ITI Solar Technician Electrical (AITT Theory) flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

Voltage equals current multiplied by resistance (V = I × R). Used to calculate any one of the three quantities when the other two are known in a DC circuit.

Power formula

Electrical power P = V × I, also written P = I²R or P = V²/R. Measured in watts for DC and in watts of real power for AC.

Series resistance

In a series circuit the total resistance is the sum of individual resistances, the current is the same through every element, and voltages add.

Parallel resistance

In a parallel circuit the reciprocal of total resistance equals the sum of reciprocals of individual resistances; voltage is common and currents add.

RMS value

Root-mean-square value of a sine wave equals peak value divided by the square root of two. A 325 V peak corresponds to 230 V RMS.

Power factor

Cosine of the phase angle between voltage and current; equals R/Z. Real power equals apparent power multiplied by power factor.

Clamp meter use

Measures current by sensing the magnetic field around a conductor without breaking the circuit. Hall-effect versions can also measure DC current.

Megohmmeter purpose

Applies a high DC test voltage, typically 500 V or 1000 V, to measure insulation resistance in megohms and detect degraded cable or module insulation.

Diode forward bias

A silicon diode conducts when the anode is about 0.7 V more positive than the cathode; below this threshold it blocks current.

Zener diode

Operates in controlled reverse breakdown to hold a nearly constant voltage; a series resistor limits current and dissipates excess power.

Bridge rectifier

Four diodes arranged so both half-cycles of AC produce DC output of the same polarity; unfiltered output is about 0.9 times the RMS input.

SCR latching

A thyristor turns on with a gate pulse when forward biased and stays on until anode current falls below the holding current.

IGBT use

A voltage-controlled switch combining a MOSFET gate with bipolar conduction. It is the standard switching device in grid-tied PV inverters.

Bypass diode role

Connected across module sub-strings so that current can bypass a shaded or faulty cell, limiting hotspot heating and power loss.

MCB function

Miniature circuit breaker trips on overload by thermal action and on short circuit by magnetic action; rated breaking capacity is stated in kiloamperes.

RCCB purpose

Residual current circuit breaker, typically 30 mA, detects earth leakage and disconnects to protect people; it does not replace an MCB for overload protection.

DC-rated devices

Switches, fuses and breakers rated for direct current must be used on the PV array; AC-rated devices may arc and fail because DC has no zero crossing.

Standard Test Conditions

Reference conditions for module ratings: 1000 W/m² irradiance, 25 °C cell temperature and AM 1.5 spectrum. Nameplate power is measured here.

Fill factor

Ratio of maximum power to the product of open-circuit voltage and short-circuit current. Crystalline silicon modules typically show 0.70 to 0.80.

Temperature effect on V_oc

Open-circuit voltage falls roughly 0.3 % per °C as cell temperature rises, so cold mornings produce the highest string voltage.

String voltage limit

Series module voltages add. The string open-circuit voltage at the coldest expected temperature must stay below the inverter or controller maximum input voltage.

Battery C-rate

Discharge or charge current expressed as a multiple of capacity. C/10 means a full discharge takes ten hours; higher rates reduce usable capacity.

MPPT controller

Maximum power point tracker adjusts operating voltage to extract peak module power, typically at 95–98 % efficiency, and accepts higher array voltages.

Anti-islanding

Protection in grid-tied inverters that disconnects from the grid when grid supply is lost, preventing back-feed that endangers utility workers.

Array sizing formula

Array watt-peak equals daily energy demand divided by peak sun hours and system efficiency, where system efficiency typically ranges from 0.7 to 0.8.

Battery bank sizing

Capacity in Ah equals daily energy times autonomy days divided by depth of discharge, battery voltage and inverter efficiency, using consistent units.

Performance ratio

Actual annual energy divided by the energy the array would produce at rated efficiency under the same irradiance; typical values are 0.75 to 0.85.

Insulation resistance test

Measures resistance from live conductors to earth with a megohmmeter; values below about one megohm indicate moisture or damaged insulation needing investigation.

Thermography

Infrared imaging that reveals hotspots, loose terminations and failing bypass diodes by their abnormal temperature rise during operation.

PV shock hazard

Modules generate voltage whenever illuminated and cannot be switched off, so DC conductors must be treated as live until proven de-energised.

Electrical fire extinguisher

Carbon dioxide or dry powder extinguishers are used on live electrical fires; water conducts and must never be applied to energised equipment.

Net metering

A billing arrangement where the bi-directional meter records both import and export, and the consumer is billed on the net energy over the period.

Single-line diagram

A schematic showing the PV array, combiner, isolators, charge controller or inverter, battery, meter and earthing on one drawing using standard symbols.