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Beyond voltage, current & interrupting rating

Consideration of all the factors related to proper application of a low-voltage circuit breaker goes beyond voltage, current, and interrupting rating. The performance of a specific type of circuit breaker may be influenced by nonelectrical factors related to the installation environment, such as ambient temperature, humidity, elevation, or presence of contaminants.

4 important factors related to proper application of a LV circuit breaker
4 important factors related to proper application of a LV circuit breaker

Enclosure type and size, service conditions, loads and their characteristics, outgoing conductors, characteristics of the electrical distribution system, other protective devices on the line side and load side of the circuit breaker under consideration, and even frequency of operation and maintenance should all be taken into account.

For this technical article, application considerations are limited to conditions involving abnormal current and to providing protection and selective coordination under these conditions.

Contents:

    1. Protection
    2. Selective coordination
    3. Power factor considerations
    4. Voltage considerations
    5. Conclusions

1. Protection

The function of system protection may be defined as the detection and prompt isolation of the affected portion of the system when a short circuit or other abnormality occurs that might cause damage to, or adversely affect, the operation of any portion of the system or the load that it supplies.

Treatment of the overall problem of system protection and coordination of electrical power systems is restricted to the selection, application, and coordination of devices and equipment whose primary function is the isolation and removal of short circuits from the system.

Short circuits may be phase-to-ground, phase-to-phase, phase-to-phase-to-ground, three-phase, or three-phase-to-ground. Short circuits may range in magnitude from extremely low-current faults having high-impedance paths to extremely high-current faults having very low-impedance paths.

However, all short circuits produce abnormal current flow in one or more phase conductors or in the ground path. Such disturbances should be detected and safely isolated.

Two types of overcurrent protection are emphasized:

  1. phase-overcurrent and
  2. ground-fault.

At the present state of the art, phase-overcurrent conditions are detected on the basis of their magnitudes. Response time is dependent upon the particular overcurrent time-current characteristic (TCC) curve.

Ground-fault currents of a sufficient magnitude may be detected by phase-overcurrent devices. Currents below the minimum current sensitivity of phase-overcurrent devices, such as arcing ground faults, are not cleared.

A separate means (either internal to the circuit breaker or externally mounted) should be provided to detect these low-level arcing ground faults. This means of detection commonly consists of current sensors that monitor each phase and the grounding conductor separately or one current sensor that monitors all phase conductors.

A single current sensor that monitors the ground-fault current in a transformer or generator neutral grounding conductor may be used. Circuit breakers have the advantage of providing a convenient means for opening all phase conductors in response to a signal from either the phase-overcurrent or ground-fault detection device. They have the additional advantage of having the current sensors and logic circuitry located internally within the breaker.

This location minimizes the need to make external connections to control components.

A fundamental rule necessary for system protection is to apply circuit breakers within their interrupting or short-circuit current ratings. The determination of available short-circuit current at the various levels throughout the electrical distribution system is a necessary step to be completed prior to selecting circuit breakers for system protection.

MCCBs are available with various interrupting ratings in the same physical frame size. Selection by frame size or continuous-current rating alone is not sufficient. The interrupting rating should also be considered.

Current-limiting fuses, integrally fused circuit breakers, or current-limiting circuit breakers may be provided to lower the let-through short-circuit current. Curves depicting let-through current and I2t are available from manufacturers to assist in the application of these circuit breakers as shown in Figure 1a and Figure 1b.

Limited peak let-through current characteristics
Figure 1a – Limited peak let-through current characteristics
Limited let-through I2t characteristics
Figure 2b – Limited let-through I2t characteristics

An alternate method is the series connection of MCCBs, that is, two MCCBs electrically in series sharing fault interruption duties.

This protection scheme is viable, provided performance is verified by testing. UL recognizes series-connected short-circuit ratings and prescribes test procedures to verify performance. Series ratings are a consequence of certain tests that are defined by UL standards, and only combinations of devices that have been appropriately tested should be used in series applications.

See Figure 3 for an example of a test setup. Selectivity is not provided at any current level where the breaker trip characteristic curves overlap, that is, both circuit breakers trip.

Series-connected ratings should be based on tests and are only valid for the specific circuit breaker types listed in the test reports.

Individual manufacturer’s series-connected ratings may be found in the UL Recognized Component Directory. Fuse and breaker coordinated combinations are also tested by UL and are applicable within their established ratings.

Series connection test circuit from UL 489
Figure 3 – Series connection test circuit from UL 489

Determination of available fault-current levels, specification of circuit breakers and associated equipment rated for those levels, and inspection to verify that properly rated equipment has been installed satisfy the basic requirement of providing adequately rated equipment for system protection.

Selection of appropriate trip unit functions and their settings to provide protection and coordination is the next consideration.

Basic rules applicable to phase overcurrent protection are as follows:


Rule #1

Select continuous-current ratings and pickup settings of long-time delay characteristics, where adjustable, that are no higher than necessary without causing nuisance tripping and that meet applicable code requirements.

The amount of time delay provided by the long-time delay characteristics should be selected to be no higher than necessary to override transient overcurrents associated with the energizing of load equipment and to coordinate with downstream protection devices.


Rule #2

Take advantage of the adjustable instantaneous trip characteristic on MCCBs and LVPCBs. Set the instantaneous trip no higher than necessary to avoid nuisance tripping. Be sure that instantaneous trip settings do not exceed the maximum available short-circuit current at the location of the circuit breaker in the system.

This point is frequently overlooked, particularly in service entrance applications.

Rule #3

Provide ground-fault protection in accordance with the NEC, where required. Ground-fault current settings should be set to minimize hazard to personnel and damage to equipment.

Time-delay adjustments of ground-fault protective devices should be set so that ground faults are cleared by the nearest device on the supply side of the ground fault.

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2. Selective coordination

When protection is being considered, the performance of a circuit breaker with respect to the connected conductors and load is a primary concern.

To achieve coordination, consideration is also given to the performance of a circuit breaker with respect to other protective devices on the supply side or load side of it.

The objective in coordinating protective devices is to make them selective in their operation with respect to each other. In so doing, the effects of short circuits on a system are reduced to a minimum by disconnecting only the affected part of the system. Stated another way, only the circuit breaker nearest the short circuit should open, leaving the rest of the system intact and able to supply power to the unaffected parts.

Generally, coordination is demonstrated by plotting the time-current characteristic (TCC) curves of the circuit breakers involved and by making sure that the curves of adjacent circuit breakers do not overlap, as illustrated in Figure 4.

Coordinated tripping by overlapping TCC curves
Figure 4 – Coordinated tripping by overlapping TCC curves

Often selective coordination is possible only when circuit breakers with short-time delay characteristics are used in all circuit positions except the one closest to
the load.

This setting arrangement is particularly true when little or no circuit impedance exists between successive circuit breakers.

This condition often exists in a main switchboard or load center unit substation between the main and feeder circuit breakers. In these cases, for all levels of possible short-circuit current beyond the load terminals of the feeder circuit breakers selectivity requires that the main circuit breaker be equipped with a combination of long-time delay and short-time delay trip characteristics.

The withstand rating of associated circuit components and assemblies should not be exceeded.

Moving toward the load, on many feeder circuits sufficient impedance exists in the distribution system to appreciably lower the available short-circuit current at the next load-side level circuit breaker.

If the available short-circuit current at this circuit breaker is less than the instantaneous trip setting of the feeder circuit breaker, then selectivity is achieved (see Figure 5 below).

Coordinated tripping due to impedance in circuit
Figure 5 – Coordinated tripping due to impedance in circuit

The preceding discussion forms the basis for judging selective coordination between two circuit breakers in series. If the fault current being interrupted by a circuit breaker flows through the line-side circuit breaker for a period equal to or greater than its tripping time, the line-side circuit breaker trips. Under these conditions, the circuit breakers are not selective.

However, if because of impedance between the circuit breakers, the maximum current that can flow during short-circuit conditions is insufficient to initiate tripping of the line-side circuit breaker, selectivity exists.

An alternate method of achieving selective coordination is by selective interlocking of two or more levels of electronic trip units in a system. In a selectively interlocked system, the circuit breaker nearest to and toward the supply side of the fault senses the fault and signals other line-side circuit breakers that it is tripping.

That signal restrains circuit breakers farther to the line-side from reacting until they time out according to their settings. Because it does not receive such a restraining signal from a load-side circuit breaker, the circuit breaker nearest the fault continues to trip with minimum delay.

This method significantly limits the damaging energy delivered to a fault by permitting the circuit breaker nearest the fault to react without the short-time delay that is necessary to provide coordination by the time and pickup level method. The limitation of fault energy is even greater when the circuit breakers involved are current-limiting.

Some circuit breakers with electronic trip units incorporate an instantaneous override set above their tripping characteristic for self protection. If fault current through the circuit breaker reaches this level, the circuit breaker trips with no intentional delay even in selectively interlocked systems.

This feature should be considered in selectivity studies.

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3. Power factor considerations

Normally the short-circuit power factor of a system need not be considered when applying either LVPCBs or MCCBs. This practice is based on the fact that the test circuit power factors on which ratings have been established are considered low enough to cover most applications.

Test circuits with lagging power factors no greater than in Table 1 are used to establish interrupting ratings.

Table 1 – Test circuit power factors

Available short-circuit current (A, rms symmetrical)Lagging power factor (%)
MCCB (a)LVPCB
Unfused (b)Fused (b)
10 000 or less501520
10 001 – 20 000301520
Over 20 000201520

(a) UL 489
(b) ANSI C37.50


Where the power factor or X/R ratio for a specific system has been determined and is more inductive than the power factor used to establish the interrupting rating, the multiplying factor tabulated in Table 2 may be applied to the calculated, available short-circuit current.

These multiplying factors adjust the short-circuit current to a value equal to the maximum transient offset in the initial half-cycle of short-circuit current flow using the relation in Elements of Power System Analysis, as follows:

Initial half-cycle of short-circuit current

where

  • t is time and is 0 when voltage is applied,
  • α is the electrical angle after t = 0 at which point the circuit is closed,
  • θ is the power angle and equals tan–1(ωL / R),
  • Z is √(R2 +(ωL)2)

By making the simplifying assumption that the circuit is closed at a time t = 0 when the instantaneous voltage is zero, the following multiplier is derived:

Multiplier formulae

where

  • CIRC is the circuit under consideration,
  • TEST is the circuit used to test the circuit breaker.

Table 2 – Short-circuit current multiplying factor for circuit breakers

Short-circuit current multiplying factor for circuit breakers
Table 2 – Short-circuit current multiplying factor for circuit breakers

These multiplying factors are based on calculated values for peak currents rather than on laboratory tests. Individual manufacturers may have additional information.

Example

For example, consider a 225 A MCCB with an interrupting rating of 35 000 A to be applied on a circuit with a short-circuit availability of 24 000 A and a power factor of 10%. Select the multiplying factor of 1.13 and multiply the 24 000 A short circuit by it to arrive at the new short circuit of 27 100 A.

In this case, the MCCB is suitable for the 27 100 A short circuit because of its 35 000 A rating.

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4. Voltage considerations

The most common industrial and commercial utilization voltage by far is the solidly grounded 480Y/277 V system. Yet, a number of 600 V and 480 V delta systems are in service of both ungrounded and corner-grounded configurations.

Further, a growing number of industrial systems are using resistance-grounded 480Y/277 V systems.

Special attention should be given to resistance-grounded wye systems and delta systems with respect to ground faults and single-pole interrupting performance.

Consider the single fault to ground in View (a) of Figure 6 and the double fault to ground in View (b) of Figure 6 in delta systems. In each case, the voltage across the interrupting pole is just below line-to-line voltage.

The magnitude of the fault depends on the prospective current and the value of the impedances to ground at the respective faults.

Systems requiring special consideration for single-pole faults
Figure 6 – Systems requiring special consideration for single-pole faults

Then, consider the resistance-grounded wye system in View (c) of Figure 6. With a single fault to ground, the fault current is severely limited by the resistance grounding connection.

With two faults to ground, voltage across the interrupting pole is at some value between phase voltage and line voltage. Again, the magnitude of the fault depends on the prospective current and the value of the impedances to ground at the respective faults.

For the systems shown in Figure 6, straight-rated (or delta-rated) circuit breakers should be used. Referring to Table 3 and testing standards, it is known that each circuit breaker pole is tested at phase voltage at full prospective current as part of the three-phase test.

Also, each pole is tested individually at line voltage with test currents indicated in Table 3. When system conditions are beyond these test values, use of MCCBs tested specifically for cornergrounded delta systems and use of LVPCBs are options.

Table 3 – Single-pole short-circuit test values for MCCBs

Single-pole short-circuit test values for MCCBs
Table 3 – Single-pole short-circuit test values for MCCBs

These test values are the minimum required for certification to UL 489. They are not marked ratings and are printed here to aid the system designer who may need them for single-phase short-circuit analysis. Single-pole circuit breakers are tested at values equal to their interrupting ratings.

Table test values are the minimum required for certification to UL 489. They are not marked ratings and are printed here to aid the system designer who may need them for single-phase short-circuit analysis. Single-pole circuit breakers are tested at values equal to their
interrupting ratings.

Table 4 is provided as a guide for applying the appropriate voltage rating of the MCCB to each system.

Table 4 – MCCB voltage rating by system configuration

System configurationThree-pole MCCB voltage rating
VoltageGrounding480Y/277480600Y/347600
480Y/277Solid
480Y/277Resistance• (a)
480Ungrounded
480Corner ground• (a)
600Y/347Solid
600Ungrounded
600Corner ground

(a) Codes and standards allow 480 V rated MCCBs in these applications. Some manufacturers provide MCCBs specially rated for the corner-grounded delta system to satisfy user preference. These ratings may also be applied to resistance-grounded wye systems. LVPCBs are also an option.

Individual poles of multipole MCCBs are tested at short-circuit levels indicated in Table 4 for all values of multipole interrupting ratings. These tests are in addition to multipole tests in which the individual poles are required to interrupt under transient conditions that are more demanding than single-phase tests of the same pole at phase voltage.

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5. Conclusions

The following considerations apply to low-voltage circuit breakers for system protection:

  1. They combine a switching means with an overcurrent protective device in a compact,  generally self-contained unit
  2. No exposure to live parts is involved during operation when installed in an approved enclosure.
  3. They are resettable. Normally after tripping (and removal of the fault or overload that caused tripping), service may be restored without replacing any part of the assembly.
    Inspection of the circuit breaker assembly after fault current interruption is required to verify suitability to return the circuit breaker and/or other parts of the system to service. Inspection of the circuit breakers may require replacement of fuses or fuse assemblies after interruption of high-magnitude fault currents.
    In LVPCBs, most designs allow for replacing components, such as contacts or arc chutes, using instructions from the manufacturer.
  4. They provide simultaneous disconnection of all phase conductors.
  5. High short-circuit interrupting ratings, the availability of current-limiting circuit breakers, and series-connected interrupting ratings permit application on systems with high available fault currents.
  6. The advent of highly complex and technologically advanced electronic trip units has increased circuit breaker versatility and made selective coordination easier.
  7. Selection of MCCBs should include consideration of interrupting rating because more than one interrupting rating may be available in the same frame size.
  8. Selective coordination of ground-fault protective devices requires time-delay and pickup adjustments and may be enhanced by the presence of adjustments that provide inverse TCCs.

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Source // IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

3 Comments


  1. Jonathan Kunkle
    Oct 19, 2018

    Excellent article with very useful information on low power factor systems. Thank you for posting this.


  2. eduardo guzman
    Oct 16, 2018

    es un articulo muy completo sobre los factores a considerar en la selctividad de protecciones.
    Gracias


  3. Rodrigo do Rosário
    Oct 15, 2018

    Very good considerations ! Thank you.

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