TP EIMSA common mistakes
TP EIMSA · Understand the traps and how to avoid them.
Treating protective-device selection as a simple 'pick the next size up from the load current' exercise, and never checking that the device's rated breaking capacity exceeds the prospective short-circuit current at its point of installation.
Candidates learn the overload rule (Ib ≤ In ≤ Iz) as a formula and stop there; the short-circuit withstand/breaking-capacity condition is a separate check that is easy to overlook because it does not appear in the same one-line inequality.
What to do instead
Before finalising any device, write three lines on the drawing: design current Ib, protective device rating In, cable capacity Iz, then add a fourth line for prospective fault current at that board versus the device's breaking capacity (Icn/Ics) and its energy-limiting let-through. Practise by taking a real distribution board schedule and annotating all four values for every outgoing way.
Confusing the roles of bonding and earthing: describing main protective bonding as the means of fault disconnection, or omitting supplementary bonding in a location where simultaneous contact with exposed and extraneous conductive parts is possible.
The two functions overlap in everyday language ('everything goes to earth'), and candidates memorise 'bond the services' without linking bonding to touch-voltage limitation and earthing to automatic disconnection.
What to do instead
Build a two-column table: earthing/CPC = fault current path enabling disconnection; main protective bonding = equalising potential between extraneous parts and the MET; supplementary bonding = local equalisation where disconnection times alone are insufficient. For each column, note the conductor sizing rule and the test that verifies it (earth electrode resistance, main bonding continuity, supplementary bonding resistance).
Applying a single memorised formula (for example P = VI) to AC circuits containing inductance or capacitance, and reporting real power where reactive or apparent power was asked for.
Candidates under time pressure default to the DC relationship they know best and skip the power-triangle step; the wording 'power' is assumed to mean watts.
What to do instead
Before calculating, write down which quantity is requested (P in W, Q in var, S in VA) and which is given. Draw the power triangle and the impedance triangle side by side and state the phase angle. Then check with units: if the answer is in VA or var, P = VI is the wrong tool. Drill mixed R-L-C series and parallel circuits until the choice is automatic.
Mixing rms and peak values, or using line-to-line voltage where line-to-neutral is required, when computing three-phase current, voltage drop, or neutral current in an unbalanced load.
The √3 and √2 factors are applied mechanically from memory, and candidates do not track whether the quantity they started with was a peak, an rms, a phase, or a line value.
What to do instead
Adopt a habit of labelling every number with its type as you write it (e.g. '400 V line-line rms', '325 V peak'). For three-phase work, always write the star/delta relationship explicitly before substituting: V_L = √3·V_ph, I_L = √3·I_ph for delta. For unbalanced neutrals, resolve the three phase currents as phasors rather than adding magnitudes.
Selecting a cable on current-carrying capacity alone, ignoring derating factors for ambient temperature, grouping, thermal insulation, and installation method, and ignoring voltage drop over the actual run length.
Manufacturers' tables give a headline current for a reference method, and candidates treat that figure as the cable's capacity; the correction factors sit in a separate table that is easy to skip.
What to do instead
Use a fixed five-step worksheet for every cable: (1) installation reference method, (2) tabulated Iz, (3) multiply by Ca, Cg, Ci and any soil/thermal factors, (4) compare with In, (5) check voltage drop against the limit for the actual length and load power factor. Redo three worked examples where grouping and ambient temperature together force a larger size than the tabulated value suggests.
Specifying or installing a wiring system without matching the IP rating, mechanical protection, and material to the environment (wet, corrosive, high-temperature, or hazardous area), and mixing dissimilar metals at terminations.
Candidates focus on getting the circuit to work and treat enclosure and material choice as cosmetic; the environmental classification of the location is not carried forward from the earlier design step.
What to do instead
For each location, write the environment class first (e.g. damp, dusty, corrosive, explosive zone) and then justify the accessory: IP rating, gland type, conduit material, and whether a barrier or seal is needed. At terminations, state the metal pairing and use the correct bimetallic transition or inhibitor. Practise by classifying five described locations and listing the accessories each demands.
Terminating and torqueing conductors by feel rather than by the manufacturer's specified torque, and failing to re-check connections after thermal cycling or after the equipment has been in service.
Experienced hands 'know' when a terminal is tight, and torque values are seen as a paperwork detail; the consequence (loose neutral, overheating, arcing) appears months later and is not linked back to the installation step.
What to do instead
Carry a calibrated torque tool and the manufacturer's torque schedule on every job, mark completed terminations, and record the torque value in the installation record. Include a re-torque check in the first maintenance visit after commissioning. In exam answers, always name the torque tool and the record, not just 'tighten securely'.
Performing insulation resistance or earth fault loop impedance tests without first isolating and proving dead, or leaving sensitive equipment and surge protection connected during the test.
The test itself feels safe because the instrument is battery-powered, so the isolation step is compressed; candidates also forget that a 500 V insulation test can damage electronics.
What to do instead
Use a fixed safe-isolation sequence every time: identify, isolate, lock off, prove the tester on a known live source, prove dead at the point of work, and only then test. Before insulation testing, list what must be disconnected (SPDs, dimmers, electronics, capacitors) and record it. State this sequence explicitly in any exam question about testing, even when the question seems to be about instrument readings.
Interpreting a test result as pass/fail against a remembered number without confirming the applicable limit for that circuit type, and without checking instrument accuracy and test conditions (e.g. temperature correction, parallel paths).
Candidates memorise a handful of limits and apply whichever comes to mind; parallel earth paths and conductor temperature at the time of test are rarely considered, so a 'passing' reading may be misleading.
What to do instead
For each test, write the applicable limit and its source before you take the reading, plus the correction to be applied (e.g. conductor resistance referred to 20 °C, or the effect of parallel paths on loop impedance). Note the instrument's accuracy class and whether the reading sits close enough to the limit to need a repeat with a different method.
Diagnosing a tripping RCD or breaker by replacing the device first, rather than dividing the circuit to localise the fault and distinguishing insulation failure, borrowed neutral, N-E fault, and genuine overload.
Replacement is fast and sometimes appears to fix the symptom temporarily; candidates lack a structured decision tree, so they jump to the component they can see.
What to do instead
Work a fixed sequence: confirm the symptom and when it occurs, isolate and test insulation resistance to earth and between conductors, test for N-E continuity at the board, check for shared neutrals across circuits, measure load current against the device rating, then substitute a known-good device only as a confirmation step. Write the decision tree out once and apply it to practice scenarios until the order is automatic.