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1. INTRODUCTION
Fuses act as a weak link in a circuit. These reliably rupture and isolate the faulty circuit under overload and short circuit fault conditions so that equipment and personnel are protected. Following fault clearance they must be manually replaced before that circuit may be put back into operation. Striker pins are available on some designs such that remote alarms may be initiated on fuse operation.
Miniature circuit breakers (MCBs) or molded case circuit breakers (MCCBs) are also overcurrent protection devices often with thermal and magnetic elements for overload and short circuit fault protection. Earth leak age protection, shunt trip coils and undervoltage releases may also be incorporated in the designs. As a switch they allow isolation of the supply from the load. Normally, the MCB requires manual resetting after a trip situation but solenoid or motor driven closing is also possible for remote control.
This Section describes the various types of fuse and MCB together with their different uses and methods of specification. Examples and calculations for correct selection of different applications are also given.
2. FUSES
2.1 Types and Standards
2.1.1 General
Table 1 gives a summary of different fuse types, their uses, advantages and disadvantages. Table 2 summarizes some current relevant standards covering fuses. There are various categories ranging from subminiature electronic and solid-state device protection fuses, power types (expulsion and high rupturing capacity (HRC)) to "high voltage" fuses suitable for operating at voltages up to 72 kV. Rewireable fuses to BS3036, which are now of limited new application, and are not discussed in detail but are illustrated in Fig. 1b.
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TABLE 1 Summary of Fuse Types
Category -- Types -- Use -- Advantages and Disadvantages
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1. High voltage fuses above 1,000 V AC Expulsion types IEC 60282-2 Outdoor and indoor network protection Cheap replaceable element.
Arc extinguished by expulsion effect of gases and therefore needs good clearances. Not current limiting.
Capacitor protection IEC 60549, 60871, 60143 Protection of shunt and series power capacitors Unit fuses.
Clearance of faults within capacitor unit. Permits continued operation of residue of shunt capacitor bank. Line fuses to isolate faulted bank from the system.
Refillable types available Resist a stated level of repetitive discharge I 2 t.
System requires mechanical switching devices.
Transformer protection IEC 60787 Transformer circuit protection and co ordination Good selection guide.
Motor circuit applications IEC 60644 For use with direct online (DOL) AC motors Withstands motor starting currents.
Slow operation in the 10 s region (high K) combined with fast operation below 0.1 s to retain good short circuit limitation. Usually back-up types.
Current limiting types IEC 60282-1 Networks and high power industrial uses with strikers Limits short circuit energy.
Switchgear tripping Cheaper than circuit breakers.
Prevents single phasing Special types may be oil immersed.
For correct striker pin operation, see IEC 62271-105 Gives indication of operation Takes time to replace fuses when restoring supply but exact restoration of characteristics ensured, whereas circuit breakers may need maintenance.
Isolating switch required.
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2. Low-voltage fuses below 1,000 V AC - IEC 60269-1 HRC types IEC 60269-2 Supply networks Interchangeable ratings within range of fuse carrier size.
Industrial protection with ratings up to 1,250 A, and breaking capacity 80 kA Comparatively inexpensive limitation of short circuits.
Disadvantages
- BS88 For use by authorized persons Accurate time/current characteristics for a variety of applications.
Quick and easy replacement with cartridge of the correct type but longer than reclosing circuit breakers.
Special semi-conductor protection IEC 60269-4 Very fast acting on short circuit I 2 t and overvoltage very carefully controlled.
Protection of consumer units Breaking capacity to 33 kA Ratings may not be interchangeable for safety reasons when replaceable by a domestic consumer.
Domestic types IEC 60269-3, BS 1361 Special types for electricity supply utility replacement ensure discrimination, giving negligible chance of anomalous rupture of supply utility fuse.
Fuse links in plugs BS 1362 Ratings interchangeable - 13 A for power to 3 A for lighting with 5, 2 and 1 A ratings for other applications.
High breaking capacity for small size. Cheap and easy to replace.
Remain stable when carrying current for long periods.
Semi-enclosed rewirable types BS 3036 Protection of subcircuits, breaking capacity 1-4kA Economical where frequent short circuits occur. High fusing factor, low breaking capacity.
Less efficient limitation of short circuits. Variability of characteristics after rewiring.
Deterioration in use.
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3. Miniature fuses.
Higher breaking capacity sand-filled cartridge types IEC 60127-2 Protection of electronic and similar apparatus Cheap, large range of characteristics from quick acting to long time delay.
Breaking capacity below 2kA - IEC 60127 Interchangeable.
Low breaking capacity air-filled cartridge types IEC 60127-2 Assist rapid maintenance by isolating parts of electronic circuitry.
Low breaking capacity subminiature types IEC 60127-3 Avoid use on high prospective fault current circuits and replacement by incorrect types.
Fuse holders IEC 60127-6
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TABLE 2 Summary of IEC, BS and North American Fuse Standards
Description IEC BS USA
Definitions 60050-441 EN 60269 UL 248.1 2692 ANSI/IEEE C37.40 Low voltage 60269 88 UL 248 Semi-enclosed - 3036 - LV contactors 60947-4 5424 Industrial 60269-1 and 2 88-2 Fuse switchboard - 5486 High voltage 60282 EN 60282-1 IEEE C37.1 Motor circuits (HV) 60644 EN 60644 IEEE C37.46 HV contactors 60470 EN 60470 HV starters 60470 EN 60470 Distribution type 60282-2 2692-2 ANSI/IEEE C37.40, 41, 42, 47 Semiconductors 60269-4 88-4 UL 248-13 Capacitors (HV) 60549 5564 Isolators and switches 60129 EN 60129 60265-2 EN 60947-3 ANSI/IEEE C37 series EN 60265 Oil immersed type ANSI/IEEE C37.44 Design tests ANSI/IEEE C37.42
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A trend to harmonize fuse types (National Standards - BS, etc.; European - CENELEC; and International - IEC) is currently in progress speeded up by the pan-European mergers between large manufacturers. For example, revisions to BS88 Parts 1-5 were introduced in 1988 to coincide with IEC Standards and an additional Part 6 introduced. General purpose fuses are given the classification 'gG', where 'g' indicates full range breaking capacity and 'G' indicates general application. Fuses for application in motor circuits are given the classification 'gM' and are characterized by having essentially two current ratings, In and Ich. In denotes the rated current of the associated fuse holder and the second value, Ich gives the operational characteristics. For example, a 32M63 fuse link has the operational characteristics of a 63 A fuse link but its continuous rating and size is restricted to that of a 32 A fuse holder.
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TABLE 3 Useful Terms and Definitions
Item
Fuse Fuse link Fuse holder Ambient air temperature Switch fuse Fuse switch Fusing current Fuse breaking capacity rating Prospective current Minimum fusing current Current rating Cut-off
Pre-arcing time Arcing time Total operating time Let through I2 t (Joule integral)
Fusing factor (at present fuses tend to have different characteristics depending not only upon type but also on the standard to which they are manufactured.
IEC 60269 sets time 'gates' for maximum and minimum fusing currents at set times (see Fig. 2).
Gate
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Description
The complete device including the fuse holder and fuse link. Fig. 1 shows a semi-enclosed rewirable fuse and a filled cartridge type with bolted end connection arrangements.
The replaceable part, normally in cartridge form, containing a fuse element that melts under overload or short circuit conditions.
The combination of fuse base and fuse carrier.
The temperature of the air outside the fuse enclosure. Note that the performance of fuses, and to an even greater extent MCBs, is affected by the ambient temperature and the type of thermal characteristics of the enclosure. Cartridge fuses have different characteristics when mounted in a fuse holder compared to the standard (IEC 60269) test rig.
A switch in series with a fixed fuse.
A switch where the fuse link (or carrier) forms the moving contact of the switch.
The rms current that will melt the fuse element in any specified time from the commencement of current flow.
The maximum prospective current that can be broken by a fuse at its voltage rating under prescribed conditions.
The rms value of the alternating component of current that would flow in the circuit if the fuse were replaced by a solid link.
The minimum current capable of causing the fuse to operate in a specified time.
The current that the fuse link will carry continuously without deterioration.
If the melting of a fuse element prevents the current reaching the prospective current then the fuse link is said to 'cut off'. The instantaneous minimum current obtained is then the 'cut-off current.'
The time between the commencement of a current large enough to cause a break in the fuse element and the instant that the arc is initiated.
The time between the instant when the arc is initiated and the instant when the circuit is broken and the current becomes permanently zero.
The sum of the pre-arcing and arcing times.
The integral of the square of the current over a given time.
A fuse must reliably carry full load current and small overloads such as transformer magnetizing inrush currents, capacitor charging currents and motor starting currents for a short time. The ratio between the rated current and the minimum fusing current is the fusing factor and is normally 1.45 or as low as 1.25. At such overloads the fuse will melt in about 1 h and at higher currents more quickly.
Limiting values within which the characteristics (e.g. time/current characteristics) shall be contained (see Fig. 2)
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2.1.2 Standard Conditions of Operation
As the behavior of fuses is affected by environmental conditions, it is important to check this aspect before determining ratings. The following are usually included in a specification:
- Ambient temperature - the IEC standards call for low-voltage fuses to be suitable for ambient temperatures between -5 and 140 - C, while high voltage fuses must operate satisfactorily from -25 to 140deg. C.
- Humidity - a typical requirement is that satisfactory operation should be obtained in relative humidities up to 50% at 140 - C (and higher levels at lower temperatures).
- Altitude - sometimes overlooked - LV fuses meeting IEC specifications must be suitable for operation up to 2,000 m, but the IEC specification for HV is only 1,000 m.
- Pollution - it is usual for standards to contain statements to the effect that the ambient air 'should not be excessively polluted' by dust, smoke, corrosive or flammable gases, vapor or smoke. Specifiers should there fore make special note of coastal or industrially polluted atmosphere.
2.2 Definitions and Terminology
The major terms and definitions associated with fuses are described in Table 3. A fuller range is provided in Ref. [5], which in turn derives its list from IEC standards 60127, 60269 and 60282.
2.3 HRC Fuses
The high rupturing capacity (HRC) fuse has excellent current and energy limiting characteristics and is capable of reliable operation at high prospective rms symmetrical current fault levels (typically 80 kA at 400 V and 40 kA at 11 kV). Fuses are available in ratings up to 1,250 A at low voltages and, say, 100 A at 11 kV, and normally packaged in cartridge format. The fuse operates very rapidly under short circuit fault conditions to disconnect the fault within the first half cycle and therefore limit the prospective peak current.
The fuse element traditionally consists of a silver element. Recent research and development by some manufacturers has allowed copper to be used when problems of increased pre-arcing I 2 t, less pronounced eutectic alloying ('M') effect and surface oxidation are overcome. In some cases the performance of the copper element fuses actually surpasses that of the silver types (Figs. 1 and 2).
The silver or copper strip element is perforated or waisted at intervals to reduce power consumption and improve the tolerance to overloads as shown in Fig. 3. The fuse operation consists of a melting and an arcing process.
Under high fault currents the narrow sections heat up and melt. Arcing occurs across the gaps until the arc voltage is so high that the current is forced to zero and the fuse link ruptures. The operation of a typical 100 A HRC-rated fuse under short circuit conditions is shown in Fig. 4.
Under low fault current or overload conditions the whole centre part of the fuse element heats up uniformly as heat is conducted from the narrow sections to the wider parts. The centre section then eventually melts. Low melting point alloys deposited at points on silver or copper fuse elements ( Fig. 3) are used to delay the fuse operation. Alloys with melting points of approximately 180 and 230 deg. C are used for silver- and copper- (melting points approximately 1,000 deg. C) based fuse elements. When the alloy reaches its melting point it combines, after a delay, with the main fuse element material to produce a eutectic with a slightly higher melting point than the alloy alone but a much reduced overall fuse material melting point. This allows the main fuse element to melt at low overcurrents.
Especially fast acting, low I 2 t let through (low Joule constant, see Sub-Section 3.2.1) and high-current HRC fuses are required to protect power semiconductor devices because of the low thermal mass and very short times for the semiconductor devices to achieve thermal runaway to destruction.
FIG. 1 (a) Rewirable semi-enclosed fuse; (b) quartz sand-filled cartridge
fuse.
FIG. 2 IEC 60269 time/current gates for type gG fuses.
FIG. 3 Techniques of time delay on a selection of types of fuse element.
FIG. 4 Short circuit fuse operation for 100 A HRC cartridge fuse.
2.4 High Voltage Fuses
2.4.1 HRC Types
The construction is similar to the low-voltage type except that the element must be longer, with more constrictions because of the higher arc voltage that must be developed to interrupt the current. Such designs must have safe low-overcurrent operation, time/current characteristics to suit the application (for example, distribution transformer HV protection), fully adequate current and energy limitation characteristics under short circuit conditions, be of robust mechanical construction and available in standard fuse dimension packages. The element is commonly helically wound on a ceramic former.
Such an arrangement is not particularly suitable for motor protection fuse applications at, say, 3.3 kV to 11 kV, because of the thermal stresses imposed upon the element under frequent starting conditions. Fuses required for such applications (for example, in series with vacuum contactors of insufficient fault rating) have straight corrugated self-supporting elements to accommodate the stresses involved ( Fig. 5).
FIG. 5 Main constructional features of GEC high voltage fuses: (a) typical
distribution fuse; (b) typical motor circuit fuse. Helically wound silver
strip elements; Self-supporting silver strip elements with stress relief
form
2.4.2 Expulsion Types
Unlike the HRC type the fuse element is contained within a narrow bore tube surrounded by air. Under fault conditions the fuse element melts and an arc is struck across the break. The heat of the arc vaporizes a material such as resin-impregnated fiber lining the inner tube wall and this, added to the arc vapor, rushes out of the tube ends at high velocity. The gas movement assisted by the cooling and de-ionizing effect of the vaporized tube wall pro ducts extinguishes the arc. Suitable dimensioning allows reliable fault clearance down to the minimum fuse element melting current. The spring tension arrangement shown in Fig. 6 allows the fuse break to enlarge when arcing commences, thus increasing the arc voltage and aiding extinction. Such fuse types are usually employed on outdoor distribution equipment and overhead line poles. The mechanism allows the top contact to disengage on fuse operation so that the fuse carrier tube falls outwards about the lower hinge. This allows isolation and avoids leakage along the tube due to build up of arc deposits. It also makes it easy to spot fuse operation by the overhead line inspection/repair team. Expulsion fuses are not silent in operation and additional clearances are required to avoid ionized gases causing flashover.
FIG. 6 Expulsion fuse arrangement.
The advantage of the expulsion fuse element is that it has characteristics well suited to small distribution transformer protection. Slow and fast blowing characteristics are available ( Fig. 7). Its small surface area and air surround produces rapid operation under moderate fault conditions. Lack of current restrictions gives a much slower high fault current operation time.
The device is not current limiting and therefore has a rather low breaking capacity limit.
FIG. 7 Time/current characteristics of fast and slow blowing HV fuses.
2.4.3 Maximum Instantaneous Short Circuit Current, IS Limiter
A practical difficulty exists in producing high voltage HRC fuses at the higher current ratings. Following the installation of additional generation onto a system or perhaps the reinforcement of a system by the introduction of various interconnections the fault levels will inevitably increase. Sometimes this increase is beyond the capability of the existing switchgear. A choice then has to be made on whether to replace the switchgear or introduce fault limiting devices such as series reactors or the IS limiter.
The IS limiter acts like an HRC fuse and may be placed in series with the equipment to be protected. It is available for rated voltages in the range 0.75 to 36 kV. It limits the mechanical stresses on equipment by limiting the maxi mum instantaneous short circuit current. The cut-off is very rapid such that the short circuit current reaches only about 20% of the unrestrained prospective current peak and is completely cut off in typically 5 ms with a low resulting overvoltage. The AC component of the fault, which stresses the equipment thermally by the heat generated, is also minimized. An oscillogram of the interruption of a single phase with an IS limiter is shown in Fig. 8.
FIG. 8 Oscillogram of a single-phase interruption with an IS limiter.
1 Time base; 2 Voltage across lS limiter insert bridged by copper bar; 3 Short-circuit current without lS limiter; 4 Tripping pulse; 5 Voltage across l S limiter under operation; 6 Short circuit current with lS limiter
The IS limiter essentially consists of three components as shown in Fig. 9:
- An adjustable electronic sensing circuit and integral current transformer which are set to interrupt the fault depending on the rate of rise of fault current and a minimum fault current value.
- A main current conductor which contains a small explosive charge. When the tripping signal is received from the sensing circuit the main conductor is interrupted by this charge.
- A quenching circuit consisting of a lower current-carrying capacity HRC fuse which is connected in parallel with the main current conductor.
FIG. 9 IS limiter components.
1) A CT measures the short circuit current
2) An electronic circuit controls the tripping
3) A pulse condenser provides the power via a pulse transformer for firing the detonator
4) IS limiter
1-Insulating tube 2-Bursting bridge 3-Detonator cap 4-Indicator 5-Quenching material 6-Fuse element
Following breaking of the main current conductor the fuse circuit rapidly (0.5 ms) completes the fault clearance.
The advantages of the IS limiter are:
- Considerable cost savings compared to alternatives such as replacement of existing switchgear with higher fault-rated equipment or the introduction of fault limiting reactors into a system.
- Operating costs for the IS limiter are nil. Reactors would introduce losses into the system.
- Further increases in current rating may be obtained by operating individual units in parallel per phase.
The disadvantages of the IS limiter are:
- Control circuitry is somewhat complex and a risk of maloperation does exist.
- Replacement of inserts in the event of operation involves expenditure and is necessary before supply can be restored. A spares holding is therefore necessary.
- Compliance with the Health and Safety Executive Electricity at Work Regulations in the UK and similar regulations abroad requires periodic testing of the units and a record of test results to be maintained. A test kit is available from the manufacturers for this purpose.
2.4.4 Automatic Sectionalizing Links (ASLs)
These devices, sometimes referred to as 'smart fuses', are not fuses in the sense of operating through the fusion of metallic elements. However, they are designed as retrofit replacements for conventional expulsion fuse carriers and are installed and maintained using fuse operating poles; so they are included in this Section.
The ASL fulfills the same function in a distribution network as an automatic sectionalizing switch or sectionalizer. It is installed in an overhead sys tem, downstream of an auto-reclose circuit breaker, and minimizes the extent of the system outage in the event of a permanent network fault.
Making use of advances in solid-state microelectronics, a miniaturized logic circuit is fitted inside the dimensions of the carrier tube of a conventional expulsion fuse, and installed accordingly. The tube is a conductor instead of an insulator, so the current flows through the tube and encircling current transformers feed information regarding the state of this current into the logic circuit within.
On the occurrence of a transient overcurrent, the upstream auto-recloser or circuit breaker will trip and then reclose. The ASL records this event and retains it in its memory for several seconds. If on re-closure the load current has returned to normal, the ASL erases that memory and the circuit returns to normal. If however the fault is permanent, the fault will be present when the recloser re-energizes the circuit. The ASL logs this second event and awaits the second trip of the recloser. When the ASL detects that the line current has fallen to zero, it fires a chemical actuator (similar to a fuse striker) which de-latches the ASL carrier tube so that it swings down and provides safe isolation of the downstream fault. The remainder of the net work reverts to normal operation. In some versions of the ASL, a resettable magnetic latch is in place of the chemical actuator. This obviates the need to replace the actuator, but is more expensive.
2.5 Cartridge Fuse Construction
The cartridge fuse consists of a fuse element surrounded by a pure quartz granular filler contained in a tough ceramic enclosure (see Fig. 1b). The filler allows the fuse element arc vapors rapidly to condense and avoid pressure build-up within the enclosure. It also aids heat dissipation from the fuse element thereby allowing a smaller quantity of fuse element material to be used, again reducing the pressure in the cartridge. Having a number of fuse elements within the cartridge in parallel increases the surface area in contact with the filler, assists heat dissipation and helps arc extinction. Good filler material quality control is essential for repeatable minimum fusing cur rent characteristics.
The striker-pin fuse is a variation of the standard cartridge fuse link. A high resistance wire in parallel with the fuse element melts when the fuse operates and detonates an explosive charge. The charge fires out the striker pin from the fuse end cap ( Fig. 10). The striker-pin operation from any one phase is normally arranged in the associated three-phase switchgear to trip out all three phases virtually simultaneously.
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FIG. 10 Fuse trip striker-pin arrangements.
Ceramic barrel - Element former - High resistance wire in parallel with element - Quartz filler - Explosive charge - End cap - Striker pin - Seal Striker pin - Capsule - Outer housing - Ignition wire - Powder charge - Temped fiber plug Insulating bush Ignition circuit make-off wire
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