Prospective fault current

Moulded case circuit breakers are available in two basic modes of operation, current limiting and non-current limiting. It is difficult to design a moulded case circuit breaker to have a cut-off characteristic that is less than 0.01 second when a fully asymmetrical short-circuit current flows. However, there are such circuit breakers available, and care is needed when selecting these devices for a circuit that has a high prospective fault current. Some manufacturers are able to provide a cut-off in the order of 0.006 second. 

From a manufacturer s data sheet a 125 A MCCB has a let-through current Ip of 25 kA gat for a prospective fault current C of 100 kA m. ... 

Historically early designs began to fail nntil it was realised that the prospective fault currents in typical power systems had gradually increased. This was due to the natural development and expansion of those systems. Reference 1 gives a good description of the Pt characteristic. 

The actual value of let-through current for a given fuse will depend upon the nature and magnitude of the prospective fault current e.g. asynunetrical or synunetrical. This is because a greater current has to be reached in the symmetrical case than in the asynunetrical case to create the same amount of melting energy. This is due to the shape of the current waveform in the first cycle, which can be seen in Figure 8.1. 

The first step is to find the peak asymmetrical prospective fault current seen by the 200A fuses. 

It can be seen that the prospective fault current required to trip an MCB in the required time is a multiple of the current rating of the device. The multiple depends upon the characteristics of the particular devices. Thus ... 

For final circuits less than 32 A the maximum operating time of the protective device is 0.4 s. From Fig 3A2(a) it can be seen that a current of about 90 A will trip the 15 A fuse in 0.4s. The small insert table on the top right of Fig 3A2(a) of the lET Regulations gives the value of the prospective fault current required to operate the device within the various disconnection times given. 

The section entitles a supplier to rely on the safety-related research and testing already done by others. He does not have to repeat it. For example, an electrical contractor who installs electrical equipment such as a circuit breaker can relay on the assurances of the device s manufacturer that it has been made and tested to an appropriate British Standard. The contractor, however, retains responsibility for selecting the correct type of circuit breaker for the application ensuring, for example, that it has adequate breaking capacity for the prospective fault current. However, importers of foreign apparatus must satisfy themselves that the designer s or maker s assurances are valid. 

The Regulation requires the protective conductors to be effective under fault conditions. To ensure compliance, the prospective fault currents need to be ascertained and conductors selected which will carry these fault currents without damage until the protection operates to clear the fault. 

The difficulty about the calculation method is in determining the prospective fault current when the supply is derived from a supply company s LV network. Changes to the network and to the installation can affect the earth loop impedance and the prospective fault current. On the whole it is probably better to use Table 54G than do the calculation and employ a smaller section conductor at what may be a small cost saving, only to find later that it was a false economy. 

The earth loop impedance should be measured so as to enable the prospective fault currents to be calculated. These currents have to be sufficient to operate the protective fuses or circuit breakers within the specified time in the event of an earth fault. Again, the readings obtained indicate the current position, which may alter subsequently. In most cases load growth tends to increase the prospective fault current, but in areas subject to derehction the load may decrease and the consequent network alterations may result in a fall in the prospective fault current. 

This test is conducted to verify the suitability of the equipment to withstand a prospective short-circuit current that may develop on a fault. It may also be termed the steady slate symmetrical fault current or the short-time (withstand current) rating of the equipment. When the equipment is an interrupting device, it is referred to as its symmetrical breaking current. 

For a prospective symmetrical fault current of 100 kA rms the upstream fault source impedance... 

Step 3. Calculate the prospective symmetrical and asymmetrical fault currents. 

For the selection of fuses the prospective symmetrical rms value of the off-set fault current is calculated as,... 

Protection against overcurrent and short circuit current is required and is mainly provided by circuit breakers fitted with overload trips. At the initial planning stage the supply company should be asked for the prospective fault level at the supply intake so that adequately rated equipment may be selected. At this position, if a switchfuse is used to control the installation and the BS 88 FIBC fuses specified in BS EN 60439-4 are employed, there should be no problem as the breaking capacity is not less than 80 k A at 400 V. If a moulded case circuit breaker is used, however, it is necessary to select one of adequate fault breaking capacity as they are made for a range of ratings. 

At the initial planning stage the prospective fault level should be calculated for several locations in the 400 V distribution system so that apparatus of adequate fault rating may be used. Inadequately rated apparatus, for instance an automatic motor starter, can often be protected by back-up HBC fuses, providing they have a suitable time/current characteristic so that they... 

Loop impedance tests are carried out to determine the loop impedance between the power source(s) and the point in the installation where the test is done. The device employed measures the current which passes through a resistor and displays the result in ohms. It is used to determine the loop impedance between phases, phase to neutral or any phase to earth. Some instruments incorporate a transformer to enable the neutral/earth loop impedance to be measured. From Ohm s law these readings can then be expressed in prospective short circuit fault currents. 

Examples are HRC fuses (both LT and HT) and MCCBs and MCBs (LT only), which are available with current limiting features and are in extensive use. The tripping time of these devices is extremely low and much less than one half of a cycle of the current wave. They therefore do not allow the fatilt ctirrent to rise to its prospective peak. The protected devices and components can thus be selected based on the let-out energy of such devices on fault, which is extremely low, than the fault level of the system. If... 

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