fault occurs.
12 GROUND-FAULT PICKUP SETTING
The nominal value of the ground-fault current at which the ground-fault delay function is initiated.
13 INSTANTANEOUS OVERRIDE
A fixed current level at which an adjustable
circuit breaker will override all settings and will trip instantaneously.
14 INSTANTANEOUS PICKUP SETTING
The nominal value of current that an adjustable circuit breaker is set to trip instantaneously.
15 INSTANTANEOUS TRIP
A qualifying term indicating that no delay is purposely introduced in the automatic tripping of the circuit breaker.
16 INSTANTANEOUS TRIP CIRCUIT BREAKER (motor circuit protector or circuit interrupter)
Is one intended to provide short circuit protection only. Although acting instantaneously under short circuit conditions, these circuit breakers are permitted to include a transient dampening action to ride through initial motor transients.
17 INTERCHANGEABLE TRIP UNIT
One which can be interchanged by a user among circuit breaker frames of the same design (to see also definition 32 RATING PLUG).
18 INTERRUPTING RATING
The highest current at rated voltage that a device is intended to interrupt under standard test conditions.
19 INVERSE TIME
A qualifying term indicating that there is a purposely introduced delayed tripping in which the delay decreases as the magnitude of the current increases.
20 I2t (AMPERES SQUARED SECONDS)
An expression related to the circuit energy as a result of current flow. With respect to circuit breakers, the I2t [A2s] is expressed for the current flow between the initiation of the fault current and the clearing of the circuit.
21 LONG-TIME DELAY
An intentional time delay in the overload tripping of an adjustable circuit breaker’s inverse time characteristics. The position of the long time portion of the trip curve is normally referenced in seconds at 600 percent of the current setting (Ir).
22 LONG-TIME PICKUP
The current at which the long-time delay function is initiated.
23 MOLDED-CASE CIRCUIT BREAKER (MCCB)
A circuit breaker which is assembled as an integral unit in a supportive and enclosed housing of insulating material.
Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short-circuit, or ground-fault.
Operation of equipment in excess of normal, full-load rating or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating.
26 PEAK CURRENT
The maximum instantaneous current that flows in a circuit.
27 PROSPECTIVE CURRENT (AVAILABLE CURRENT)
The current that which would flow in a circuit if a short circuit of negligible impedance were to occur at a given point.
28 RATED CURRENT (In)
The marked current rating and the maximum RMS current a circuit breaker can carry continuously without tripping and the maximum current the circuit breaker will carry without changing, deleting, or adding a part or parts such as trip units and rating plugs. See also current setting (Ir).
29 RATED FREQUENCY
The service frequency of the circuit for which the circuit breaker is designed and tested.
30 RATED VOLTAGE
The rated voltage is the nominal RMS voltage for which the circuit breaker is designed to operate.
The designated limit or limits of the rated operating characteristic(s) of a device.
32 RATING PLUG
A self-contained portion of a circuit breaker that is interchangeable and replaceable in a circuit breaker trip unit by the user. It sets the RATED CURRENT (In) of the circuit breaker.
33 SHORT CIRCUIT
An abnormal connection (including an arc) of relatively low impedance, whether made accidentally or intentionally, between two points of different potential.
34 SHORT CIRCUIT CURRENT RATING
The maximum RMS prospective (available) current to which a device can be connected when protected by the specified overcurrent protective devices. The rating is expressed in amperes and volts.
35 SHORT-TIME DELAY
An intentional time delay in the tripping of a circuit breaker between the overload and the instantaneous pickup settings.
36 SHORT-TIME PICKUP
The current at which the short-time delay function is initiated.
37 THRESHOLD CURRENT
The RMS symmetrical prospective current at the threshold of the current limiting range, where:
- The peak current let-through in each phase is less than the peak of that symmetrical prospective current, and
- The I2t in each phase is less than the I2t of a 1/2 cycle wave of the symmetrical prospective current.
The opening of a circuit breaker by actuation of the release mechanism.
39 TRIP UNIT
A self-contained portion of a circuit breaker that is interchangeable and replaceable in a circuit breaker frame by the user. It actuates the circuit breaker release mechanism and it sets the RATED CURRENT (In) of the circuit breaker unless a rating plug is used (to see also definition 32 RATING PLUG).
Resource: Low voltage circuit breakers – Working with trip characteristic curves by ABB
We need to make UL489 certification of MCB. Which items need to be tested? What are the standards for each item?
Thank you very much
Just wanted to share my personal opinion on this topic. The molded case circuit breakers,, have proven to be very unreliable in pole mounted substations for transformers from 150 kva to 250 kva. They were unable to swich off the LV part of the transformer, when there was failure on one of the LV cables which led to whole switch melting and eventualy fuses exploded and turned off faulty cable.
This problem you describe is usually due to faulty grounding of the secondary side of the transformer. Separate bonding jumpers must be sized and installed in accordance with NEC standards in order to facilitate the proper opening of the over-current device.
i have a question for you… How can we increase the magnetic threshold current of a MCCB?? I am currently working on this topic as my major project for my master degree need some help..
Edvard, please clarify that MCCB provides short circuit and over current protection and if I need additional ground fault protection than MCCB has to be with microprocessor release or it can also be thermal magnetic release?
MCCB does provide overcurrent protection through overcurrent trip unit and short circuit current up to 120kA with TMF thermomagnetic trip unit with fixed or adjustable thermal and magnetic threshold (I3 = 10 x In) or with microprocessor unit as you stated above.
MCCB needs microprocessor unit to provide ground fault protection, of course with with toroid current transformer.
But, if you take a look at the nowdays modern circuit breakers, they all have advanced microprocessor units.