ITI Solar Technician Electrical (AITT Theory) common mistakes
ITI Solar Technician Electrical (AITT Theory) · Understand the traps and how to avoid them.
Treating current as something that is 'used up' in a series circuit, so the answer for current after each resistor is reduced step by step (e.g. 6 A, then 3 A, then 1.5 A).
Everyday language about 'consuming' electricity and battery 'draining' encourages a water-tank mental model in which charge is spent. Candidates then apply arithmetic division rather than the series-current rule.
What to do instead
State the rule aloud before calculating: in a series circuit the same current flows through every element; only voltage divides, in proportion to resistance. Verify with Ohm's law on each resistor (V=IR) and check the individual voltages sum to the source EMF.
Connecting an ammeter in parallel with a load or a voltmeter in series with it when sketching a measurement circuit.
Both instruments are drawn as circles in the circuit, so the placement distinction is memorised as a label rather than as a reason. Candidates who never learned why the ammeter is low-resistance and the voltmeter high-resistance cannot reconstruct the correct connection.
What to do instead
Anchor on resistance: the ammeter must carry the current, so it goes in series and must have near-zero resistance; the voltmeter must sense a potential difference without diverting current, so it goes in parallel and must have very high resistance. Draw the internal resistance symbol inside each meter when practising.
Calculating the DC output of a PV module by simply multiplying nameplate current by nameplate voltage and then assuming the same power at the battery or load, ignoring the difference between STC and NOCT and between Imp and Isc.
The datasheet lists many numbers (Voc, Isc, Vmp, Imp, Pmax, temperature coefficients) and candidates grab the largest ones or the first ones. They also forget that module temperature in the field is far above 25 degrees C.
What to do instead
Use Vmp x Imp = Pmax for operating power, and reserve Voc x Isc for the 'do not exceed' check on controller and cable ratings. Apply the power temperature coefficient (typically about -0.3 to -0.4 %/degree C) for expected cell temperature, and use NOCT-based or derated values for realistic yield calculations.
Sizing a battery bank by dividing daily energy by battery voltage and stopping there, without applying depth of discharge, inverter efficiency, or temperature derating.
The first formula learned (Ah = Wh / V) is satisfyingly simple and gives a small, attractive number. Candidates under pressure skip the derating chain because each factor feels like an optional refinement.
What to do instead
Always write the full chain: required Ah = (daily Wh x days of autonomy) / (system voltage x DoD x inverter efficiency x temperature derating). State each assumed factor explicitly in the answer so the examiner can see the method, and sanity-check that the resulting bank is larger than the naive value.
Selecting a cable cross-section from current-carrying capacity alone and ignoring voltage drop over the run length.
Ampacity tables are the most visible part of cable selection and give a single clear number, whereas voltage drop requires a formula and a length. Candidates also underestimate how long PV source and array-to-controller runs actually are.
What to do instead
Check both criteria and take the larger cable: ampacity with all derating factors, and voltage drop (typically kept under 3 percent for PV DC circuits and under 5 percent overall). Practise the two-wire drop formula Vdrop = 2 x I x L x rho / A and convert the permitted percentage into volts before solving for area.
Describing earthing by bonding the module frames to the negative conductor or by using the same conductor for protective earth and neutral without explaining the separation.
The words 'earth', 'ground' and 'negative' are used loosely in training material, and grid-tied inverter topologies (transformerless versus transformer-isolated) blur the distinction. Candidates conflate functional earthing of the array with system grounding of a current-carrying conductor.
What to do instead
Separate the concepts in writing: equipment earthing bonds exposed metal (frames, rails, enclosure) to the earth electrode for fault protection; system grounding deliberately references one live conductor and depends on inverter type. Use the correct colour code and state that protective earth must never carry operating current. Mention equipotential bonding and the earth resistance target in the same answer.
When asked to identify a thyristor or IGBT circuit, describing the SCR as controlled by base current or the IGBT as a current-controlled device.
BJT terminology is learned first and sticks; the three-terminal symbols look similar in a hurry. Candidates also forget that an SCR latches once triggered and cannot be turned off by its gate.
What to do instead
Build a one-line comparison for each device: SCR - gate pulse triggers, anode-cathode conduction latches until current falls below holding level; MOSFET - voltage-controlled gate, majority carrier; IGBT - voltage-controlled gate with low saturation loss, used in inverters. Match the device to its application (SCR for controlled rectification, IGBT for PV inverter switching).
Answering net-metering questions by describing the meter as running backwards for the whole billing period, or by assuming export is always paid at the retail tariff.
Popular explanations of net metering oversimplify. The actual settlement depends on import/export registers, time-of-use or feed-in arrangements, and the local interconnection rules, which candidates have not separated from the physics.
What to do instead
Distinguish the two registers: energy imported from the grid and energy exported to it. Explain that billing nets or separately prices these flows according to the applicable tariff and regulation, that a bidirectional meter is required, and that export may be limited or curtailed. Also mention anti-islanding as a mandatory interconnection requirement.
Commissioning a system by checking open-circuit voltage only, then declaring it ready without an insulation resistance test, polarity check, or short-circuit current comparison between strings.
Voc is easy, safe and gives a satisfying reading. The other tests need a megohmmeter, a clamp meter and a plan for irradiance correction, so they are skipped or described vaguely.
What to do instead
Write a fixed commissioning sequence: visual inspection, polarity and labelling, open-circuit voltage per string, short-circuit or operating current per string compared across strings, insulation resistance to earth with the specified test voltage, then inverter start-up and monitoring. Note that measured values must be corrected for irradiance and temperature before being compared with the datasheet.
In engineering-drawing or estimation questions, reading a dimension from a drawing without checking the stated scale, or omitting waste and contingency from a bill of quantities.
Candidates treat a drawing as a picture rather than a controlled document, and treat estimation as pure measurement. Scale factors and allowance percentages feel like minor details rather than the point of the question.
What to do instead
Read the title block first for scale, units and revision, then convert measured lengths using the scale before any arithmetic. In estimates, show quantity take-off, then add explicit allowances for cable routing slack, connectors, fixings and contingency, and total the labour and material separately so the method is visible.