Oil Cooled Cathodic Protection Transformer Rectifiers: Why, when and where…

Oil Cooled Cathodic Protection Transformer RectifiersThis article describes the components of a cathodic protection rectifier, and when to use oil cooled cathodic protection transformer rectifiers vs. air cooled rectifiers.

When it comes to cathodic protection power supplies, conventional transformer rectifier circuits have long been employed by the cathodic protection industry for impressed current CP systems.  These power supplies (commonly referred to as rectifiers in the CP world) consist of three main components; the transformer, the rectification stack, and a cabinet to house these components.  The transformer takes the input AC voltage on the primary side and controls the output AC voltage on the secondary side.  The rectification stack, typically silicon diode stacks which have largely replaced older less efficient selenium stacks, convert the AC input wave form into a DC wave form by cycling the AC flows in one direction and blocking in the other.  Additional components typically include circuit breakers, fuses, voltage and current output meters, lightning arrestors, surge suppressors, transformer tap bars, and monitoring systems.

Air-cooled Rectifier
Typical Air-cooled Rectifier

The majority of these Rectifiers are housed in air-cooled NEMA 3R enclosures – these enclosures are typically constructed of hot dipped galvanized steel, aluminum, stainless steel or painted steel.  NEMA 3R enclosures are intended for outdoor use.  They provide a degree of protection against falling rain and ice formation but are not completely water tight or weather proof and could be subjected to beating rain or streams of water, under certain conditions, entering the enclosure.  This is the most common type of rectifier enclosure in the industry.

When and Where to Use Oil Cooled Cathodic Protection Transformer Rectifiers


Oil Cooled Rectifier
Oil Cooled Rectifier

For some applications; however, the use of air cooled NEMA 3R enclosures is not recommended or not suitable.  The three most common reasons not to use air-cooled NEMA 3R enclosures are:

  1. Rectifier transformer size is too large to support an air cooled enclosure. For a small percentage of impressed current CP systems where the power requirements (measured in DC Watts) are sufficiently high that the cooling capacity of the enclosure is insufficient for the heat generated by the transformer (typically anything more than 12kW for single phase and 18kW for three phase.)
  2. Severe environment locations where high humidity, dust or other situations could shorten the life of a standard air cooled rectifier. Marine and near shore applications often fall into this category.
  3. The enclosure must be in a hazardous classified location requiring Class 1 Div. 2, Group D compliant enclosure – commonly referred to as Explosion Proof.
Oil Cooled Rectifier for Hazardous Locations
Oil Cooled Rectifier for Hazardous Locations

For these applications, oil cooled cathodic protection transformer rectifiers are typically specified.  As implied in the name, the oil cooled rectifier utilizes an enclosure that has a sealed reservoir which houses the transformer and transformer tap bars and is filled with a special transformer oil.  The transformer oil provides better heat transfer and dissipation and the larger case facilitates improved heat removal.

It is very important to note that standard oil cooled rectifiers are NOT explosion proof.  For an oil cooled rectifier to be considered Explosion Proof, the components that are not immersed in the transformer oil reservoir must be housed in special Explosion Proof fixtures.  Simply specifying oil cooled when ordering a rectifier does not satisfy the requirements for locating the rectifier in a hazardous Class 1 Div.2 location without also including the additional provisions required for the explosion proof fittings.


To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote for cathodic protection materials or related construction services, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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Helium Leak Testing for Cathodic Protection Anodes

MATCOR is pleased to announce that we are now capable of performing Helium leak testing on our full range of linear anode products as an optional testing service. This is a common practice among companies and product developers that provide products that could potentially leak gas or that require water tightness. Products commonly leak tested include refrigeration lines, vehicle brake lines, and devices that contain potentially harmful or deadly substances. Helium is the second smallest element (Hydrogen is the smallest), which means that it is valuable for leak testing. Smaller molecules naturally can find smaller gaps or defects from which to leak. Unlike hydrogen, however, helium is a noble gas and is therefore unreactive due to its complete valence electron shell. As a result, helium is the most viable gas for use in leak testing.

Helium leak testing is now available for all MATCOR linear anode products, however our Kynex connection technology has zero reported failures since it was introduced a decade ago.
Helium leak testing is now available for all MATCOR linear anodes, however our patented Kynex technology has zero reported failures since it was introduced a decade ago.

MATCOR has enjoyed an outstanding record as the world’s leading supplier of MMO anodes/Titanium linear anodes with over 25 years of linear anode experience supplying our industry leading SPL™ family of linear anode products for pipelines, tanks and other applications around the world. Our patented automated injection molded Kynex® connection technology has an outstanding track record with no known connection failures since this technology was introduced in 2009.

We do, however, see some client specifications calling for 100% connection testing and helium leak testing is the most effective means to test an entire anode assembly.


For more information, please feel free to contact your local MATCOR representative or contact us at the link below.

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