Well done , you have made a determined effort to think about the problems and how you would go about resolving them.
Definitely worthy of support I feel.
Just my thoughts on what and how you may be able to approach this.
Always remember what training you have undergone.
Use approved terminology wherever possible, make a point of ensuring the you are working dead at all times, minimise the live testing required.
Always work logically in a fault diagnosis situation.
Consider the circuit design and function to ensure you know what items you are testing and why you are testing. Also you should be aware of the nature and range of values you expect to find and the units in which those values are measured.
Loss of supply.
This is rather wide ranging and could cover local or installation wide loss of supply.
An initial practical assessment of items of functional equipment could indicate the range of the loss of supply.
If the entire installation has lost power then a test for voltage with an approved voltage tester at the closest accessible point to the incoming supply should indicate if the fault lies with the supply equipment or the installation equipment.
If a protective device has operated this should be, on a well designed system, the closest protective device to the fault.
Depending of the type of protective device the testing on the safely isolated circuit may be low ohm continuity to determine a short circuit fault or an overload fault where a circuit breaker has tripped, or insulation resistance testing, possibly followed by low ohm continuity testing, to determine an earth fault where an RCD has tripped.
Testing continuity across the main fuse would require breaking of security seals for the DNOs equipment and would be an offence.
Resetting or replacing a protective device before resolving the fault would be generally a futile exercise and may be considerably dangerous.
Overload
Where a circuit breaker has tripped due to overcurrent an initial assessment of the load of connected equipment may permit an initial diagnosis. Failing that a low ohm continuity test on the safely isolated circuit between line and neutral would allow calculation of the expected current draw using I=V/R. Further splitting the circuit in stages sand testing would narrow down the area to the point of the fault. It should be noted that electronic equipment may give a false indicating of the connected load.
Your answer was good though fairly informal and lacking detail on the nature of “all test results” you were performing on the circuit. The example is a good touch though most MCBs do not come in 5A any more.
Short circuit and earth fault
I believe this would mean faults of negligible impedance between line and neutral or between line and earth (or both).
Testing of the safely isolated circuit with a low ohm continuity tester and splitting down the circuit logically and testing would narrow down the area to the point of the fault.
If earth faults means a sufficiently low resistance to permit an RCD to trip then this may be possible to determine with a suitably accurate insulation resistance tester, loose terminations could cause short circuits if the cable had become removed from the terminal, reversed polarity would not normally cause short circuits but would generate a dangerous situation by leaving equipment live when switched off.
Transient voltage
Often a high transient voltage generated from electronic equipment generating harmonic currents or unstable voltage conditions, supply network faults, high voltage atmospheric conditions or rapid heavy load switching (that can also cause transient low voltage on the supply).
A data logger may be employed to determine voltage transients experienced over time and by the speed and regularity of the transients may permit a diagnosis of the cause.
Loss of phase / line
Los of line in a single phase system will be similar to loss of supply.
Loss of a single phase in a three phase system can be indicated by motor faults, poor or none existent stating and irregularity in operation. Voltage testing using a suitable volt meter with probes meeting GS38 requirements to determine to voltage at each phase should identify the faulty phase, or continuity tests on a safely isolated circuit can identify the break point in a line conductor.
Overall good answer
Incorrect phase rotation can be identified as it can cause motors to run in reverse.
I test with a phase rotation tester at the supply and load ends of a circuit can confirm incorrect phase rotation. If the circuit is safely isolated then a low ohm continuity cheek of the line conductors from supply to load can identify reversed phases.
A good answer perhaps slightly specific to the equipment you have, but correct.
High resistance joints
High resistance joints are characterised by an increased temperature at the point of resistance due to power dissipation in the resistance and in severe cases heat damage and burning of the accessorises and cables, which may then be identified by sight and smell.
Use of thermo graphic surveying may indentify such areas of increased temperature and logical staged measurement of low ohm continuity on a safely isolated circuit will also permit identification of the pint of the fault.
If you are doing R1+R2 testing or end to end resistance on ring final circuits then these problems should come to light, but may not be part of general diagnosis techniques. Correct use of a low ohm continuity tester to identify.
Component, accessory or equipment failure
Component, accessory or equipment faults can be indentified by loss of power and / or intermittent or uncharacteristic operation of equipment.
Total loss of power to equipment can be narrowed down to the point of fault by the use of voltage indicating equipment, suitable for the supply voltage, at accessible points on the supply route up to the equipment.
Monitoring or receiving accurate reports of the nature and frequency of intermittent or uncharacteristic operations can give an indication of the type and location of the fault.
Component or equipment failure can be determined by a logical step by step approach that is dependent on the nature of the equipment and the manufacturers instructions. Wherever possible diagnosis techniques should be restricted to work on safely isolated equipment.
Some useful examples in your answer and visual inspection is often a very simple and clear cut method of fault finding.
I should stress that these are only my off the cuff thoughts about possible ways of responding not approved answers and not necessarily correct, particular requirements or items on which you have been trained could well override these thoughts.
Someone closer to training may have better ideas
Thinking is also a good method of fault diagnosis!!!