29. Typical example of where a device for protection against fault current is not required Conductors having a reduction in current-carrying capacity between the busbars and the switchboard
30.
31.
Hinweis der Redaktion
This slide marks the start of Section 434 relating to fault protection. This is worth pointing out saying that the previous slides related to overload and now fault current protection was going to be discussed. Discuss the important of determining prospective fault current in relation to protection of electrical equipment and personnel etc. Discuss the high values that can occur. Calculate typical current that could flow on an earth fault at a socket close to an industrial intake position (Earth fault loop 0.23 Ω gives 1000 A at 230 V). Discuss the difference between line to earth, line to neutral and line to line fault currents and the fact it is important to determine the highest value. Discuss the different methods listed in BS 7671 for determining prospective fault current. Explain that enquiry normally only provides a standard maximum figure and not one relating specifically to the installation in question
Explain that in this diagram that where a system is TN-C-S the fault current at the origin for line to neutral and line to earth will be the same. With TN-S system they could well be different. Also point out that prospective fault current measurement should be taken with the main protective bonding in place which can have the effect of lowering the impedance but raising the fault level.
The values of line to line fault cannot be measured with some instruments. Where a prospective fault current measurement has been taken with a single-phase instrument the value for a three-phase system should be taken as double the single-phase value (unless more accurate methods can be proven)
Like overload protection, and except where not required, a device providing protection against fault current is to be installed at the point where a reduction occurs in the cross-sectional area or other change causes a reduction in the current-carrying capacity of conductors
Explain that the intention of this calculation is to ensure conductors are not overheated when a fault occurs. Refer to Table 43.1 and discuss the different conductor insulation temperatures and their k values t is the duration of the fault, in seconds, s is the conductor cross-sectional area in mm 2 I is the effective fault current, in amperes, expressed for a.c. as the rms value, due account being taken of the current limiting effect of the circuit impedances k is a factor, taking account of conductor material, insulation material and the initial and final conductor temperatures See Table 43.1 for values of k for common materials Provide sample example (from Learning Guide or make up example)
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