8. A final circuit is the wiring between the
a. main cut-out and the energy meter
b. main switch and the distribution board
c. distribution board and the current-using equipment (Definitions page 24)
d. consumer's main earth terminal and circuit protective device
Well c would have to be selected but that is not the definition given. Where for example a FCU is used the final circuit would be between the FCU and the current - using equipment. Not the distribution board and the current-using equipment. This is why the definition says:-
Final circuit. A circuit connected directly to current - using equipment, or to a socket-outlet or socket-outlets or other outlet points for the connection of such equipment.
One has to remember it says:-
Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).
Now where the FCU supplies for example 3 sockets then the wiring between the FCU and the three sockets is a final circuit.
So if I wanted I could run a 6mm cable around a factory in ceiling space Reference Method C* (Clipped direct) and as it passed over each room I could use a FCU to supply that room and supply the cable with a 45A RCBO. The final circuits in this case would be FCU to sockets. This is why the BS7671 is written so carefully.
All the exams I have taken both 2381 and 2382 were written exactly as in the book. In fact I think this is wrong approach and questions should more represent true life situations. But I question as to if these are real questions or something someone has made up to give an idea of what may be asked?
With question 10 they have shortened the answer also as a system can be "a single source or multiple sources running in parallel"
Question 9. The prospective short circuit current at the origin of the consumer's installation must be taken into account when
a. estimating the external earth loop impedance
b. applying diversity factor for the installation
c. selecting the system of earthing for the supply
d. selecting the type of overcurrent protective device to be installed
First we must remember that the prospective short circuit current and the loop impedance are the same thing really. So if the highest loop impedance is for example 0.3 then the prospective short circuit current will be 230/0.3 = 766 Amp. We should measure Line - Neutral and Line - Earth and use highest reading. But also we must consider the let through value of any proceeding protective device.
Well b is out and c would be true but for the supply authority who are not controlled by BS7671. d is not true as it is the prospective short circuit current at the socket which is important. As to a which is only one left well I calculate or measure I don't estimate.
I scrolled to the end and the answer it gives is d. However it seems to have missed the let through value of the supply fuse. Again incomplete question or answer as the option.
If these questions are really from the 2382 exam well just glad I have already passed it.
First of all the prospective short circuit current (pscc) and the loop impedance are 2 completely different things. pscc and pefc are measured where ever protective devices are installed, in an installation with 1 consumer unit they are measured at the incoming supply. they are measured as near to the origin as possible because this point will have the lowest impedance, if the circuit hase the lowest impedance then the maximum current reading will be obtained. on a single phase suply pscc and pefc are measured and the highest is recorded as the pfc. this pfc figure is compared to the braking capacity of all protective devices (the number is a rectangle on circuit breakers e.g. 10000) the braking capacity of all protective devices must be greater than the pfc to ensure that they will not be damaged when disconnecting any fault current. therefore D is the correct answer