GFCI or GFI (Ground Fault Circuit Interrupter) - A specific type of circuit protection (commonly required in kitchens & bathrooms) that helps safeguard against shocks. GFCI protection can come from an outlet or a breaker.
Ground Fault - Current misdirected from the hot (or neutral) lead to a ground wire, box, or conductor.
Grounds or Grounding - Connecting one side or neutral of a circuit to the earth through low resistance or low impedance paths, to help prevent transmitting shocks to personnel.

Hot, Neutral, Ground - The three most common circuit wires. The hot brings the current flow in, the neutral returns it to the source, and the ground is a safety route for returning current. The ground and neutral are joined only at the main service panel.
Hertz (Hz) - Expression of ac frequency in cycles per second, e.g., 60 Hz.
High Voltage and Low Voltage Windings - Terms used to distinguish the winding having the greater voltage rating from that having the lesser in two-winding transformers. The terminations on the high voltage windings are identified by H1, H2, etc., and on the low voltage by X1, X2, etc.
HV - High voltage

Impedance - The obstruction to the flow of current in an ac circuit. The transformer impedance is made up of the internal resistance and reactance which creates a voltage drop caused by current flowing in the windings. Impedance losses are a combination of heat (I2R) losses in the conductor, eddy current loss, and losses in other parts of the transformer caused by stray currents.
Impulse - A current surge
Insulation - A material having a high resistance to the flow of electric current; insulation over underground conductor is made of either EPR or XLPE material.
Insulator - Any material which does not allow electrons to flow through it.