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Glossary of Electrical Terms

50+ electrical design terms explained in simple words

А

Circuit Breaker (MCB)
An electrical device that automatically opens a circuit during overload or short circuit. The primary protective element of an electrical network.

В

Main Switchboard (MSB) / Input-Distribution Device
The panel at the building power supply entry point. Receives electricity from the external grid and distributes it to consumers. Contains main breaker, metering devices, RCDs.
Two-Way Switch / Intermediate Switch
A switch that allows controlling lighting from two (two-way) or three+ (intermediate) locations. Used in corridors, staircases, bedrooms.

Г

Final Circuit / Group Circuit
The line from the panel to end consumers (outlets, lights). Each group circuit is protected by a separate MCB. No more than 20 points per outlet group is recommended.
MDB (Main Distribution Board)
The central panel of a building, receiving power from the transformer substation. Distributes electricity to floor and sectional panels. The highest-power panel in the hierarchy.

Д

RCBO (Residual Current Circuit Breaker with Overcurrent)
A device combining MCB (overload and short-circuit protection) and RCD (leakage current protection) functions. Saves space in the panel.

З

Earthing / Grounding
Intentional electrical connection to earth to protect people from electric shock. Can be functional (TN-S, TN-C-S) and protective (PE conductor).
Protective Earth Conductor (PE)
A conductor connecting equipment enclosures to earth. Ensures safety: during insulation failure, current goes to earth rather than through a person. Yellow-green insulation.

К

Cable
One or more insulated conductors in a common sheath. For building wiring, cables like VVG, NYM, PVC are used. Selection depends on load and installation conditions.
Short Circuit (S/C)
An unintended connection between phase and neutral/earth conductors with very low resistance. Causes a sharp current spike, heating and fire risk. Protection — circuit breakers and fuses.
Power Factor (cos φ)
Ratio of active power to apparent power. Shows how efficiently electricity is used. For household loads cos φ ≈ 0.9-1.0, for motors — 0.7-0.85.
Grounding Electrode / Earth Loop
A system of metal electrodes buried in the ground to create reliable earthing. Loop resistance — no more than 4 Ohm for 380/220 V per PUE/IEC.
Demand Factor (Kd)
Ratio of design load to installed capacity. Accounts for the fact that not all loads run simultaneously. Kd = 0.3-0.8 for residential buildings (the more apartments, the lower the Kd).
Reactive Power Compensation
Installation of capacitor banks to improve cos φ. Reduces line losses and transformer load. Relevant for workshops with motors — raises cos φ from 0.7 to 0.95.
Cable Gland
A device for sealed cable entry into a panel or equipment enclosure. Ensures IP protection and mechanical cable fixation. Required to maintain the panel's protection rating.

Н

Rated Current (In)
The current at which electrical equipment can operate indefinitely without overheating. For MCBs, standard ratings: 6, 10, 16, 20, 25, 32, 40, 50, 63 A.

О

Single-Line Diagram (SLD)
A simplified power supply diagram where all phases are shown as one line. Contains load data, breakers, cables, S/C currents. The main project document.

П

Conductor
A material (usually copper or aluminum) that conducts electric current well. Copper conductors are used in building wiring — they are more reliable than aluminum.
PUE (Electrical Installation Code)
The main regulatory document of Russia for design and installation of electrical installations up to 500 kV. Contains requirements for wiring, earthing, protection, lighting. 7th edition is current.
Voltage Drop
Voltage difference between the beginning and end of a line due to conductor resistance. Per PUE/IEC — max 5% from main panel to end consumer. The main criterion for cable cross-section selection.

Р

Distribution Board (DB)
A metal or plastic enclosure with DIN rails housing MCBs, RCDs and other devices. Distributes electricity across consumer groups.
Design Current (Icalc)
Current calculated for selecting cable cross-section and MCB rating. Icalc = Pcalc / (√3 × U × cos φ) for three-phase or Icalc = Pcalc / (U × cos φ) for single-phase.
Short-Circuit Calculation
Determination of short-circuit current to verify MCB breaking capacity and cable thermal withstand. Per GOST 28249 (Russia) or IEC 60909 (international).
Voltage Relay / Undervoltage-Overvoltage Relay
A device that disconnects power when voltage goes beyond acceptable limits (e.g., <170 V or >270 V). Protects home appliances from voltage surges and neutral failure.
Neutral Earthing Mode
The method of earthing the transformer neutral: solidly earthed (TN), isolated (IT), through impedance. For residential buildings — solidly earthed neutral (TN-C-S).
Power Calculation / Load Calculation
Determination of design load (Pcalc) for selecting cable cross-section and transformer. Considers installed capacity and demand/simultaneity factors per SP 256 / IEC.

С

Selectivity / Discrimination
A protection property where only the breaker closest to the fault trips during a short circuit, while the rest remain on. Ensures continuity for other consumers.
Protection Degree (IP Code)
See IP Rating — international standard IEC 60529. E.g., bathroom outlet must be IP44, outdoor light — IP65, indoors — IP20.
Enclosure Protection Degree
See IP Rating. Applied to panels, outlets, lights. Determines suitability for installation in dry/damp/dusty rooms and outdoors.

Т

Transformer
A device for converting AC voltage. Steps down high voltage (6-10 kV) to household level (220/380 V). The basis of substations.
Cable Thermal Withstand
The cable's ability to withstand heating by S/C current during the protection tripping time. Verified: S ≥ Isc × √t / k, where k is the insulation material coefficient (115 for Cu/PVC).

У

RCD (Residual Current Device)
A protective device that disconnects the circuit when current leaks to earth (e.g., through a person). Does not protect against overload or S/C — an MCB is needed for that.
Installed Capacity (Pinst)
The sum of rated powers of all electrical loads at a facility. Differs from design load (Pcalc) by the demand factor (Kd < 1), as all equipment rarely runs simultaneously.

Ф

Phase (L1/L2/L3)
A conductor with voltage relative to earth and neutral. In a three-phase network — three phases (L1, L2, L3) shifted 120°. Household outlets — 220 V (phase-neutral), industrial — 380 V (phase-phase).

Ш

Busbar
A conductor for distributing electricity inside a panel. Made of copper or aluminum. A comb busbar is a type of busbar for connecting MCBs of the same phase.

Э

Electrical Wiring
The totality of wires and cables with fixings and protective structures. Can be concealed (chased in plaster) or exposed (in trunking, on trays).

B

BIM (Building Information Modeling)
Building Information Modeling technology. Electrical is part of the BIM model along with architecture and structures. Exchange via the IFC format. Mandatory for government orders in Russia since 2022.

C

Cable Tray
A metal structure for exposed cable routing. Used in industrial and commercial buildings. Convenient for maintenance and adding new cables.
Cross-section (Conductor Cross-Section)
Conductor cross-sectional area in mm². Determines the allowable current. Standard range (Cu): 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50 mm². Outlets — 2.5 mm², lighting — 1.5 mm².

D

Derating Factor
A coefficient reducing cable ampacity depending on: number of cables in a bundle, ambient temperature, installation method. Per IEC 60364 / IS 732 derating tables.

E

ELCB (Earth Leakage Circuit Breaker)
An earth leakage protection device. In modern installations, replaced by RCDs. Distinguish between voltage-operated ELCB (obsolete) and current-operated (RCD/RCCB).

I

IP Rating (Ingress Protection)
International classification of enclosure protection against dust and moisture. First digit (0-6) — dust protection, second (0-9) — water. IP44 — splash protection, IP65 — full dust and water protection.
IEC (International Electrotechnical Commission)
International Electrotechnical Commission. Develops standards (IEC 60364 — low-voltage installations, IEC 60909 — short-circuit currents). PUE and IS are based on them.

L

LED Lighting
Lighting based on light-emitting diodes. Consumes 5-10 times less electricity than incandescent lamps. Service life — 30,000-50,000 hours. Standard for modern projects.

M

MCB (Miniature Circuit Breaker)
Miniature circuit breaker. Standard protection for group circuits in panels. Characteristics: B (household), C (commercial/industrial), D (motors with high inrush current).
MCCB (Molded Case Circuit Breaker)
Molded case circuit breaker. Used on main incomers and for high-power consumers (currents from 100 to 2500 A). Has adjustable trip settings.

P

PEN Conductor
Combined protective earth and neutral conductor in the TN-C-S system from substation to main switchboard. Separated into PE and N at the entry. Minimum cross-section — 10 mm² (Cu) or 16 mm² (Al).

S

SPD (Surge Protective Device)
Surge protective device. Protects equipment from lightning strikes and switching surges. Classes: Type I (lightning), Type II (switching), Type III (fine protection).

T

TN-C-S System
An earthing system where neutral (N) and protective earth (PE) conductors are combined at the entry (PEN) and separated in the main switchboard. The most common system in Russia.

V

VDE (Association for Electrical, Electronic & Information Technologies)
German association for electrical engineering. VDE standards are among the strictest in the world. Cables with VDE marking guarantee top insulation quality.

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