I.S. 10101 / ET101 Wiring Rules common mistakes
I.S. 10101 / ET101 Wiring Rules · Understand the traps and how to avoid them.
Treating the scope of ET101 as covering every electrical installation without checking the stated exclusions and the boundary with supply-authority equipment and with equipment covered by other standards.
Candidates memorise the general title of the standard and skim the scope clause, so they assume universal application and miss the stated limits and interfaces.
What to do instead
Read the scope clause line by line and write a one-line summary of what is included, what is excluded, and where responsibility transfers to the supply authority or another standard; test yourself by classifying borderline examples.
Choosing a protective measure (automatic disconnection, double insulation, SELV, etc.) without first establishing the relevant external influences and the characteristics of the supply, then applying the disconnection-time table to the wrong system type.
Candidates jump straight to a familiar measure and a memorised table without first classifying the earthing system (TN-S, TN-C-S, TT) and the circuit, so the wrong time or condition is applied.
What to do instead
Adopt a fixed sequence: identify the earthing system, then the external influences, then select the measure, then look up the disconnection time for that system and circuit type. Practise classifying single-line diagrams before calculating anything.
Confusing basic protection with fault protection and misapplying the touch-voltage limits, treating a 230 V nominal system limit as if it applied to a special location or a reduced-voltage arrangement.
The two layers of protection are learned as a single topic, and the tabulated voltage limits are recalled without their conditions of application.
What to do instead
For each protective measure, state separately how basic protection is achieved and how fault protection is achieved, and quote the touch-voltage limit together with the condition (normal or special location, AC or DC) under which it applies.
Sizing a protective conductor by copying the phase conductor size instead of applying the tabulated relationship between phase and protective conductor cross-sectional area, or ignoring the adiabatic check for the fault current and disconnection time.
Candidates default to 'same size as the phase' because it is safe-looking, and skip the k-squared-S-squared energy check because it needs the fault current and the k value.
What to do instead
Learn the phase-to-protective-conductor size relationship as a table and always follow it with the adiabatic check using the actual prospective fault current and disconnection time; show both steps in worked answers.
Calculating voltage drop or current-carrying capacity using the nominal current only, omitting the correction factors for ambient temperature, grouping, thermal insulation and protective device type.
Correction factors are a list to be remembered rather than a procedure to be applied, so candidates compute Iz from In and stop there.
What to do instead
Use the full design sequence every time: design current, then protective device rating, then apply every relevant correction factor to obtain the required tabulated capacity, then check voltage drop separately for the actual load and length.
Specifying an RCD rating or type by habit without matching its residual operating current and type (AC, A, B, or with time delay) to the protective measure, the equipment downstream, and the need for selectivity.
Candidates associate RCDs only with socket outlets and bathrooms and do not connect the device type to the nature of the residual current or to discrimination with upstream devices.
What to do instead
For every RCD, state its purpose (additional protection, fault protection, or both), its rated residual current relative to the touch-voltage and earth-electrode resistance, its type relative to the load, and its time delay relative to upstream devices.
Locating or omitting isolation and switching devices without distinguishing isolation, emergency switching, functional switching and emergency stopping, and without checking that isolation is capable of being secured and interrupts all live conductors.
The four functions are learned as synonyms, so candidates place one switch and claim it satisfies every requirement.
What to do instead
Build a four-column table (isolation, emergency switching, functional switching, emergency stopping) and for each list the device requirement, the conductors switched, and the securing/labelling requirement; apply it to each circuit in practice questions.
Applying the general rules of the standard to a bathroom, construction site or agricultural location without adjusting for the zone boundaries, the reduced touch-voltage limits, the increased mechanical protection or the additional bonding requirements of that special location.
Candidates revise special locations as a list of extra facts rather than as modifications to the general rules, so they answer with the general requirement and lose the location-specific mark.
What to do instead
For each special location, write the general rule and then the specific amendment side by side (zones and their limits, permitted equipment, supplementary bonding, RCD requirements, IP ratings) and practise spotting which rule the question is testing.
Describing inspection and testing as a single act, listing tests without the required order (visual inspection before dead testing, then live testing), and omitting the continuity of protective conductors and main bonding checks.
Candidates memorise a list of instruments and readings but not the sequence or the reason each test precedes the next, so they propose live tests before confirming dead-test results.
What to do instead
Learn the verification sequence as a numbered procedure with the purpose of each step, and always state that visual inspection precedes testing and that continuity and polarity results are established before energising.
Recording test results and completing certification without comparing measured values against the tabulated maxima, and without noting departures, limitations or the extent and limitations of the inspection.
Candidates treat certificates as a record of readings rather than a judgement, so they report a value without stating whether it complies or what was not inspected.
What to do instead
For every recorded result, write the measured value, the applicable limit, and a pass or fail judgement; and always state the extent of the installation covered, the limitations of the inspection, and any departures recorded.