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Frequently Asked Questions

What is a Zig-Zag Grounding Transformer?

A Zig-Zag grounding transformer is a special three phase autotransformer with no secondary or output. It is used to establish a neutral on a 3 wire system. There are several reasons why a zig zag autotransformer may be used include ground fault protection, neutral grounding (solid or via a resistor), and harmonic filtering.

What is an Isolation Transformer?

All transformers other that autotransformers are Isolation Transformer. This is due to the fact that the primary and secondary windings are electrically isolated from each other (they are not physically connected to each other). The transformation in voltage and current between primary and secondary windings occurs as a result of the shared magnetic field in the core (Mutual Inductance).

What is an Autotransformer?

An autotransformer is a transformer that has only one winding per phase, part of which is common to both the primary and secondary circuits. Transformers wired in a “Buck-Boost” configuration are autotransfomers. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. An autotransformers can be used in either a step-up or step-down application unlike isolation transformers. Autotransformers can also be used as part of a reduced voltage starter to reduce motor inrush current (see Motor Starting Autotransformers)

What are the standard service conditions for a dry type distribution transformer?

Unless designed for special service conditions / environments, below are the standard service conditions for dry type distribution transformers:

  • Ambient Temperature: -20°C to + 30°C (max peak +40°C)
    • Note: dry type transformers should not be energized at ambient temperatures below 0°C
  • Relative Humidity: < 70%
  • Altitude: ≤ 1000m ASL (3300 ft.)

Avoid rain, moisture, high temperature, intense heat or direct sunlight. Maintain the minimum recommended clearances and ensure that all ventilated panels are clear of obstructions.

What is required clearance around a dry type distribution transformer?

The minimum required clearances of a dry type transformer to walls, floors or other equipment must adhere to the local electrical code requirements.

In the absence of such requirements, Rex Power Magnetics recommends that dry type transformers be mounted so that there is an air space of no less than 150mm (6”) between the enclosures, and between the enclosure and any adjacent surface except floors. When the adjacent surface is a combustible material, the minimum permissible separation between the transformer enclosure and the adjacent surface should be 300mm (12”). Where the adjacent surface is the wall on which the transformer is mounted, the minimum permissible separation between the enclosure and the mounting wall should be 6mm (0.25”) so long as the surface is of a non-combustible material.

What do I need to consider for low ambient temperature conditions?

Low ambient temperatures do not generally affect an energized transformer. The transformer's energization / No-load losses typically generate enough heat to operate effectively in low ambient temperatures down to -20°C.

The main issue with low ambient temperature conditions is when the unit is left de-energized or stored in lower temperatures. Energizing and loading a transformer that has been left in low ambient temperatures can stress the transformer and lead to failure. The insulation can become brittle in low temperature conditions, and as the windings warm up quickly and begin to expand, the insulation can crack, leading to a failure.

Furthermore, low temperatures can cause moisture/frost to form on the unit which can be absorbed into the insulation system. Anti-condensation heaters can be supplied within the enclosure to help keep the ambient temperature above 0°C when the transformer is de-energized, however due to the fact that the enclosures are typically ventilated and uninsulated, they are only effective for ambient down to -10°C. If ambient goes below -30°C, special designs and cold start procedures may be necessary. Care should be taken to store transformers in dry areas with temperature control.

Rex's installation, operation and maintenance manuals recommends that transformers be tested (meggered), brought above 0°C and/or go through a dry-out process if moisture is suspected to be present. Damage and injury can result from energizing a transformer which has had its insulation system compromised by moisture.

What is the terminal temperature rating of ventilated dry type distribution transformers?

Rex Power Magnetics’ ventilated distribution transformer terminals are rated 90°C. Conductors with at least a 90°C insulation rating at or below their 90°C ampacity rating should be utilized.

What is the odor emitting from my new transformer? 

It`s normal for new transformers to release some harmless odors from the varnish impregnation used in the coils for a week or two after energization. Older Transformers can also release some odor if loaded to a higher level than they have experienced previously in their history.

R.C. Snubber

Product Overview & Application

SNUBBER open FrontR.C. snubbers are a passive electrical devices, comprised of appropriately selected resistors and capacitors, which are used for protecting medium voltage power transformers from electrical switching transients. While lightning arresters help protect the transformers from the high voltage spikes that appear on the line after a lightning strike, snubbers are better suited for the more common problem of circuit breaker switching transients. Examples of the switching operation include instances where a circuit breaker switches between transformer primaries, or automatically triggered switching between grid and reserve power.

When a circuit breaker interrupts current flow, an arc develops across its contacts. The rapid current interruption usually occurs somewhere other than the current zero-crossing point, giving rise to a Transient Recovery Voltage (TRV), which has significant high-frequency content. If the transients are at the natural resonance frequency of the system, then there is a possibility of internal oscillatory voltages developing in the primary windings of the transformer. A high turn-to-turn voltage will result, which will lead to damage of the insulation.

The problem is further exasperated with modern vacuum-type circuit breakers, which have shorter distances between contacts. This serves to increase the speed of the interrupting cycle. The transient level and frequency are functions of the physical distance between the breaker and transformer, transformer construction, the type of load being switched, and the switching characteristic of the breaker.

The RC-snubber network lowers the frequency of the transient voltage applied to the transformer primary below the resonance frequency of the circuit. It reduces the development of the oscillatory voltages and provides a low impedance path to ground for the transients.

Snubbers are a low cost means for protecting a transformer against a costly repair and related downtime. In designing medium voltage power systems, it is a good practice to at least leave space for future installation of a snubber network.

Product Specification

Voltage Class  2.4kV to 34.5kV (150kV BIL)
Frequency  60Hz (50/60Hz Optional)
Fuse  6A Full Range Current Limiting Fuse
Resistor  20 - 50 Ohm (+/- 10%) Tubular Non-Inductive Resistor
Capacitor  0.13uF, 0.25uF, 0.5uF Surge Capacitor
Enclosure Type  Type 1 Indoor
Enclosure Finish  ANSI/ASA 61 Grey
Mounting  Floor Mounting
Warranty  12 Months (See Warranty and Limitations)
Quality System  ISO 9001:2015 Quality Management System
Optional Features & Accessories
  • Snubber Ground Current Monitoring Circuits
  • Distribution, Intermediate or Station Class Surge Arresters
  • Voltage Indicators / Glow Tubes
  • Type 1, 3R or 4X Enclsoure
  • 304 & 316L Stainless Steel Enclosure
  • Custom Enclosure Finish (Color)


Canadian Sales Representatives


Southern Ontario

Ontech Sales Inc.
1892 Baseline Rd. W.
Courtice, ON L1E 0X6
Darrell Mcneill: This email address is being protected from spambots. You need JavaScript enabled to view it.  
Tel: 416-576-5114 
Lynn Konowal: This email address is being protected from spambots. You need JavaScript enabled to view it.  
Tel: 905-828-4730 

Northern Ontario (Thunder Bay, Ft. Francis, Terrace Bay)

Integra Agencies
301 E Weston Street
Winnipeg, MB R3E 3H4
Tel: 204-694-1339
Fax: 204-694-8299


Northern Alberta

Brodwell Industrial Sales
7 Rowland Cres
St.Albert ,AB T8N 5B3 
Tel: 780.458.1500
Fax: 780.458.0334

Southern Alberta

Brodwell Industrial Sales
#1010, 4385 104 Ave SE
Calgary, AB T2C 5C6
Tel: 403.253.7702
Fax: 403.253.2341

British Columbia

Arbutus West Agency Ltd.
1424 Regan Ave
Coquitlam, BC V3J 3B5
email: This email address is being protected from spambots. You need JavaScript enabled to view it.
Tel: 604-521-5180


Québec - Estrie, Centre du Québec & Mauricie

Agence Béliveau Turmel et Associés
115 de la Riviere
Racine, QC J0E 1Y0
Tél: 450-532-3578
Cell: 819-239-6591 
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Québec - Rive-Sud de Montréal, Abitibi

Eric Turmel
Tél: 450-349-7453
Cell: 514-515-7453
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Québec - Montréal, Laval, Laurentides, Outaouais & Lanaudière

Eric Massé
Cell: 514-247-1909
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Québec - Québec, Saguenay, Cote-Nord, Beauce, Gaspesie & Bas St-Laurent

Serge Maheu
Cell: 418-933-4513
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Québec - Coordonnateur Technique

Stephane Brouillette
Cell: 438-300-8801 #101
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Québec - Support Interne

Yanick Turmel
Tél: 438-300-8802 #106 
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Yvan Turmel 
Tél: 438-300-8802 #107
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Integra Agencies Man. Ltd.
301 E Weston Street
Winnipeg, MB R3E 3H4Tel: 204-694-1339
Fax: 204-694-8299

New Brunswick, Nova Scotia, Prince Edward Island

ELP Marketing
32 McQuade Lake Crescent
Halifax, NS B3S 1G8
Tel: 902-450-5155
Fax: 902-450-1222
email: This email address is being protected from spambots. You need JavaScript enabled to view it.



Advance Electrical Agency Ltd.
2717B Wentz Ave.
Saskatoon, SK S7K 4B6
Tel: 306-653-0313
Fax: 306-244-4070
email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Newfoundland & Labrador

Vigilant Technical Sales Ltd.
Contact: Jon Morgan
28 Symonds Avenue
St.John's, NL A1E 5B1
Tel: 709-753-6685
Fax: 709-753-7759

AC Line / Load Reactor

Inductors placed at the input and output of electrical equipment can provide protection and improve performance. Line Reactors absorb many power line disturbances which could damage or shut down your inverters, order variable speed controllers, or other voltage-sensitive equipment.


Read more: AC Line / Load Reactor

General Purpose Isolation Transformer

transformerGeneral Purpose Isolation transformers are used to lower distribution voltages (480 or 600V) to lower voltages (typically 120/240 or 208Y120) for power distribution within commercial or industrial buildings. These “two windings” transformers provide the electrical isolation required, and allow for short runs of high current, low voltage cables, reducing losses in the system, and reducing costs.


Read more: General Purpose Isolation Transformer

Mini Power Center

4-1Mini Power Centers combine three individual components, prewired into one NEMA 3R enclosure: a primary main breaker, a single-phase or three-phase dry-type encapsulated transformer (Type EP), and a secondary distribution load center with main breaker. Inter-connecting wiring is completed at the factory. Mini Power Centers are used wherever there is a 480V or 600V distribution system and loads requiring 208Y/120V, three-phase or 120/240V single-phase.

Read more: Mini Power Center



Autotransformers are transformers which have a single winding per phase, part of which is common to both primary and secondary circuits. Using an Autotransformer is an economical and compact means of connecting electrical equipment to a power supply or a different voltage. Part of the winding is common to both primary and secondary circuits so there is no isolation between the two. This may be acceptable on some power systems that do not have a grounded neutral on the secondary side of the main power transformer. Typical applications include motor loads of industrial machinery, electric heating, air conditioners, etc. Designed for installation where a reliable power source is required for lighting and other low voltage equipment and machinery. Indoor and outdoor installation is possible depending on the enclosure.

Read more: Autotransformer

Industrial Control Transformer

Industrial control transformers are used to convert the available supply voltage to the voltage that is required to supply industrial control circuits and motor control loads.

These loads consist of a combination of electromagnetic devices such as solenoid operated valves, switches, bells, alarms, and other components such as indicator lights, timers, electronic equipment, and logic boards.

Some components of a circuit, especially electromagnetic devices require a very high level of power at start-up.  This start-up power requirement is called inrush VA. After their initial start-up, these components settle down to a lower power requirement for normal continuous operation. This lower operating power requirement is called Sealed or Steady State VA. Some devices can draw up to 10 times the normal operating or sealed current for periods of up to 50 milliseconds upon start up.

Control transformers must be designed, constructed, and selected to ensure that they provide the output voltage stability needed for trouble free operation of all circuit components.

Rex control transformers incorporate several features to optimize output voltage stability and regulation when supplying industrial control circuits with high inrush current requirements.

Read more: Industrial Control Transformer

Harmonic Mitigating Transformer

Harmonic Mitigating Transformers (HMT's) are specially designed for non-linear loads which draw significant amounts of harmonic current. While K-factor rated transformers are simply designed to handle the additional losses of supplying non-linear loads, HMT's actually minimize the voltage distortion and power losses that result from harmonic load currents.

Read more: Harmonic Mitigating Transformer

Encapsulated Transformer


Encapsulated (Potted) Transformers (also referred to as Epoxy Resin Encapsulated) are designed for applications where the enviromental conditions would not permit a general purpose ventilated dry type transformer. The entire tranformer core & coil is encased in a silica sand / polyurethane mixture which protects the windings from any airborne contaminants and moisture.

Read more: Encapsulated Transformer

Cast Coil Power Transformer

Cast Coil2Cast coil power transformers feature primary and secondary windings which are vacuum cast inside a solid block of epoxy resin. The epoxy resin is introduced under vacuum in order to penetrate all the spaces within the winding, ensuring a void free casting with minimal partial discharges. The sold casting provides the highest degree of protection to the windings, making them ideally suitable for harsh environmental conditions which include airborne dust & high humidity. Furthermore, the fiberglass reinforcement which is incorporated into the casting provides the transformer with the highest mechanical strength which makes it resilient to short circuit forces.

Read more: Cast Coil Power Transformer

Motor Guarding Transient Filter

LineFilters catThe steep voltage wave fronts of the Pulse Width Modulated (PWM) output of Adjustable Frequency Drives (AFD’s) produce high frequency effects which may damage the insulation of motors operated by the equipment. The problems result from two distinct effects.


Read more: Motor Guarding Transient Filter

Air Core Reactor

ir-Core reactors provide a linear impedance with respect to current which is crucial in many applications where an iron core reactor would otherwise saturate.

Read more: Air Core Reactor

Control Power Transformer

Product Overview & Application

CPT webMedium Voltage Control Power Transformers (CPT) are single phase transformers used to supply control power in medium voltage switchgear and transformers. Nominal primary voltages range from 2.4kV to 15kV. The secondary voltage is typically configured for 120/240V (series parallel) operation. Note: Fuse and fuse clips shown in image are optional accessories.

Product Specification

Capacity  500VA to 3,000VA Single Phase
Voltage Class  up to 7.2kV (45kV BIL)
Cooling  Self Cooled (ANN / AN)
Frequency  60Hz (50/60Hz Optional)
Conductors  Copper (Cu) Windings
Insulation System  180°C (115°C rise) 
Enclosure Type            Open Construction With Encapsulated Coil
Warranty  12 Months (See Warranty and Limitations)
Quality System  ISO 9001:2015 Quality Management System
  • CSA Certified
  • UL Listed
Reference Standards
  • CSA C22.2 No 47
  • CSA C22.2 No 66.1
  • CSA C9
  • IEEE C57.13.2
Optional Features & Accessories
  • Suitable For 50Hz Nominal Frequency
  • Primary Fuse Clips & Fuses




Drive Isolation Transformer

transformerDrive Isolation Transformers are designed for supplying power to SCR (Silicon Control Rectifier) motor drives which provide convenient variable speed motor control and can save on energy. Unfortunately, the drawback of these variable frequency drives (VFD) is that they produce harmonic voltage distortion and draw harmonic currents from the source which can be damaging both to the transformer supplying the power, and the other equipment that is connected to that supply.


Read more: Drive Isolation Transformer

VPI/VPE Power Transformer

CoreAndCoilVacuum Pressure Impregnated (VPI) power transformers utilize an open winding design, where the coil insulation is impregnated with an epoxy resin under vacuum and pressure, providing the windings a degree of protection against moisture and dirt, and improving the thermal conductivity of the winding which reduces localized hot spots. Furthermore, the VPI process removes air voids which deep within the windings, filling them with epoxy instead. These air voids, if not eliminated, can lead to higher partial discharges which can damage the transformer insulation over time, leading to premature failure. For a higher degree of protection, vacuum pressure encapsulated (VPE) coils can be utilized which involves repeated VPI and bake cycles

Read more: VPI/VPE Power Transformer

Hazardous Location Transformer

Product Overview & Application

“Hazardous Locations and Harsh Industrial Environments” typically refers to areas which may contain dangerous and corrosive, explosive or combustible gasses, liquids, or dusts. Typical applications include the mining, petrochemical, and pulp and paper industries. Rex Power Magnetics has been designing and producing a line of dry type transformers for Hazardous Locations and Harsh Industrial Environments. Rex’s product offering in this category includes three varieties, known as the HLA, HLB & HLC product lines, which are CSA and UL approved for CLASS I, DIVISION 2, GROUPS A, B, C, & D.

  • Group A: Atmospheres containing acetylene;
  • Group B: Atmospheres containing butadiene, ethylene oxide, hydrogen (or gases of vapours equivalent in hazard to hydrogen, such as manufactured gas) or propylene oxide
  • Group C: Atmospheres containing acetaldehyde, cyclopropane, diethyl ether, ethylene, hydrogen sulphide, or unsymmetrical dimethyl hydrazine (UDHM), or other gases or vapours of equivalent hazard
  • Group D: Atmospheres containing acetone, acrylonitrile, alcohol, ammonia, benzene, benzol, butane, ethylene, dichloride, gasoline, hexane, isoprene, lacquer solvent vapours, naphtha, natural gas, propane, propylene, styrene, vinyl acetate, vinyl chloride, xylenes, or other gases or vapours of equivalent hazard

*NEW* A variant of Rex’s HLA & HLB transformers is available with IECEx approved CLASS 1 Zone 2 certification. Please contact our sales office for more information on IECEx approved transformers CLASS I, ZONE 2, GROUPS IIA, IIB, IIC — IECEx CSA 13.0011X

  • Group II A: Atmospheres containing acetaldehyde, acetone, cyclopropane, alcohol, ammonia, benzene, benzol, butane, ethylene, dichloride, gasoline, hexane, isoprene, lacquer solvent vapours
  • Group II B: Atmospheres containing acrylonitrile, butadiene, diethyl ether, ethylene, ethylene oxide, hydrogen sulphide, propylene oxide, or unsymmetrical dimethyl hydrazine (UDHM), or other gases or vapours of equivalent hazard
  • Group II C: Atmospheres containing acetylene, carbon disulphide, or hydrogen or other gases or vapours of equivalent hazard

Product Specification

Construction Features
  • ANC design
  • Encapsulated Transformer
  • Bottom/Front Access (based on kVA)
  • Conduit K/O on sides
  • nameplate marking
  • ANC design
  • Encapsulated Transformer
  • Bottom/Front Access (based on kVA)
  • Conduit K/O on sides
  • nameplate marking
  • ANC design
  • Ventilated Transformer
  • Bottom/Front Access (based on kVA)
  • Conduit O/O on sides
Insulation Class  180C  185C  220C
Temperature Rise  115C Maximum  115C Maximum  80C, 115C or 150C
Temperature Code  T3  T3  T3C, T3, or T2C
  • ASA #61 Grey
  • Type 3R outdoor
  • ASA #61 Grey
  • Type 3R outdoor
  • ASA #61 Grey
  • Type 3R outdoor
Certification CSA & UL approved for Class 1, Division2 CSA & UL approved for Class 1, Division2 CSA & UL approved for Class 1, Division2
Application Windings are encapsulated in epoxy, preventing airborne contaminants from reaching the coils, and ths preventing damate to electrical insualtion. As with HLA, the windings are encapsulated. Additionaly, teh connection between the leads and the coils are encapsualted. Wiring to the rest of the system is done at the leads. Conventional construction: non-encapsualted, vented design. Designed to conform to specified temperature code.


K-Factor Rated Transformer

Today's modern electronic, electric components and circuitry such as computers, copiers, printers, fax machines and display terminals utilize switching mode power supplies for their operation. These switching mode power supplies are non-linear in nature and unlike a linear load which uses current from the power source continuously over the sinusoidal cycle, a non-linear load draws current in pulses from the power source thereby creating harmonic distortion. These harmonics currents cause significant power system problems such as:

  • Circuit breakers and fuses blowing far below current ratings
  • Neutrals in transformers and panel boards are much hotter than their ratings
  • Distributions Transformers are overheating even when operating well within their specified nameplate rating

Read more: K-Factor Rated Transformer

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International Reps

International Sales Representatives


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