SOLAS Electrical Apprenticeship common mistakes
SOLAS Electrical Apprenticeship · Understand the traps and how to avoid them.
When asked to confirm a protective device will disconnect a fault within the required time, the candidate compares the tabulated disconnection time against the circuit's nominal current (In) instead of the actual fault current at the far end of the circuit, or reads the device's time/current curve at the wrong multiple of rating.
Candidates memorise '0.4 s for final circuits' and then treat the device rating as the fault current, forgetting that disconnection time depends on prospective fault current (Zs) at the point of fault, and that the curve is logarithmic so misreading a decade gives wildly wrong times.
What to do instead
Practise the full sequence on at least ten worked circuits: compute Zs from external loop impedance plus R1+R2 (or measure it), divide nominal voltage by Zs to get fault current, then enter the manufacturer's time/current curve at that current and read the time. Always state the assumed supply voltage and whether the value is measured or calculated.
On cable-sizing questions the candidate selects a conductor from the current-carrying capacity table using the design current directly, skipping the correction factors for ambient temperature, grouping, thermal insulation, and the device's derating factor (Ca, Cg, Ci, and In ≤ It after correction).
The tabulated It values look like a simple lookup, and under exam time pressure the candidate jumps to the nearest rating above Ib. The correction-factor step is where most marks are lost because it is arithmetic the candidate has not rehearsed under time limits.
What to do instead
Write the design inequality out every time: Ib ≤ In ≤ It(corrected) and I2 ≤ 1.45 × It, and show each correction factor as a separate line with the table reference. Do five full sizing calculations end-to-end, including voltage drop check, before the exam.
The candidate confuses the roles of earthing and bonding, or states that the main protective bonding conductor size is chosen from the circuit protective conductor table, rather than from the supply neutral/phase conductor size per the standard's table.
Both are green/yellow conductors and both terminate at the main earthing terminal, so candidates conflate 'earthing' (fault current path for protective disconnection) with 'protective bonding' (equalising potential between extraneous-conductive-parts and the MET). The sizing rules are different and the candidate has learned only one.
What to do instead
Build a one-page comparison: purpose, where each conductor connects, what it carries in normal and fault conditions, and the sizing basis for each. Then answer ten short questions that mix the two so the distinction becomes automatic.
When describing insulation resistance testing, the candidate quotes a test voltage and minimum acceptable value without stating the conditions: which voltage for which circuit (500 V DC for circuits up to 500 V, 250 V DC where sensitive equipment may be damaged), the requirement to disconnect sensitive equipment and remove lamps, and that the reading is taken after the test voltage has been applied and the value is in megohms not ohms.
Candidates remember the number '1 megohm' but not the conditions attached to it, and they often write the test procedure as a single sentence, losing the marks for isolation, safe isolation, and restoration of the circuit afterwards.
What to do instead
Write the insulation resistance test as a numbered procedure from safe isolation to restoration, with the voltage, the minimum value, the reason for each disconnection, and the record kept. Repeat until you can write it from memory in under five minutes.
The candidate applies Ohm's law correctly to a pure resistor but then applies the same arithmetic to an AC circuit containing inductance or capacitance, adding impedance magnitudes arithmetically (e.g. Z = R + XL) instead of combining them as phasors, or ignoring that power factor changes the current for a given kW load.
The DC model is taught first and reinforced by repetition, while the phasor/vector nature of AC impedance is introduced later and practised less. Under exam pressure the candidate reverts to the simpler, more familiar arithmetic.
What to do instead
Redo every AC calculation with a sketch of the phasor triangle before substituting numbers, and check the answer's plausibility (Z must be at least as large as the largest single component). Practise converting between kW, kVA and kVAr using the power triangle until it is routine.
On motor and control questions the candidate describes the power circuit and the control circuit as one, or cannot explain what a contactor's normally-open auxiliary contact does in a direct-on-line starter's hold-in (self-holding) circuit, so the answer omits how the motor keeps running after the start button is released.
Candidates learn the component names but not the sequence of operation. The hold-in contact is the single concept that separates a working DOL starter from a push-and-hold circuit, and it is the most commonly omitted element in written answers.
What to do instead
Draw the DOL starter from memory with main circuit, control circuit, and the auxiliary contact across the start button; then annotate each conductor with its state (energised/de-energised) at rest, during start, and while running. Do the same for a forward/reverse interlocked starter.
The candidate answers a safe-isolation question by describing switching off at the breaker only, omitting lock-off/tagging, proving the tester on a known source before and after testing, proving dead at the point of work, and identifying the isolation point unambiguously.
Candidates know 'isolate' as a word and reproduce it as a word. The procedural steps feel like padding to a candidate focused on technical content, so they are compressed into one line and lose the marks that carry the safety domain.
What to do instead
Learn the isolation sequence as a fixed seven-step checklist (identify, isolate, secure/lock, prove tester, prove dead, work, restore) and write it out in full in every practice answer, even when the question seems to ask only about the test itself.
When explaining semiconductor devices, the candidate describes a diode or thyristor as a switch that is simply on or off, without stating the forward voltage drop, the gate/trigger requirement for a thyristor to conduct and the fact that it latches until current falls below the holding value, or the difference between a transistor as a current-controlled and a MOSFET as a voltage-controlled device.
Circuit symbols are easy to recognise, so candidates assume they understand the device. The behaviour that matters in fault-finding and control (latching, holding current, gate drive, voltage drop) is learned as a list of facts rather than as an operating principle.
What to do instead
For each device write three lines: what turns it on, what turns it off, and what it does to voltage and current in each state. Then apply each device to a real circuit (rectifier, lamp dimmer, relay driver) and explain the circuit using those lines.
On renewable-energy and inverter questions the candidate treats a PV array as a constant-voltage source, or ignores the difference between the DC side (array, isolator, string fusing, DC-rated switching) and the AC side (inverter output, grid protection, anti-islanding) when describing a typical installation or a fault.
The candidate's mental model comes from battery-based DC circuits where voltage is fixed, and the PV array's current-limited, irradiance-dependent behaviour (I-V curve, maximum power point) is unfamiliar. The DC/AC boundary is also where terminology is weakest.
What to do instead
Sketch a grid-connected PV system from array to meter, labelling the DC isolator, inverter, AC isolator and protective devices, and state what changes on the DC side when irradiance falls. Learn three points on the I-V curve (open circuit, short circuit, maximum power) and what the inverter does to track the last of them.
The candidate uses a regulation number as the whole answer ('because of BS 7671') without stating the requirement it imposes, the condition under which it applies, or the reason behind it, and mixes up which edition or which part of the standard governs the point being asked about.
Regulation numbers are memorable and feel authoritative, so they are used as a substitute for understanding. Where the question asks 'why', the number alone scores nothing, and misattributed numbers actively harm credibility.
What to do instead
For every regulation you cite, write the requirement in your own words in one sentence plus the reason it exists, and check the number against the current edition before the exam. In answers, lead with the requirement and the reason, then cite the regulation as supporting evidence.