Instrument Transformer Construction

Instrument Transformer Construction – Indoor vs. Outdoor

Instrument transformers are used in both indoor and outdoor applications.  The function of the instrument transformer is essentially the same but there is a significant difference in the construction of indoor vs. outdoor units.

First, the insulating material will be different. Outdoor instrument transformers are designed to be used in outdoor environments and must be protected against environmental conditions which includes possible contaminants.  This means the material used to build outdoor units must be able to minimize tracking. These units are typically cast in materials like Hydrophobic Cycloaliphatic Epoxy (HCEP), Cycloaliphatic Epoxy (CEP), UV-Resistant Polyurethane, or Butyl Rubber.

Second, outdoor units will have a greater spacing between line and ground.  They will also have sheds on the primary connections.  Sheds provide larger surface creepage distances from the primary connections to the secondary connections and the grounded base plate.

Next, for outdoor units, the hardware and baseplates must be able to handle outdoor environmental conditions. They must be constructed of corrosion-resistant materials like aluminum and stainless steel.

Finally, another feature that differentiates indoor from outdoor units is the orientation of the primary terminals. Indoor units are designed to connect to the bus bar in switchgear applications while outdoor units are set-up for cable connections on pole-top installations.

Article written by Jon Rennie

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