Fuses and Overcurrent Protectors PDF

1 (18) FUSES AND OVERCURRENT PROTECTORS CONCEPTS: OVERCURRENT PROTECTION IS A GENERIC TERM FOR OVERLOAD AND SHORT CIRCUIT PROTECTION According to the standards, cables must normally be equipped with overcurrent protection to prevent excessive heating of the cables. However, an overcurrent protect...

1 (18) FUSES AND OVERCURRENT PROTECTORS CONCEPTS: OVERCURRENT PROTECTION IS A GENERIC TERM FOR OVERLOAD AND SHORT CIRCUIT PROTECTION According to the standards, cables must normally be equipped with overcurrent protection to prevent excessive heating of the cables. However, an overcurrent protector must not be used if turning off the current proves more dangerous than heating the conductors. E.g. The zero protection and grounding conductor must not have overcurrent protection. Protective devices must operate at currents, voltages and for such a period of time as is appropriate to take into account the characteristics of the circuits and the possibility of hazards. Overcurrent protectors are divided into overload and short circuit protectors. Usually, the same device is not good at both, but both protections use their own, such as a fuse and a motor protection circuit breaker. Short circuit protection is used to cut short circuit currents. It must cut off as quickly as possible and have a high breaking capacity. Normally, the short-circuit protection only starts to work at such a high excess current that it does not protect the wire from overload. The most common short-circuit protectors are fuses, as well as circuit breakers controlled by relays. The short-circuit protection is intended for cutting short circuit currents that are many times higher than the normal load current. A short circuit is typically e.g. connection of a phase conductor and a neutral conductor. In the event of a short circuit, the current magnitude is e.g. 10 A behind fuse 100 A ( depends on the location of the property; urban/rural ). The fuse is a pretty good short-circuit protector OVERLOAD PROTECTION: The function of the overload protection is to protect the wire from overload currents higher than the normal load current and to cut off the current only when the temperature of the line is about to exceed the permissible value. Overload protection usually protects some actuator, e.g. electric motor. Motor overload protection is typically a thermal relay or motor protection switch. In the electrical system of a private house, the fuse acts both as short-circuit protection for the wire and as overload protection. Overload protection protects the cable from overload currents that are somewhat higher than normal load currents. Good overload protection cuts off the power only when the temperature of the wire is about to exceed the permissible value.  2 (18) a = ideal short-circuit protection, b = ideal overload protection SELECTIVITY Selectivity aims to limit disturbances caused by a fault situation in the electricity network to as small an area as possible, i.e. to a small area. disconnect as small a part of the network as possible, while keeping the rest of the network in normal use. For example, when a couple of circuit breakers are connected in a row, they are selective when the circuit breaker closest to the fault point is working. The selectivity of circuit breakers can be investigated by comparing operating time curves. In other words, the operating time curves of successive circuit breakers are placed in the current-time coordinate system, from which, by finding out their intersection, the selectivity limit is obtained, if they do not intersect with each other, the action is selective. However, it is easiest to study the tables published by manufacturers and choose according to the recommendations of manufacturers. Selectivity depends on the size of the fault current and is denoted by kA. In turn, the selectivity of fuses can be viewed from melting time curves. When these melting curves differ, they are selective.  3 (18) Consider the mutual selectivity of the fuse and the circuit breaker in the case where the fuse precedes the circuit breaker circuit breaker | fuse I = short-circuit current Icn =Nominal short-circuit current breaking capacity Is =Selective cut-off current A =Minimum fuse melting limit current and (I^2)*t value C =Maximum operating limit current of the circuit breaker and (I^2)*t value selectivity limit selectivity range | non-selective range Fuse protection A fuse is a device operating in an electrical circuit with a metal wire or tape, the material and thickness of which are selected so that when the current exceeds a certain value, the metal melts and causes the current to break down. For ordinary instrument panels, fuses ("props") with a heat- resistant porcelain shell are used; The breakage of the fuse wire is indicated by the detachment of a small colored plate on the outside of the shell. In amplifiers, etc. Electronic devices use fuses with a closed glass shell. The characteristics of fuses can be described by the following quantities: short-circuit current cutting ability Melting curve rated voltage and current  4 (18) PLUG FUSES: Plug fuses are suitable for use as overload protection and short circuit protection. The use of a plug fuse as a short-circuit protection is limited by its cutting ability. The most common plug fuses are manufactured for a rated voltage of 500V. The cutting capacity of 500V plug fuses is 20kA according to the standard The minimum requirement table of the IEC standard below shows that the cutting capacity of the plug fuse is non-existent, although the tripping capacity of plug fuses sold in Finland is higher than the values in the minimum requirement table, approximately 20kA. I ≤ 25 A, katk. ability 4 kA 25a < I ≤ 63 A, cache ability I > 63 A, katk. ability 16 kA SIZES DII ( 25 A ) AND DIII ( 63 A ) FUSE BOX AND COVER DII AND DIII -> IF DIII COVER AND FUSE LESS THAN 25A ARE USED REDUCER RING  5 (18) CONSTRICTOR RING POCELAIN RING BOTTOM CONTACT AND BOTTOM CONTACT WRENCH THERE ARE BOTH FAST (gG fuse, e.g. lighting groups) AND slow ( aM - fuse, e.g. circulation pump)  6 (18) MORE SPECIAL PLUG FUSES DI FUSE DO fuse trays Sizes: D01(16A), D02(63A), D03 Applications: Protection of industrial circuits and electrical equipment.  7 (18) CIRCUIT BREAKERS A circuit breaker, i.e. an automatic fuse, is used to protect the electrical network. It protects wires, cables and equipment. A circuit breaker breaks the circuit in the event of an overload or short circuit, thus preventing major damage. They are also used for protection against excessively high contact voltages due to insulation failures. Circuit breakers are current-limiting and equipped with magnetic quick release and slow-motion thermal triggering Circuit breakers are classified into several classes based on their trip curves. K, Z and A are manufacturer- specific trigger curves. B-curve for resistive loads, heating, lighting C-curve like B, but additionally for both slightly inductive and capacitive loads. D-curve for strongly inductive and capacitive loads, are intended for applications with higher starting current peak. The K-trip curve is like the D-curve and is used for cable and equipment protection such as motors, transformers, automation. Selected based on the rated current of the motor. With magnetic tripping, the starting current of the motor does not cause tripping. Strongly inductive loads, motors, transformers, discharge high pressure sodium lamps, power tools, welding machines The Z and A tripping curve is intended for the protection of semiconductors, thyristors, diodes, voltage and dimensional transformers, as well as those with low contact currents, such as control circuits. CHARACTERISTICALLY TYPE B CIRCUIT PROTECTION AUTOMATIC HIGH-SPEED 3*IN -5*IN TYPE C CIRCUIT PROTECTION AUTOMATIC SLOW 5*IN-10*IN TYPE D LINE PROTECTION AUTOMATIC EVEN SLOWER 10*IN-20*IN  8 (18) Their choice is influenced by the rated current (0.2-125A), cutting ability and voltage. The rated voltage is normally 400VAC and maximum allowable operating voltage 440VAC. Of course, there are also circuit breakers up to 690V on the market, but when the voltage increases, it is also important to consider lowering the cutting capacity. launch time nominal current multiple nominal current multiple Trigger curve K "slow" Trip curve B Trip curve C CIRCUIT BREAKER FOR OVERLOAD PROTECTION The use of circuit breakers as overload protection has increased dramatically in recent years. When using circuit breakers, there are a few differences that need to be taken into account compared to fuses. The power losses of a circuit breaker are greater than those of a similarly sized fuse, Circuit breakers for wire protection are compact in design, that is, they fit into a smaller space. For this reason, the temperature of a final panelboard can be significantly higher than that of a similar fuse switchboard, especially if the panel includes several groups with a high load. The temperature also affects the operating values of circuit breakers and the load capacity of the wires. Groups loaded close to their rated current should not be installed right next to each other.  9 (18) WITH HIGHER CURRENTS, HANDLE FUSES ARE USED Handle fuses are very suitable for short circuit protection due to their cutting properties. Handle fuses are most commonly manufactured for a rated voltage of 690V. The cutting capacity of handle fuses is at least 50kA, unless a higher rated cutting capacity has been declared by the ready-made. Table 3.7 Fuse sizes and current ranges.... Size current range of the fuse [A] Table 3.7 Fuse sizes and current ranges  10 (18) INDICATION OF BLOWN HANDLE FUSE SWITCH FUSE HANDLE ON THE DOOR  11 (18) EXCHANGE HANDLE Cut-off area and use class The cut-off area and operating class of fuses shall be indicated by the following abbreviations: g suitable as short circuit and overload protection fuse, The operation of such a fuse is guaranteed over the entire overcurrent range a fuse suitable only for short circuit protection G "fast fuse", suitable for wire protection M "slow fuse" suitable for motor circuit protection, durable motor starting current impetus R "High speed fuse", suitable for protection of power semiconductors  12 (18) The fuse cut-off area is described by the letter g or a: The cutting capacity of the g-fuse covers the entire current range The cutting capacity of an a-fuse covers a certain area The second letter indicates the class of use. This letter accurately defines the power-time characteristics. For example: gG means a general-purpose fuse with a cutting capacity covering the entire current range gM means a protective fuse in the motor circuit, the cutting capacity of which covers the entire current range aM means a protective fuse in a motor circuit whose cutting capacity covers a specific area of current OVERLOAD PROTECTION WITH FUSES For fuses, the upper melting limit current is the current at which the fuse will work confidently. As a rule, it is greater than 1.45 times the rated current of the fuse. In this case, the overload protection cannot be selected directly according to the load on the conductor, but the formula must be used for dimensioning: k * In ≤ 1.45 * Iz IN is the rated current of the protective device Iz continuous load capacity of the conductor k The ratio of the upper melting limit current of the fuse to the rated current of the fuse With type D plug fuses k= 2.1 when In ≤ 4A => In ≤ 0.69 * Iz 1.9 when 4A < In ≤ 10A => In ≤ 0.76 * Iz 1.75 when 10A < In ≤ 25A => In ≤ 0.82 * Iz 1.6 when In > 25A => In ≤ 0.90 * Iz with gG type fuses k= 2.1 when In ≤ 4A => In ≤ 0.69 * Iz 1.9 when 4A < In ≤ 10A => In ≤ 0.76 * Iz 1.6 when In ≤ 16A => In ≤ 0.90 * Iz For fuses, the lower melting current is the current at which the fuse can operate. Ia is at least 1.13 *In. For fuses, the upper melting limit current is the current at which the fuse will work confidently. As a rule, it is greater than 1,45 times the nominal current of the fuse."  13 (18) For example, in the trip curves found in Schneider electric's free "curve direct" program, the melting limit currents of the 25A gG fuse per hour are: - lower melting limit current 31A - upper melting limit 40A In other words, these 25A fuses may trip when loaded with 31A for an hour. And they are sure to trigger when loaded with 40A current for that same hour Example: The load capacity of the line adjusted by correction factors is 26A. For overload protection, gG fuses are used. Let's solve the maximum rated current of the fuse: k-value=1,6 Iz=26A In=? From the above formula, we get In < (1.45/1.6) * 26A=23.5A So, overload protection can be implemented with a 20A gG fuse. Nominal current of a fuse of type gG, A| Minimum value of the load capacity of the conductor, A Example of loadability of the conductor and choice of overload protection  14 (18) Differences from a fuseless system The maximum operating limit current of fuses is not as close to the rated current as that of circuit breakers or other circuit breakers. With the exception of the old L- and U-type protectors, a fuse always needs more load capacity from the wire than a line protector. FUSE DIMENSIONING IF THE 1-PHASE POWER OF THE DEVICE P = 2200 W -> I = 2200 W / 230 V = 9.6 A, THAT IS, THIS IS THE LOAD CURRENT, WHICH SHOULD BE SMALLER THAN THE FUSE TO BE CHOSEN, THAT IS, IN THIS CASE, THE FUSE IS 10 A THREE-PHASE LOADS: KITCHEN STOVE , with a typical power of 8,1 kW SAUNA STOVE with a typical power of 6 kW However, both of these are devices with 3 pcs of 1-phase loads. E.g. sauna heater 6 kW = 3 x 2 kW IN THE CASE OF CORRECT 3-PHASE POWER, THE CALCULATION FORMULA IS AS FOLLOWS: P = √3 x U x I x COSφ = U = main voltage 400 V I = rated current of the device ( from the plate value of the device ) COSφ = cosine fii, which for motors is of the order 0,8. With a purely resistive load, COSφ = 1 Resistive loads include, for example: stove resistors, stove resistors , electric radiators, etc. With inductive loads (e.g. electric motors, fluorescent lamps, etc.) COSφ < 1. THE FOLLOWING IS A RULE OF THUMB FOR FUSE DIMENSIONS IN FIXED INSTALLATIONS 1.5 mm2 CU MINIMUM CROSS- SECTION, FUSE 10 A -> 1.5 mm2 CU fuse 16 A -> 2.5 mm2 fuse 25 A -> 6 mm2 fuse 35 A -> 10 mm2 fuse 50 A -> 16 mm2 FOR LARGER CROSS-SECTIONS, ALUMINUM CABLES ARE USED ( CHEAPER )  15 (18) WHEN SWITCHING FROM COPPER TO ALUMINIUM, USE TRANSITION CONNECTORS  16 (18) WHEN CHOOSING A FUSE AND WIRE, LOAD CONDITIONS MUST BE TAKEN INTO ACCOUNT. WIRES HEAT DIFFERENTLY IN SURFACE MOUNTING, FLUSH MOUNTING, CABLE WRECKAGE OR IF THE CABLE IS INSTALLED IN THE GROUND The heating of the cable depends on the following factors, among other things: 1. Ambient temperature and thermal conductivity 2. Warming effect of nearby electrical or heating lines 3. Limited movement of air in the duct and shaft 4. Cable gutters, protective pipes, etc. additional heat resistance caused 5. Previous load on the line 6. Underground cable installation depth 7. Specific thermal resistance of the earth Fuseless protection Circuit breaker selection In addition to the rated current and release, the key factors affecting safety and price are the correct cutting capacity. For it, it is necessary to calculate the maximum value of the short-circuit current occurring at the location of the circuit breaker. Compact circuit breakers Compact circuit breakers are plastic framed and insulated. They are used as main circuit breakers, motor outputs, as well as cable protection. They are smaller than air circuit breakers, as well as more modest in performance. In general, they meet the main clutch requirements, but for smaller frames this qualification must be checked with the manufacturer. Their current limiting properties range from the so-called. from zero-point circuit breakers to current-limiters. The cutting capacity increases as the current limiting capacity improves. They have a voltage resistance of up to 690VAC, rated current 125-1600A and pole number 3 or 4. The cut- off ability is selected based on the short-circuit current that occurs. Cutting abilities range from 16 to 100kA. Installation methods include: fixed, extinguishing and pull-out.  17 (18) Circuit breakers below 125 A include, depending on the model, a fixed or adjustable thermomagnetic relay. The 250 A relay is adjustable and the latest circuit breakers over 400 A have processor-based relays. Accessories for these include working current and voltage coils, auxiliary and alarm contacts, motor controllers and door wrenches. Air circuit breakers Air circuit breakers have either plastic or metal bodies. They are used as main circuit breakers in industry and commercial properties where high mechanical and electrical reliability is required. In terms of current limitation properties, air circuit breakers are zero-point circuit breakers. When using a current-limiting circuit breaker, the dynamic stress on the switchgear can be reduced when operated at high short-circuit currents to limit its peak value. The rated voltage is up to 690V, 1000V models are also available, and the rated current is 800-6300A. The cutting abilities range from 40 to 150kA and their installation methods are fixed and pull-out. Air circuit breakers have fully electronic relays, so it allows wide adjustment ranges. Residual current circuit breaker AC-type fault current protection switch, works with alternating current Type A residual current protection switch, works with alternating current and pulsed direct current B-type residual current protection switch, works with alternating current and also with smoothed direct current Retarded residual current circuit breaker Frost-resistant device Surge resistant device Residual current circuit breaker refers to an automatically operating protective device. Its operation can be either independent of the mains voltage or dependent on it. As a rule, it does not need auxiliary voltage, but is triggered by spring force. RCD is used to protect against excessively high contact voltages in case of failure, as well as against dangerous leakage currents, it also provides good protection against ground faults passing through the human body. The switch starts working when the residual current exceeds its nominal value. The residual current circuit breaker can be used in all grounded networks. The minimum current at which the switch must operate is called fault current. The residual current circuit breaker monitors the amount of current entering and leaving the electrical device. As long as these quantities are equal, the electric current can pass unhindered. If part of the electrical current is transferred to the ground as residual current, the residual current circuit breaker detects the difference between the amount of incoming and exiting current.  18 (18) Motor protection switch The motor protection switch is used for overload and short circuit protection. The motor protection switch has cut-off properties that limit short-circuit current. When designing the switch, the magnitude of the current boost at engine start (8-12*In) and the current (5..7-fold) lasting several seconds during the acceleration phase shall be taken into account. Motor protection uses standard coordination classes 1 and 2, on which instrument selections are based. Protection coordination takes into account the compatibility of the circuit breaker, contactor and thermal relay. The coordination and instrument selection tables are manufacturer-specific. Differences from a fused system Fuseless protection has pros and cons: +two-pole cut-off +fully remote controlled +can be reactivated after a fault +safe to use +Easy access to alerts +Plenty of accessories -cutting ability decreases with voltage increase -In motor protection, costs increase as voltage rises -complete selectivity is difficult to implement -requires familiarity from the designer