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

3. Cont…Types of Overcurrent protection devices
Electrical Installation Designs
Cont…Types of Overcurrent protection devices
Cartridge fuses for use in plugs to BS 1362
These are used in 13A plug tops and have a maximum fault capacity of about 6kA
High breaking capacity (HBC) fuses to BS 88
industrial applications
maximum fault capacity of about 80kA
MCBs to BS 3871
miniature circuit breakers which may be used as an alternative to fuses for some installations
The British Standard includes ratings up to 100A and maximum fault capacities of 9kA
graded according to their instantaneous tripping currents – that is, the current at which they will trip within 100ms
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4. will trip instantly at between 2.7 and four times its rated current
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
more suitable on loads within minimal or no switching surges such as domestic or commercial installations
MCB Type B to BS EN 60898
will trip instantly at between three and five times its rated current
suitable for domestic and commercial installations.
PPKSE

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

6. will trip instantly at between five and ten times its rated current.
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.
suitable for highly inductive commercial and industrial loads.
MCB Type 3 to BS 3871
will trip instantly at between seven and ten times its rated current.
suitable for protecting highly inductive circuits and is used on circuits supplying transformers, chokes and lighting banks.
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7. will trip instantly at between 10 and 25 times its rated current.
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.
suitable for welding and X-Ray machines where large inrush currents may occur.
MCB Type 4 to BS 3871
will trip instantly between 10 to 50 times the rated current
suitable for special industrial applications such as welding equipment and X-Ray machines.
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8. Semi-enclosed Fuses (BS 3036)
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.
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.
When a fault occurs an overcurrent flows and the fuse element becomes hot and melts or “blows”.
PPKSE

9. Cont…Semi-enclosed Fuses (BS 3036)
Electrical Installation Designs
Cont…Semi-enclosed Fuses (BS 3036)
The designs of the fuse carrier and base are also important.
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.
Also if over enclosed, heat will not escape the fuse will blow at lover current.
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.
PPKSE

10. Cont…Semi-enclosed Fuses (BS 3036) Current Rating (A)
Electrical Installation Designs
Cont…Semi-enclosed Fuses (BS 3036)
Current Rating (A)
Wire Diameter (mm) 5
0.20 10
0.35 15
0.50 20
0.60 30
0.85
Refer Appendix 3 in IEE Regulation
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11. Cont…Semi-enclosed Fuses (BS 3036)
Electrical Installation Designs
Cont…Semi-enclosed Fuses (BS 3036)
Advantages of Semi enclosed Fuses
They are very cheap compared with other protective devices both install and to replace.
They are no mechanical moving parts.
It is easy to identify a blown fuse.
Disadvantages of Semi enclosed Fuses
The fuse element may be replaced with wire of the wrong size either deliberately or by accident.
The fuse element weakens with age due to oxidization, which may result in a failure under normal operating conditions.
The circuit cannot be restored quickly since the fuse element requires screw fixing.
PPKSE

12. Cont…Semi-enclosed Fuses (BS 3036)
Electrical Installation Designs
Cont…Semi-enclosed Fuses (BS 3036)
Cont…Disadvantages of Semi enclosed Fuses
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.
There is a danger from scattering hot metal if the fuse carrier is inserted into the base when the circuit is faulty.
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13. Electrical Installation Designs
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14. Electrical Installation Designs
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15. 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

16. Advantages of Cartridge Fuses
Electrical Installation Designs
Advantages of Cartridge Fuses
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.
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17. Disadvantages of Cartridge Fuses
Electrical Installation Designs
Disadvantages of Cartridge Fuses
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.
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18. High Breaking Capacity Fuses (BS 88)
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.
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.
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.
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19. Conti…High Breaking Capacity Fuses (BS 88)
Electrical Installation Designs
Conti…High Breaking Capacity Fuses (BS 88)
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20. Advantages of HBC Fuses
Electrical Installation Designs
Advantages of HBC Fuses
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
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21. Disadvantages of HBC Fuses
Electrical Installation Designs
Disadvantages of HBC Fuses
They are very expensive compared to semi enclosed fuses
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22. Miniature Circuit Breakers (BS 3871)
Electrical Installation Designs
Miniature Circuit Breakers (BS 3871)
The disadvantages of all fuses is that when they have operated the must be replaced.
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.
An MCB of the type shown in figure below incorporates a thermal and magnetic tripping device.
PPKSE

23. Cont…Miniature Circuit Breakers (BS 3871)
Electrical Installation Designs
Cont…Miniature Circuit Breakers (BS 3871)
The load current flows through the thermal and electromagnetic mechanisms.
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.
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24. Cont…Miniature Circuit Breakers (BS 3871)
Electrical Installation Designs
Cont…Miniature Circuit Breakers (BS 3871)
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.
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25. Cont…Miniature Circuit Breakers (BS 3871)
Electrical Installation Designs
Cont…Miniature Circuit Breakers (BS 3871)
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.
PPKSE

26. The circuit protection is difficult to interfere with.
Electrical Installation Designs
Advantages of MCBs
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.
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27. They are very expensive compared to rewirable fuses.
Electrical Installation Designs
Disadvantages of MCBs
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.
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28. 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.
This level of protection or fusing performance is given by the fusing factor of the device:
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29. 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).
The current rating of a device is the current which it will carry continuously without deteriorating.
Thus, 10A fuse which operates when 15A flows will have a fusing factor of 15 ÷ 10 = 1.5.
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30. Semi enclosed fuses: between 1.5 and 2.
Electrical Installation Designs
Conti…Fusing Factor
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 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.
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31. In order to give protection to the conductors of an installation:
Electrical Installation Designs
Conti…Fusing Factor
In order to give protection to the conductors of an installation:
The current rating of the protective device must be equal to or less than the current carrying capacity of the conductor;
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.
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32. 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.
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.
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33. However, semi-enclosed fuses can have a fusing factor of 2.
Electrical Installation Designs
Conti…Fusing Factor
However, semi-enclosed fuses can have a fusing factor of 2.
The wiring regulations require that the rated current of a rewirable fuse must not exceed times the current carrying capacity of the conductor it is to protect. This factor is derived as follows:
The maximum fusing factor of a rewirable fuse is 2.
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34. In = current rating of the protective fuse or circuit breaker
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
I2 = the operating current for the fuse or circuit breaker
Then
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35. Conti…Fusing Factor therefore
Electrical Installation Designs
Conti…Fusing Factor
therefore
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.
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36. Disconnection time calculations
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.
Those protecting fixed equipment circuits in room other than bathrooms must operate within 5 seconds (Regulation to 13 and 601).
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37. Disconnection time calculations
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.
The more rapid disconnection times for fixed equipment in bathrooms take account of a possible reduced body resistance in the bathroom environment
PPKSE

38. Disconnection time calculations
Electrical Installation Designs
Disconnection time calculations
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.
PPKSE

39. Disconnection time calculations
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.
Regulation provides an equation for calculating the maximum operating time of the protective device to prevent the permitted conductor temperature rise being exceeded as follow
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40. Disconnection time calculations
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
43A of IEE Regulations – pg 47)
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41. Time/Current Characteristics of Protective Devices
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.
These logarithmic scales are shown in the graphs of Figure 4.21 and 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.
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42. Disconnection time calculations
Electrical Installation Designs
Disconnection time calculations
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 50A device of this type will operate over the time range of 0.04s to 20s at a current of 4 x 50A = 200A.
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43. Time/current characteristics of a type 1 MCB to BS 3871
Electrical Installation Designs
Time/current characteristics of a type 1 MCB to BS 3871
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