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3. FUSE OPERATION
3.1 High Speed Operation
Normally the short circuit current will reach a very high value limited by the system source impedance to the fault and the fault impedance itself.
Where the fault impedance is approximately zero the fault current will equal the 'maximum prospective short circuit current' ( Fig. 4). Most fuses are designed to interrupt the fault so quickly that the current never reaches its maximum value and therefore acts as a current limiting device. On the other hand, an expulsion fuse acts quite slowly and does not limit the current.
On AC circuits the rise of current depends on the circuit parameters and the point in the cycle when the short circuit occurs. At high fault currents the very short clearance times will vary according to the phase angle, and below 100 ms a range of clearing times is possible for each type of fuse. Fuse performance is therefore generally tested for two onerous conditions:
- Arcing (after the fuse element melts) must commence at a point on the voltage wave between 40- and 65- to test thermal stresses.
- Between 65- and 90- to test electromagnetic stresses.
3.2 Discrimination
3.2.1 Joule Integral
The energy required to melt the fuse element varies only slightly with the prospective fault current and is almost constant for a particular type of fuse. This is measured by a constant called the 'Joule integral' or 'I^2t' value. Therefore for short operating times below 100 ms, the 'I^2 t' value is used for grading series fuses. Discrimination is achieved when the total I^2t of the minor (down stream - load side) fuse link does not exceed the pre-arcing I^2t of the major (upstream - power source side) fuse. I^2t characteristics and tabulated values for low-voltage fuses in the range 125-400 A are given in Table 4.
At longer operating times above 100 ms cooling occurs and more energy has to be input into the fuse element so that current/time discrimination curves must be used. A variety of fuse characteristics are available. An example of typical time/current characteristics for general purpose low-volt age HRC fuses in the range 125-400 A is given in Fig. 11.
Satisfactory discrimination time/prospective current curves between fuses and between an IDMTL relay and fuses in two alternative 11 kV/415 V radial connected circuits are shown in Fig. 12. It should be noted that an 'extremely inverse' IDMTL relay characteristic is available for grading between relay operated circuit breakers and fuse protected circuits.
Cut-off characteristics for 4-1,250 A, Cooper-Bussman HRC fuses are given in Fig. 13a. For comparison, Merlin Gerin Compact series MCCB current limitation curves are given in Fig. 21.
TABLE 4 Tabulated I^2t Characteristics for HRC Fuses, 125-400 A (courtesy
of Cooper-Bussmann)
3.2.2 Earth Loop Impedance
IEC 60364-1 gives guidance for safe installations and maximum permissible disconnection time/touch voltages in building services applications. The earth loop impedance must be such that under earth fault conditions sufficient fault current flows to trip the circuit breaker or operate the fuse and isolate the fault in time. Where this is not possible residual current circuit breakers (RCCBs) must be used to ensure rapid isolation of the fault. RCCBs designed for domestic applications only have a low fault-breaking capability (typically 1 kA at 0.8 pf). Therefore it is very important to check this parameter before applying such devices to high fault level industrial applications.
FIG. 11 Time/current characteristics for HRC fuses, 125-400 A. (courtesy
of Cooper-Bussmann).
3.3 Cable Protection
3.3.1 Overload
The IEC requirements for low-voltage cable overload protection (IEC 60364-4-43) are that the characteristics of a device protecting a cable against overload (for example, a fuse) shall satisfy the following conditions:
IB <In <Iz
I2 <1.45 x Iz
where IB is the design current of that circuit;
In is the continuous current-carrying capacity of the cable under specified conditions;
Iz is the nominal current of the protective device;
I2 is the current ensuring effective operation in the conventional time of the protective device.
Type gG fuse links to IEC 60269 are tested to a conventional cable overload test at 1.45xIz. Therefore compliance with the first equation inherently satisfies the requirement of the second equation. Typical fuse protection of 3 core copper conductor PVC/SWA/PVC LV cables is shown in Table 5.
3.3.2 Short Circuit
Cable manufacturers give short circuit current/time curves which must not be exceeded for different cable constructions and insulating materials. An example of a 10 mm2 PVC insulated copper conductor cable is shown by the solid curve in Fig. 14, together with four fuse current/time characteristics (shown by the dotted curves) to be selected for short circuit protection of this cable.
FIG. 12 Discrimination on a radial feeder.
FIG. 13a Cut-off characteristics for HRC fuses, 4-1,250 A (courtesy Cooper-Bussman).
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FIG. 13b 415 V network with three MCCBs in series and installation examples
for correct cascading (courtesy EMMCO/Merlin Gerin based upon current limiting
capacity).
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3.4 Motor Protection
It is normal to use contactor motor starters to control motor circuits. In some instances these do not have adequate fault rating characteristics to withstand short circuit conditions or break high fault currents. A series fuse capable of withstanding the repeated motor starting current stresses is therefore added with the contactor in the motor control centre (MCC). Such fuses are designed to operate quickly at high overcurrents and dissipate low power. Therefore motor starting fuses may be made physically smaller than those designed for protection over a wide range of fault currents. In motor protection applications fuse ratings from two to three times the motor feeder cable rating are necessary, as shown in Table 6.
Guidance on fuse application taking into account the recent amendments to motor control gear IEC 60947-4-1 standard is available from leading fuse manufacturers.
A 3.3 kV, 950 kW (196 A full load current) motor running and starting current/time characteristic is shown in Fig. 15. In order to check adequate motor and cable protection and discrimination the following characteristics are shown superimposed on this diagram:
- A typical contactor maximum fault interrupting capability of 7 kA.
- The hot and cold motor thermal overload relay characteristics.
- 95 mm2 PVC insulated copper conductor motor feeder cable short circuit current/time capability.
- 250 A motor fuse protection characteristic.
On large motor drives the fuses will incorporate striker pins which trip all three phases of the contactor to prevent single phasing.
TABLE 5 Conventional Cable Overload Fuse Protection
FIG. 14 Short circuit protection of 10 mm2 PVC insulated cable by type
T fuses.
TABLE 6 Short Circuit Fuse Protection of PVC/SWA/PVC Copper Cables in
Motor Circuits
FIG. 15 Motor starting characteristic showing motor thermal overload
and graded fuse short circuit protection.
FIG. 16 Comparison between semiconductor and normal industrial HRC 63
A fuse characteristics (operating on 80 kA, 750 V system, 0.5 pF fault).
3.5 Semiconductor Protection
Special care is necessary because of the low tolerance of semiconductor devices to high-overcurrent conditions. Semiconductor fuses are therefore designed to be faster acting than conventional HRC industrial fuses ( Fig. 16). In addition, it should be noted that semiconductors often operate in a switched mode fashion with high current fluctuations but relatively low rms current values. The fuse must be selected such that the I 2 t value is not exceeded in order to avoid anomalous fuse operation.
4. MINIATURE CIRCUIT BREAKERS
4.1 Operation
The miniature circuit breaker (MCB) and molded case circuit breaker (MCCB) offer the overload protection characteristics of the fuse, good short circuit current limiting protection together with the advantage of a switching function. If correctly specified the MCB also has the added advantage of not requiring replacement after breaking a short circuit within its rated capability.
To achieve good fault current limitation the current-carrying contacts are arranged such that a magnetic repulsion effect proportional to the square of the fault current rapidly separates the contacts. An arc is then developed and extended across arc chutes ( Fig. 17). Typical contact opening times are of the order of 0.5 ms and total fault clearance time 8 ms with a 50% reduction in prospective current peak for a modern 15 kA MCB. Such devices do not therefore meet the very fast fuse fault clearance times and prospective short circuit current limitation. Enhanced current limiting characteristics are, how ever, available from some manufacturers. Improved contact layouts and gas producing polyamide arc chutes which smother the arc give 0.2 ms opening times and total clearance times of only 2.5 ms. For reliable repeated operation up to at least 10 times at a 150 kA prospective short circuit current, the installation protected by such an enhanced modern breaker would see less than 9% of the peak prospective current and less than 1.3% of the calculated thermal stresses. Careful MCB selection may therefore offer short circuit protection characteristics nearly as good as a fuse.
Overload protection is achieved by the thermal distortion effects produced by a bimetallic element. After a preset, and often adjustable, amount of thermal overload the main current-carrying contacts are arranged to open rapidly. Manual or motor driven reset is then necessary to restore supply.
Single, double and three pole MCBs are shown in Fig. 18. Fig. 19 shows typical time/current characteristics of a 160 A HRC fuse and high speed current limiting 160 A MCCB. At high fault levels the MCB or MCCB has a definite minimum time characteristic. Therefore special care must be paid to achieve adequate discrimination between breakers on radial circuits at these higher fault levels.
A modern 250 A MCCB with enhanced discrimination characteristics (courtesy of Merlin Gerin) is shown in Fig. 20. These devices are designed to ensure protection discrimination for short circuit currents greater than the rated breaking capacity of the circuit breaker. The devices are characterized as follows and may be fitted with electronic tripping units (see Fig. 20) to allow a wide degree of adjustment for the tripping current thresholds and times:
In - circuit breaker current rating
Ir - overload protection current tripping threshold
Im - short circuit current tripping threshold
tr - overload tripping time adjustment
tm - short circuit tripping time adjustment
FIG. 17 Current limiting effect of the miniature circuit breaker (MCB)
(courtesy of EMMCO/Merlin Gerin).
FIG. 18 Single, double and three pole MCBs for low-voltage applications
(courtesy of Merlin Gerin).
FIG. 19 Time/current characteristics of an HRC fuse and MCCB both rated
at 160 A. I^2t and temperature characteristics must also be investigated.
FIG. 20 Adjustable characteristics of a modern MCCB fitted with an electronic
tripping unit.
FIG. 21 Thermal stress and current limitation MCCB curves.
4.2 Standards
At low-voltage domestic and distribution levels there is a trend away from the use of fuses in favor of MCBs in order to avoid the inconvenience and cost of replacing cartridge or rewirable fuses. At the higher voltages and at high fault currents the fuse remains a highly cost effective solution to the protection of equipment. IEC 60947-2 has now replaced the older IEC 157 as the present standard covering MCBs up to 1,000 V AC. It should be very carefully noted that MCBs are given a short circuit category (P1, P2, etc.), depending upon their capability to operate repeatably under short circuit conditions. Not all categories represent MCBs that are usable after clearing a fault and some, like a fuse, have to be replaced. Two utilization categories are defined:
Category A - Breakers without an intentional short time delay provided for selectivity under short circuit conditions.
Category B - Breakers with an intentional short time delay provided for selectivity under short circuit conditions.
The term molded case circuit breaker (MCCB) normally applies to the higher current-carrying capacity three pole units typically in the range 100-1,250 A at up to 1,000 V. Miniature circuit breakers (MCBs) are applied at the final distribution feeder level in single, two, three and four pole varieties up to 100 A. In comparison, the traditional air circuit breaker (ACB) has low voltage (,1,000 V) current-carrying capacity ratings at least up to 6,300 A.
Advice must be sought from the manufacturers for operation in other than temperate climates. The current/disconnection time characteristics are sensitive to wide temperature variations and current-carrying capacity derating factors should be checked for operation above 40deg. C. Application of several MCBs in close proximity may also require grouping derating factors to be applied.
4.3 Application
4.3.1 Cascading and Prospective Fault Current Limitation
Application principles, especially for the modern enhanced current limiting MCB types, are similar to those mentioned for fuses in Section 3.
In a radial network, upstream circuit breakers, installed near to the source, must be selected to have an adequate fault-breaking capacity greater than the prospective short circuit current at the point of installation. Two or more breakers may be cascaded in a network in this way such that the cur rent limiting capacity of the upstream devices allows installation of lower rated and therefore lower cost downstream (away from the source) circuit breakers. This is a recognized and permitted technique under IEC 60364-1 and related national LV installation standards: 'A lower breaking capacity is permitted if another protective device having the necessary breaking capacity is installed on the supply side. In that case the characteristics of the device shall be coordinated so that the energy let through of these two devices does not exceed that which can be withstood without damage by the device on the load side and the conductors protected by these devices.' Correct cascading characteristics may only be obtained after laboratory testing and details of possible combinations for a particular application are detailed in manufacturers' literature. Consider the 415 V network and associated MCCB current limitation curves shown in Fig. 13b and Fig. 21 respectively.
For correct cascading the following two criteria must be met:
1. The upstream device, A, is co-ordinated for cascading with both devices B and C even if the cascading criteria is not achieved between B and C.
It is simply necessary to check that the fault-breaking capacity combinations A1B and A1C meet the downstream requirements.
2. Each pair of successive devices is co-ordinated i.e. A with B and B with C, even if the cascading criteria between A and C is not fulfilled. It is simply necessary to check that the combinations A1B and B1C have the required breaking capacity.
The results of this approach to the examples shown in Fig. 13b are shown in Table 7.
TABLE 7 Reinforced Cascaded MCCB Breaking Capacities in a Radial Network
A contactor or switch disconnector with limited breaking capacity and short circuit withstand, or a cable with a thermal stress limitation, may be short circuit and overload protected by a suitably rated series MCCB or MCB. The approach is the same as that shown in Sub-Section 3.4 for fuse protection.
Using the Merlin Gerin Compact series MCCB 415 V system voltage current limitation curves shown in Fig. 21, the following typical design questions may be answered:
1. To what value is a prospective short circuit current, ISC 5100 kA rms, limited when upstream protection is provided by using a C400L type MCCB (660 V-rated voltage, 400 A-rated current at 20 - C, 150 kA rms breaking capacity, IEC Class P1)?
- From the current limitation curves at 415 V, approximately 42 kA peak.
2. A 25mm2 , aluminum conductor PVC insulated cable has a maximum permissible thermal stress limit of 3.613106 A2 s. Will the cable be adequately short circuit protected by a C250H type MCCB (660 V-rated volt age, 250 A-rated current at 40 - C, 85 kA rms breaking capacity, IEC Class P1)?
- From the thermal stress limitation curves at 415 V, the protection is limited to a short circuit current of approximately 38 kA.
3. A C161L type MCCB (660 V-rated voltage, 150 A-rated current at 40 deg.C, 150 kA rms breaking capacity, IEC Class P1) feeds via a long length of cable, a distribution board with one 120 A and one 30 A outgoing way.
The prospective short circuit level at the C161L MCCB is ISC 540 kA and this is reduced by the cable impedance to ISC 524 kA at the distribution board busbars. Check if it is possible to install a C45N MCB as the 30 A breaker and a C125N MCCB as the 120 A breaker. The maximum operating temperature inside the cubicle and at the busbar connections is 40 deg. C.
- From Fig. 21 and the data given above, the C161L MCCB will limit the 40 kA rms prospective short circuit current to approximately 15 kA peak. A C125N MCCB has a current rating of 125 A at 40 - C and from Fig. 21 a short circuit breaking capacity of approximately 22 kA. It is therefore suitable for this application.
- A C45N MCB has a current rating of 60 A at 40 - C and from manufacturers' literature a breaking capacity of only 6 kA rms. However from Fig. 21, the C161L MCCB will limit the 24 kA prospective short circuit level to 13.5 kA peak and from manufacturers' literature (not included here) it is recommended by reinforced cascaded fault limitation as adequately rated for this application.
In both cases a further check is necessary to ensure adequate discrimination with both up- and downstream protective devices.
4.3.2 Discrimination
Discrimination or selectivity is the co-ordination of automatic devices in such a manner that a fault appearing at a given point in the network is cleared by the protective device, and by that device alone, installed immediately upstream of the fault. A full explanation is given in Section 10. If the loading conditions of two circuit breakers connected in series in a radial circuit are similar, then a comparison of the cold MCCB time/current characteristic curves will provide a reasonable assessment of overload discrimination.
In order to determine short circuit discrimination the relevant I^2t let through and operating thresholds must be considered. In the example given in Fig. 13b for a fault on busbar 2 only breaker B must operate such that other supplies fed from busbar 1 are maintained. This discrimination must be satisfied up to the full short circuit levels and may be difficult to achieve owing to the similar definite minimum time characteristics of the cascaded MCBs at high fault levels.