ITI Solar Technician Electrical (AITT Theory) cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R.

Series: R_total = R1 + R2 + …; same current through all elements; voltages add.

Parallel: 1/R_total = 1/R1 + 1/R2 + …; same voltage across all; currents add.

Two resistors in parallel: R = (R1 × R2)/(R1 + R2).

Energy: W (Wh) = P (W) × t (h); 1 kWh = 3.6 × 10⁶ J.

Conductor resistance R = ρL/A; ρ(copper) ≈ 1.72 × 10⁻⁸ Ω·m at 20 °C.

Temperature coefficient of copper ≈ +0.00393 per °C; resistance rises with temperature.

Coulomb: 1 C = 1 A × 1 s; charge Q = I × t.

RMS = peak/√2 for a sine wave; average of a full sine cycle = 0; average of half cycle = 0.637 × peak.

Frequency f = 1/T; Indian grid supply is 230 V RMS, 50 Hz, single phase.

Inductive reactance X_L = 2πfL; capacitive reactance X_C = 1/(2πfC).

Impedance Z = √(R² + (X_L − X_C)²); power factor = cos φ = R/Z.

Real power P = V I cos φ; reactive power Q = V I sin φ (var); apparent power S = V I (VA).

Ammeter connects in series, voltmeter in parallel; an ideal ammeter has zero resistance, ideal voltmeter infinite resistance.

Clamp meter measures current via magnetic field without breaking the circuit; works on AC and, with Hall sensor, DC.

Megohmmeter (insulation tester) applies 500 V DC to measure insulation resistance in MΩ; multiplier reading × range.

Diode: forward biased when anode > cathode by ≈0.7 V (Si) or 0.3 V (Ge); reverse biased blocks current.

Zener diode operates in reverse breakdown to regulate voltage; series resistor limits current.

Half-wave rectifier: output ripple frequency = supply frequency; full-wave = 2 × supply frequency.

Bridge rectifier uses 4 diodes; each pair conducts on alternate half cycles; output ≈ 0.9 × V_rms (unfiltered).

Filter capacitor reduces ripple; larger C or larger load resistance lowers ripple.

BJT: NPN conducts when base-emitter forward biased; β = I_C/I_B; used as switch or amplifier.

MOSFET is voltage-controlled; low gate current; common in MPPT and inverter switching stages.

Logic gates: AND (all 1s), OR (any 1), NOT (invert), NAND and NOR are universal gates.

SCR (thyristor) latches on when gate pulse arrives and anode is forward biased; turns off only when current falls below holding current.

TRIAC is a bidirectional thyristor used for AC phase control; two SCRs back-to-back in behaviour.

IGBT combines MOSFET gate with BJT conduction; standard switch in grid-tied PV inverters.

Diode forward drop ≈ 0.7–1.0 V; conduction loss = V_f × I_avg.

Bypass diode across a module sub-string limits hotspot losses when shaded; blocking diode prevents reverse current at night.

PWM switching controls average output voltage: V_avg = D × V_dc, where D is duty cycle.

Boost converter steps voltage up: V_out = V_in/(1 − D); buck steps down: V_out = D × V_in.

Heat sink and thermal resistance θ (K/W) determine junction temperature: T_j = T_a + P × θ_total.

Indian cable sizes: 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35 mm²; current capacity rises with cross-section.

Voltage drop limit for wiring design: typically ≤ 3 % for a PV feeder, ≤ 5 % overall.

MCB trips on overload (thermal) and short circuit (magnetic); rated breaking capacity in kA.

RCCB/ELCB (30 mA) protects people against earth leakage; does not replace MCB for overload.

Fuse rating must be ≥ design current and below cable current-carrying capacity.

Earthing: earth continuity conductor resistance to earth should be low (commonly ≤ 1 Ω for grid substations; ≤ 5 Ω for small installations).

DC side of a PV array requires DC-rated isolators, fuses and MCBs - AC devices must not be used on DC.

Cable derating factors apply for ambient temperature, grouping, and conduit/enclosure installation.

Standard Test Conditions (STC): 1000 W/m² irradiance, 25 °C cell temperature, AM 1.5 spectrum.

Irradiance is measured in W/m²; energy in kWh/m²/day (peak sun hours).

Open-circuit voltage V_oc falls ≈ 0.3 %/°C as cell temperature rises; I_sc rises slightly with temperature.

Fill factor FF = (V_mp × I_mp)/(V_oc × I_sc); typical crystalline Si FF is 0.70–0.80.

Efficiency η = P_max/(irradiance × area).

Air mass AM 1.5 is the reference spectrum; AM 1 is overhead sun.

Albedo and tilt affect plane-of-array irradiance; optimum tilt in India is roughly latitude ± 5°.

Shading of one cell can reduce string output disproportionately; bypass diodes limit the loss to a sub-string.

Cell types: monocrystalline (highest efficiency, ~20–23 %), polycrystalline (~17–20 %), thin film (a-Si, CdTe, CIGS, lower efficiency).

Typical cell V_oc ≈ 0.6–0.7 V; a 60-cell module gives V_oc ≈ 36–40 V; 72-cell module ≈ 44–46 V.

Cells in series add voltage; cells in parallel add current.

Module I-V curve: I_sc, V_oc, P_max at the knee; MPP tracker holds the operating point at the knee.

Series string voltage must stay below the inverter/charge controller maximum input voltage at the coldest expected temperature.

String current must stay below the module/string fuse rating at the hottest temperature and highest irradiance.

Hot-spot heating occurs when a shaded cell is reverse biased; bypass diodes and string fuses mitigate it.

Array = series strings in parallel; total power = sum of module powers when matched.

Battery types: flooded lead-acid, VRLA/AGM/GEL, lithium-ion (LFP). LFP has higher usable depth of discharge (≈80–90 %) than lead-acid (≈50 %).

C-rate: C/10 discharge takes 10 h; C/5 takes 5 h; higher C-rate reduces usable capacity.

Battery capacity in Ah; energy in Wh = Ah × V.

PWM charge controller: module voltage must match battery voltage; simple, low cost.

MPPT charge controller: tracks maximum power point; typical efficiency 95–98 %; can accept higher array voltage.

Inverter efficiency typically 90–96 %; standby/self-consumption loss must be included in energy estimates.

Off-grid inverter output is often a modified sine wave (cheap) or pure sine wave (recommended for motors and electronics).

Grid-tied inverter must match grid voltage and frequency and disconnect on loss of grid (anti-islanding).

Daily energy demand E = Σ (P_device × hours/day)/inverter efficiency.

Array size (Wp) = E_daily / (peak sun hours × system efficiency); system efficiency includes soiling, temperature, wiring and inverter losses (typically 0.7–0.8).

Battery bank Ah = (E_daily × autonomy days)/(DoD × V_battery × η_inverter).

Charge controller current rating ≥ array I_sc × 1.25 (safety factor per IEC).

Inverter rating ≥ peak simultaneous load; check surge capability for motors.

Cable voltage drop: V_drop = 2 × L × I × ρ/A (two-way length), keep below 3 % for PV feeders.

Energy yield estimate: annual kWh = array kWp × PSH/day × 365 × performance ratio (PR ≈ 0.75–0.85).

Performance ratio PR = actual energy / (irradiance × array rating).

Pre-commissioning checks: open-circuit voltage per string, short-circuit current per string, polarity, insulation resistance to earth (typically ≥ 1 MΩ at 1000 V test).

I-V curve tracer and irradiance meter are used to verify performance at commissioning.

Torque module clamp bolts to manufacturer specification; verify mounting structure and wind loading.

String voltage measured at the combiner should match the expected sum of module V_oc within tolerance.

Periodic maintenance: clean modules, inspect connectors for corrosion/arcing, check torque, inspect junction boxes and cable trays.

Thermography detects hotspots and loose terminations during operation.

Record commissioning data: date, irradiance, module temperature, string voltages/currents, insulation resistance.

Follow lockout/tagout on the DC side before servicing; treat DC conductors as live until proven de-energised.

DC shock hazard: PV modules cannot be switched off; they generate voltage whenever illuminated.

Body current thresholds: ~1 mA perception, ~10 mA 'let-go', ~30 mA respiratory difficulty, ~50 mA ventricular fibrillation risk.

First aid for electric shock: isolate supply, call for help, then CPR if breathing/ heartbeat absent; do not touch the casualty while still energised.

Fire on electrical equipment: use CO₂ or dry powder extinguisher; never water on live electrical fire.

Arc flash: DC arcs do not self-extinguish at zero crossing as AC does; use DC-rated devices and PPE.

Working at height: harness, anchor point, and fall-arrest per site procedure; roof edges and fragile roofs are the main hazards.

Lockout/tagout: isolate AC and DC, verify dead with tester, discharge capacitors in inverters before opening.

PPE for PV work: insulated gloves (1000 V rated), arc-rated clothing, safety footwear, eye protection.

Solar water heating: flat-plate or evacuated tube collectors, typically 60–80 °C output; thermosiphon or pumped.

Solar PV water pumping: VFD or MPPT controller drives an AC or DC pump; typical agricultural use.

Solar street lighting: PV module + battery + LED luminaire + controller; autonomy typically 2–3 days.

Solar home systems (SHS): small PV + battery + charge controller for lighting and phone charging.

Solar dryer and solar cooker: direct thermal applications, low cost, used in rural settings.

Rooftop grid-tied PV: self-consumption plus export via net metering.

Solar–diesel hybrid: PV reduces diesel consumption; controller dispatches to maximise PV fraction.

Solar cold storage and telecom power: PV + battery with high autonomy requirements.

Net metering: consumer's meter records import and export; billing is on net kWh over the billing period.

Net feed-in tariff (gross metering): all PV generation is exported and paid at a fixed tariff; consumption billed separately.

Anti-islanding protection: inverter must disconnect when grid is lost, per IEEE 1547 / IEC 62116.

Grid-tied inverter must match grid voltage, frequency, phase and power factor within specified limits.

Typical Indian residential net-metering cap is often 100–150 % of sanctioned load; state regulations vary.

Bi-directional meter (import/export) is required for net metering; a standard meter is insufficient.

Interconnection requires approval from the distribution licensee; application, feasibility, and meter testing precede commissioning.

Synchronisation: inverter output must be synchronised before closing the AC contactor; grid-tied inverters do this automatically.

Single-line diagram (SLD) of a PV system shows modules, strings, combiner, isolators, charge controller/inverter, battery, meter, earthing.

Schematic symbols: PV cell (diode with arrows), battery (long/short lines), earth (three decreasing lines), fuse (rectangle with line through).

Scale drawings: 1:50 means 1 unit on drawing = 50 units actual.

Bill of Materials (BOM) lists module, inverter, cables, connectors, mounting, fuses, isolators, meters with quantities.

Cable length estimate = route length + 10–15 % for slack and terminations; DC and AC runs costed separately.

Trade calculation: number of modules = ceil(array Wp / module Wp); number of strings = ceil(modules per string / modules per string capacity).

Estimation includes labour, transport, taxes (GST in India), and contingency; typical contingency 5–10 %.