Technical Glossary

A

ANSI

American National Standards Institute. Coordinates the U.S. standards system; many of the standards we work with (ANSI/IEEE C57, C37) are jointly developed with IEEE.

Arc Flash

The high-energy explosion of plasma that can occur during an electrical fault. Arc-flash incident energy is calculated per IEEE 1584 and PPE selected per NFPA 70E.

ASTM

American Society for Testing and Materials. Source of test methods like ASTM D877 (oil dielectric strength) and D923 (oil sampling).

B

BIL (Basic Insulation Level)

The peak voltage a transformer or switchgear can withstand without insulation breakdown — typically expressed in kV (e.g., “150 kV BIL” for 15 kV equipment). Higher BIL = more margin against lightning surges.

Bushing

The insulated terminal that lets a high-voltage conductor pass safely through the grounded tank or enclosure. Most catastrophic transformer failures begin at a bushing.

C

Cast Resin Transformer

Dry-type transformer with windings encapsulated in vacuum-cast epoxy. Excellent fire performance (F1 class), high humidity tolerance. See our types comparison.

Coordination Study

An engineering analysis that ensures protective devices (breakers, fuses, relays) trip in the right order — clearing a fault as close to its source as possible without nuisance tripping upstream.

CSA

Canadian Standards Association. CSA marking is required for most electrical equipment installed in Canada.

CT (Current Transformer)

An instrument transformer that steps current down to a level safe for relays and meters (typically 5 A or 1 A secondary). Reverse polarity is a classic commissioning miss.

D

Delta (Δ)

A three-phase winding configuration where the windings are connected in a closed triangle. Common on the primary side of distribution transformers.

DGA (Dissolved Gas Analysis)

Lab analysis of gases dissolved in transformer oil — primarily H₂, CH₄, C₂H₄, C₂H₂, CO. Different gas patterns indicate different fault types (overheating, partial discharge, arcing).

DOE 2016

U.S. Department of Energy efficiency standard for distribution transformers, in effect since January 2016. Sets minimum efficiency at full load.

E

Enclosure (NEMA Type)

NEMA 250 ratings define how an enclosure protects against environmental hazards. Common ratings: NEMA 1 (indoor general), 3R (outdoor rain-tight), 4X (outdoor corrosion-resistant), 12 (industrial dust-tight).

F

F1 Fire Class

The highest fire-performance classification for dry-type transformers per IEC 60076-11 — self-extinguishing, low smoke, low toxicity. Required in many high-rise, hospital, and tunnel applications.

G

Ground Resistance

The resistance of the equipment ground path to remote earth. Best practice: < 5 Ω for general installations, < 1 Ω for substations and critical facilities.

H

Harmonics

Distortions of the AC waveform at multiples of the fundamental frequency (60 Hz). Caused by non-linear loads (VFDs, UPS, LED drivers). Can cause transformer overheating — drives K-factor selection.

Hi-Pot Test

High-potential test that applies an over-voltage (typically 75% of factory test) to verify dielectric integrity in the field. Done as part of NETA acceptance testing.

I

IEEE

Institute of Electrical and Electronics Engineers. Issues most of the technical standards we follow — IEEE C57 for transformers, IEEE C37 for switchgear, IEEE 1584 for arc flash.

IEC

International Electrotechnical Commission. The international counterpart to ANSI/IEEE/UL — IEC 60076 for transformers, IEC 62271 for HV switchgear.

Impedance (Z)

The percentage voltage that must be applied to the primary to circulate full-load current in a short-circuited secondary. Common values: 5.75% for distribution transformers, 5–8% for power transformers. Lower Z = higher fault current; higher Z = better fault limiting.

IP Rating

Ingress Protection rating per IEC 60529. Two digits: first = solids (0–6), second = liquids (0–8). Common ratings: IP54 (dust + splash), IP65 (dust-tight + jets).

K

K-Factor / K-Rating

A multiplier (K-1, K-4, K-13, K-20, etc.) that indicates a transformer’s ability to handle harmonic currents without overheating. K-1 = standard linear load. K-13 typical for data centers. K-20 for heavy non-linear loads.

kAIC (Short-Circuit Rating)

Short-circuit current rating in kiloamps. The amount of fault current the equipment can interrupt or withstand without damage. Common LV ratings: 65 kAIC, 100 kAIC, 200 kAIC.

kVA / MVA

Kilovolt-amps / megavolt-amps. The apparent power rating of a transformer (1 MVA = 1,000 kVA = 1,000,000 VA).

L

LV (Low Voltage)

In North American practice, ≤ 1,000 V AC. Below this threshold, switchgear is built to UL 891 / UL 1558. See switchgear article.

Loss Evaluation

The practice of capitalizing transformer no-load and load losses over project life (typically 30 years) and adding that cost to the purchase price for true lifecycle comparison.

M

MCC (Motor Control Center)

An assembly of motor starters, contactors, and protective devices serving multiple motor loads. Distinct from switchgear — typically lower voltage, designed for motor starting duty.

Megger / Insulation Resistance Test

A high-voltage DC test that measures insulation resistance to ground in megohms. Trended against commissioning baseline to detect insulation deterioration.

MV (Medium Voltage)

1 kV to 38 kV in North American practice. Switchgear built to ANSI/IEEE C37.20 series.

N

NEMA

National Electrical Manufacturers Association. Issues NEMA SG-5 (switchgear assemblies), NEMA TP-1/TP-2 (transformer efficiency), NEMA 250 (enclosures).

NETA

InterNational Electrical Testing Association. Issues NETA ATS (Acceptance Testing Specifications) for new equipment commissioning and NETA MTS (Maintenance Testing Specifications) for periodic maintenance.

NFPA 70 / NEC

National Electrical Code. The U.S. standard for electrical installation. Issued by the National Fire Protection Association.

NFPA 70E

Standard for Electrical Safety in the Workplace — covers arc-flash analysis, PPE selection, and lockout-tagout procedures.

No-Load Loss

The power dissipated in a transformer’s core whenever it’s energized, regardless of load. Constant 24/7 — drives loss-evaluation calculations.

O

OSHA 1910.269

U.S. occupational safety standard for electric power generation, transmission, and distribution. Governs work practices on energized equipment.

P

Pad-Mounted Transformer

Outdoor liquid-filled transformer in a tamper-resistant enclosure, mounted on a concrete pad. Common for underground residential and commercial distribution.

PI (Polarization Index)

The ratio of 10-minute to 1-minute insulation resistance readings. Acceptance criteria: PI ≥ 2.0 for liquid-filled, ≥ 1.5 for dry-type.

Pole-Mounted Transformer

Liquid-filled transformer bolted to a utility pole. Typically 10–167 kVA single-phase, used on overhead distribution.

Power Factor (Transformer)

The ratio of dissipated power to apparent power in the transformer’s insulation. Trended over time to detect insulation degradation. A jump > 0.5% from baseline warrants investigation.

PT (Potential Transformer) / VT (Voltage Transformer)

Instrument transformer that steps voltage down to a relay/meter-safe level (typically 120 V secondary). PT and VT are interchangeable terms.

R

Relay (Protective)

An automatic device that monitors current/voltage and trips a breaker when a fault is detected. Modern relays are microprocessor-based (e.g., SEL-751, GE Multilin, ABB REF). Each protection function has an ANSI device number (50 = instantaneous overcurrent, 51 = time overcurrent, 87 = differential, 27 = undervoltage).

S

SCCR (Short-Circuit Current Rating)

The maximum short-circuit current that equipment can withstand or interrupt without damage. Same concept as kAIC.

Stress Cone

A field-applied or factory-molded termination that controls the electric stress at the end of a shielded MV cable. Poor stress cone preparation is a leading cause of cable termination failure.

Substation

An assembly that transforms voltage between transmission and distribution levels. Usually contains step-down transformers, MV switchgear, protective relays, and metering.

Switchboard

An LV electrical assembly built to UL 891. Distinct from switchgear — switchboards use molded-case or insulated-case breakers; switchgear uses power circuit breakers.

Switchgear

An assembly of switches, fuses, breakers, and protective relays that controls and protects electrical equipment. Built to ANSI/IEEE C37.20 (LV or MV) or UL 1558 (LV PCB).

T

Tap Changer

A mechanism that adjusts the transformer’s turns ratio to compensate for primary voltage variations. De-energized tap changers (DETC) are adjusted with the unit off; load tap changers (LTC) operate under load.

Thermography

Infrared imaging used to find hot spots on energized equipment. Annual thermography is standard preventive maintenance.

TTR (Turns Ratio Test)

A field test that confirms the actual turns ratio matches nameplate within 0.5%. Done at every tap position during commissioning.

U

UL

Underwriters Laboratories. UL listings (UL 891 for switchboards, UL 1558 for LV switchgear) are required for most U.S. installations.

V

Vacuum Circuit Breaker (VCB)

An MV breaker that interrupts current in a vacuum-sealed bottle. Modern standard for new installations — SF6 breakers are being phased out for environmental reasons.

Vector Group

Notation that describes a three-phase transformer’s primary winding configuration, secondary winding configuration, and phase displacement. Format: HV-LV-displacement (e.g., Dyn1 = Delta primary, Wye secondary, 30° lag; Dyn11 = Delta primary, Wye secondary, 30° lead).

VPI (Vacuum Pressure Impregnated)

Dry-type transformer construction where varnish is drawn into windings under vacuum and pressure. Workhorse of indoor distribution. See types guide.

W

Wye (Y)

A three-phase winding configuration where one end of each winding connects to a common neutral. Provides a neutral for single-phase loads. Often used as the secondary of distribution transformers (480Y/277, 208Y/120).

Z

Zigzag Winding

A specialized winding configuration used to derive a system neutral or block zero-sequence currents. Common in grounding transformers.