Electrical Installation Designs Overcurrent protection devices Every electrical

Electrical Installation Designs Overcurrent protection devices Every electrical circuit and final circuit shall be protected against overcurrent by suitable protective devices. The selected protection device should be a current rating which is not less than the full load current of the circuit but which does not exceed the cable current rating. The cable is then fully protected against overload and short circuit faults (Regulation 433 -02 -01). PPKSE

Electrical Installation Designs Types of Overcurrent protection devices Semi-enclosed (rewirable) fuses to BS 3036 n called rewirable fuses n domestic installations n maximum faults capacity of about 4 k. A Cartridge fuses to BS 1361 n high rupturing capacity (HRC) types n used for ac circuits on industrial and domestic installations n fault capacity about 30 k. A PPKSE

Electrical Installation Designs Cont…Types of Overcurrent protection devices Cartridge fuses for use in plugs to BS 1362 n These are used in 13 A plug tops and have a maximum fault capacity of about 6 k. A High breaking capacity (HBC) fuses to BS 88 n industrial applications n maximum fault capacity of about 80 k. A PPKSE MCBs to BS 3871 n miniature circuit breakers which may be used as an alternative to fuses for some installations n The British Standard includes ratings up to 100 A and maximum fault capacities of 9 k. A n graded according to their instantaneous tripping currents – that is, the current at which they will trip within 100 ms

Electrical Installation Designs MCBs to BS 3871 MCB Type 1 to BS 3871 will trip instantly at between 2. 7 and four times its rated current n more suitable on loads within minimal or no switching surges such as domestic or commercial installations n MCB Type B to BS EN 60898 will trip instantly at between three and five times its rated current n suitable for domestic and commercial installations. n PPKSE

Electrical Installation Designs Cont…MCBs to BS 3871 MCB Type 2 to BS 3871 will trip instantly at between four and seven times its rated current. n It offers fast protection on small overloads combined with a slower operation on heavier faults, which reduces the possibility of nuisance tripping. n Its characteristics are very similar to those of an HBC fuse, and this MCB is possible best suited for general commercial and industrial use. n PPKSE

Electrical Installation Designs Cont…MCBs to BS 3871 MCB Type C to BS EN 60898 will trip instantly at between five and ten times its rated current. n suitable for highly inductive commercial and industrial loads. n MCB Type 3 to BS 3871 will trip instantly at between seven and ten times its rated current. n suitable for protecting highly inductive circuits and is used on circuits supplying transformers, chokes and lighting banks. n PPKSE

Electrical Installation Designs Cont…MCBs to BS 3871 MCB Type D to BS EN 60898 will trip instantly at between 10 and 25 times its rated current. n suitable for welding and X-Ray machines where large inrush currents may occur. n MCB Type 4 to BS 3871 will trip instantly between 10 to 50 times the rated current n suitable for special industrial applications such as welding equipment and X-Ray machines. n PPKSE

Electrical Installation Designs Semi-enclosed Fuses (BS 3036) consists of a fuse wire, called the fuse element, secured between two screw terminals in a fuse carrier. n The fuse element is connected in series with the load and the thickness of the element is sufficient to carry the normal rated circuit current. n When a fault occurs an overcurrent flows and the fuse element becomes hot and melts or “blows”. n PPKSE

Electrical Installation Designs Cont…Semi-enclosed Fuses (BS 3036) The designs of the fuse carrier and base are also important. n They must not allow the heat generated from an overcurrent to dissipate too quickly from the element, otherwise a larger current would be required to blow the fuse. n Also if over enclosed, heat will not escape the fuse will blow at lover current. n The fuse element should be consist of a single strand of plain or tinned copper wire having a diameter appropriate to the current rating as given in table below. n PPKSE

Electrical Installation Designs Cont…Semi-enclosed Fuses (BS 3036) Current Rating (A) Wire Diameter (mm) 5 0. 20 10 15 20 30 0. 35 0. 50 0. 60 0. 85 Refer Appendix 3 in IEE Regulation PPKSE

Electrical Installation Designs Cont…Semi-enclosed Fuses (BS 3036) Advantages of Semi enclosed Fuses n They are very cheap compared with other protective devices both install and to replace. n They are no mechanical moving parts. n It is easy to identify a blown fuse. Disadvantages of Semi enclosed Fuses n The fuse element may be replaced with wire of the wrong size either deliberately or by accident. n The fuse element weakens with age due to oxidization, which may result in a failure under normal operating conditions. n The circuit cannot be restored quickly since the fuse element requires screw fixing. PPKSE

Electrical Installation Designs Cont…Semi-enclosed Fuses (BS 3036) Cont…Disadvantages of Semi enclosed Fuses n They have low breaking capacity since, in the event of a severe fault, the fault current may vaporize the fuse element and continue to flow in the form of an arc across the fuse terminals. n There is a danger from scattering hot metal if the fuse carrier is inserted into the base when the circuit is faulty. PPKSE

Electrical Installation Designs PPKSE

Electrical Installation Designs PPKSE

Electrical Installation Designs Cartridge Fuses (BS 1361) The cartridge fuse breaks a faulty circuit in the same way as a semi enclosed fuse, but its construction eliminates some of the disadvantages experienced with an open fuse element. The fuse element is encased in a glass or ceramic tube and secured to end caps which are firmly attached to the body of the fuse so that they do not blow off when the fuses operates. PPKSE

Electrical Installation Designs Advantages of Cartridge Fuses n n n PPKSE They have no mechanical moving parts. The declared rating is accurate. The element does not weaken with age. They have small physical size and no external arching which permits their use in plug tops and small fuse carriers. Their operation is more rapid than semi enclosed fuses. Operating time is inversely proportional to the fault current.

Electrical Installation Designs Disadvantages of Cartridge Fuses n n PPKSE They are more expensive to replace than rewirable fuse elements. They can be replaced with an incorrect cartridge. The cartridge may be shorted out by wire or silver foil in extreme cases of bad practice. They are not suitable where extremely high fault currents may develop.

Electrical Installation Designs High Breaking Capacity Fuses (BS 88) For protecting circuits where extremely high fault currents may develop such as on industrial installations or distribution systems. n The fuse element consists of several parallel strips of pure silver encased in a substantial ceramic cylinder, the ends of which are sealed with tinned brass end caps incorporating fixing lugs. n The cartridge is filled with silica sand to ensure quick arc extraction. Incorporated on the body is an indicating device to show when the fuse has blown. n PPKSE

Electrical Installation Designs Conti…High Breaking Capacity Fuses (BS 88) PPKSE

Electrical Installation Designs Advantages of HBC Fuses n n n n PPKSE They have no mechanical moving parts. The declared rating is accurate. The element does not weaken with age. Their operation is very rapid under fault conditions. They are capable of breaking very heavy fault currents safely. They are capable of discriminating between a persistent fault and transient fault such as the large starting current taken by the motors. It is difficult to confuse cartridges since different ratings are made to different physical sizes

Electrical Installation Designs Disadvantages of HBC Fuses n PPKSE They are very expensive compared to semi enclosed fuses

Electrical Installation Designs Miniature Circuit Breakers (BS 3871) The disadvantages of all fuses is that when they have operated the must be replaced. n A MCB overcomes this problem since it is an automatic switch which opens in the vent of an excessive current flowing in the circuit and can be closed when the circuit returns to normal. n An MCB of the type shown in figure below incorporates a thermal and magnetic tripping device. n PPKSE

Electrical Installation Designs Cont…Miniature Circuit Breakers (BS 3871) The load current flows through thermal and electromagnetic mechanisms. n In normal operation the current is insufficient to operate either device, but when an overload occurs, the bimetal strip heats up, bends and trips the mechanism. n PPKSE

Electrical Installation Designs Cont…Miniature Circuit Breakers (BS 3871) n PPKSE The time taken for this action to occur provides an MCB with the ability to discriminate between an overload which persists for very short time, for example the starting current of a motor, and an overload due to a fault. The device only trips when a fault current occurs.

Electrical Installation Designs Cont…Miniature Circuit Breakers (BS 3871) n PPKSE This slow operating time is ideal for overloads but when a short circuit occurs it is important to break the faulty circuit very quickly. This achieved by the coil electromagnetic device.

Electrical Installation Designs Advantages of MCBs n n n PPKSE Tripping characteristics and therefore circuit protection are set by installer. The circuit protection is difficult to interfere with. The circuit is provided with discrimination. A faulty circuit may be easily and quickly restored. The supply may be safely restored by an unskilled operator.

Electrical Installation Designs Disadvantages of MCBs n n PPKSE They are very expensive compared to rewirable fuses. They contain mechanical moving parts and therefore require regular testing to ensure satisfactory operation under fault conditions.

Electrical Installation Designs Fusing Factor The speed with which a protective device will operate under fault conditions gives an indication of the level of protection being offered by that device. n This level of protection or fusing performance is given by the fusing factor of the device: n PPKSE

Electrical Installation Designs Conti…Fusing Factor The minimum fusing current of a device is the current which will cause the fuse or MCB to blow or trip in a given time (BS 88 gives this operating time as 4 hours). n The current rating of a device is the current which it will carry continuously without deteriorating. n Thus, 10 A fuse which operates when 15 A flows will have a fusing factor of 15 ÷ 10 = 1. 5. n PPKSE

Electrical Installation Designs Conti…Fusing Factor n n n n PPKSE Since the protective device must carry the rated current it follows that the fusing factor must always be greater than one. The closer the fusing factor is to one, the better is the protective offered by that device. The fusing factors of the protective devices previously considered are: Semi enclosed fuses: between 1. 5 and 2. Cartridge fuses: between 1. 25 to 1. 75. HBC fuses: less than 1. 25. MCBs: less than 1. 5.

Electrical Installation Designs Conti…Fusing Factor In order to give protection to the conductors of an installation: n The current rating of the protective device must be equal to or less than the current carrying capacity of the conductor; n The current causing the protective device to operate must not be greater tahn 1. 45 times the current carrying capacity of the conductor to be protected. n PPKSE

Electrical Installation Designs Conti…Fusing Factor The current carrying capacities of cables given in the tables of Appendix 4 of the IEE Regulations assume that the circuit will comply with these requirements and that the circuit protective device will have a fusing factor of 1. 45 or less. n Cartridge fuses, HBC fuses and MCBs do have a fusing factor less than 1. 45 and therefore when this type of protection is afforded the current carrying capacities of cables may be read directly from the tables. n PPKSE

Electrical Installation Designs Conti…Fusing Factor However, semi-enclosed fuses can have a fusing factor of 2. n The wiring regulations require that the rated current of a rewirable fuse must not exceed 0. 725 times the current carrying capacity of the conductor it is to protect. This factor is derived as follows: n The maximum fusing factor of a rewirable fuse is 2. n PPKSE

Electrical Installation Designs Conti…Fusing Factor Now, if In = current rating of the protective fuse or circuit breaker Iz = current carrying capacity of the cable I 2 = the operating current for the fuse or circuit breaker Then PPKSE

Electrical Installation Designs Conti…Fusing Factor therefore PPKSE when rewirable fuses are used, the current carrying capacity of the cables given in the tables is reduced by a factor of 0. 725, as detailed in Appendix 4 item 5 of the Regulations.

Electrical Installation Designs Disconnection time calculations The overcurrent protection device protecting socket outlet circuits and any fixed equipment in bathrooms must operate within 0. 4 seconds. n Those protecting fixed equipment circuits in room other than bathrooms must operate within 5 seconds (Regulation 413 -02 -08 to 13 and 601). n PPKSE

Electrical Installation Designs Disconnection time calculations The reason for the more rapid disconnection of the socket outlet circuits is that portable equipments plugged into the socket outlet is considered a higher risk than fixed equipment since it is more likely to be firmly held by a person. n The more rapid disconnection times for fixed equipment in bathrooms take account of a possible reduced body resistance in the bathroom environment n PPKSE

Electrical Installation Designs Disconnection time calculations n PPKSE The IEE Regulations permit us to assume that an overload protective device is also intended to provide short circuit protection, and has a rated breaking capacity greater than the prospective short circuit current at the point of its installation, the conductors on the load side of the protective device are considered to be adequate protected against short circuit currents without further proof.

Electrical Installation Designs Disconnection time calculations This is because the cable rating and the overload rating of the device compatible. However, if this condition is not met or if there is some doubt, it must be verified that fault currents will be interrupted quickly before they can cause dangerously high temperature rise in the circuit conductors. n Regulation 434 -03 -03 provides an equation for calculating the maximum operating time of the protective device to prevent the permitted conductor temperature rise being exceeded as follow n PPKSE

Electrical Installation Designs Disconnection time calculations Where t = duration time in seconds S = cross sectional area of conductor in square millimetres I = short circuit rms current in amperes k = a constant depend upon the conductor metal and type of insulation (see Table 43 A of IEE Regulations – pg 47) PPKSE

Electrical Installation Designs Time/Current Characteristics of Protective Devices Disconnection times for various overcurrent devices are given in the form of a logarithmic graph. This means that each successive graduation of the axis represents a ten times change over the previous graduation. n These logarithmic scales are shown in the graphs of Figure 4. 21 and 4. 22. The time/current characteristics of all circuit breakers in Figure 4. 21 to 4. 22 have a vertical section where there is a wide range of operating times for a certain current. n PPKSE

Electrical Installation Designs Disconnection time calculations n PPKSE Hence, with a fixed supply voltage, the maximum earth fault loop impedance is also fixed over this range of time. The operating current during the time concerned is a fixed multiple of the rated current. For example, a Type 1 MCB has a multiple of 4 referred to Table 4. 3 so a 50 A device of this type will operate over the time range of 0. 04 s to 20 s at a current of 4 x 50 A = 200 A.

Electrical Installation Designs Time/current characteristics of a type 1 MCB to BS 3871 PPKSE