SG PPE Electrical common mistakes
SG PPE Electrical · Understand the traps and how to avoid them.
When calculating a circuit's total load, the candidate adds the resistance of parallel branches (R1+R2+…) instead of using the reciprocal formula, so the total resistance comes out larger than the smallest branch.
The candidate learned the series formula first and applies it to every circuit, so they fail to notice that adding parallel resistances should always give a total smaller than the smallest branch.
What to do instead
Before calculating, state whether the circuit is series or parallel. For parallel branches, use 1/RT = 1/R1 + 1/R2 + … and check the result: it must be smaller than the smallest branch resistance. For two branches only, use the product-over-sum shortcut and verify it against the reciprocal method.
The candidate chooses a cable size from a current-carrying capacity table without applying the correction factors for ambient temperature, grouping, or thermal insulation.
The tabulated rating is treated as the final answer, and the correction factors in the appendix are skipped because the candidate does not connect them to the design sequence.
What to do instead
Work through the design sequence in order: design current, protective device rating, then It ≥ In / (Ca × Cg × Ci). Write each factor and its source table beside the calculation so the marker can see the reasoning.
The candidate sizes the protective device to the load current but does not check that the disconnection time is met for the relevant earthing system (TN or TT).
The focus stays on overload protection, and the fault-path loop impedance check is treated as a separate topic rather than part of the same design decision.
What to do instead
For each circuit, complete both checks: overload protection (Ib ≤ In ≤ Iz) and fault protection (Zs within the maximum value in the relevant table for the disconnection time). State which table and which disconnection time apply.
The candidate confuses the roles of circuit protective conductors, main protective bonding conductors, and supplementary bonding, and sizes one using the table for another.
All three are green/yellow and all are called 'earth wires' in everyday speech, so the candidate treats them as interchangeable.
What to do instead
Label each conductor by function before sizing it. Use the adiabatic equation or the standard table for the CPC, the incoming supply neutral size for main bonding, and the requirements for supplementary bonding. Write the function next to the calculation.
The candidate states a test result as a pass or fail without comparing it against the required value, or records a reading with no units or no test conditions.
The candidate is under time pressure and treats testing as a list of actions to perform rather than measurements to interpret.
What to do instead
For every test, write three things: the measured value with units, the required or expected value with its source, and the comparison. Practise saying the sentence 'measured X, required Y, therefore pass/fail' until it is automatic.
During the practical assessment, the candidate energises a circuit or removes a cover without first proving the tester on a known source and confirming dead.
The candidate wants to save time and assumes the isolation is already correct because someone else set it up.
What to do instead
Follow the safe isolation procedure every time, out loud: identify, isolate, secure, prove the tester, test for dead on all conductors, prove the tester again. Treat the sequence as non-negotiable regardless of time pressure.
The candidate diagnoses a fault by replacing components at random rather than measuring to localise it first.
Replacement feels like progress, and the candidate lacks a structured fault-finding routine to fall back on.
What to do instead
Use a divide-and-conquer routine: confirm the symptom, split the circuit at a known point, measure, and halve the suspect section each time. Record each measurement and what it rules out before touching any component.
The candidate quotes a regulation number or a required value from memory and gets the number, the table, or the units wrong.
The candidate has memorised fragments without the surrounding context, so similar values from different tables get mixed up.
What to do instead
Where the exam allows reference material, look up the value rather than recalling it. Where it does not, build a one-page summary of the values you must know and test yourself on them weekly, including units and which table each comes from.
The candidate answers a calculation question with a number only, omitting the formula, the substitution, and the units.
The candidate assumes the final answer is all that is marked and rushes to finish within the time limit.
What to do instead
Always show formula, substitution, answer with units, and a one-line sanity check. This earns method marks even when the arithmetic slips and catches unit errors before they cost the whole question.
The candidate misreads the question's conditions, for example answering for a TT system when the question specifies TN-S, or giving the requirement for a domestic installation when the question describes a commercial one.
The candidate recognises a familiar topic and answers the version they have practised, skipping the qualifying details in the stem.
What to do instead
Underline the qualifying conditions in the stem (system type, location, load type, supply characteristics) before writing anything. Restate them in the first line of the answer so the marker can see they were applied.