Fusing for Instrument Transformer Circuit Protection

Voltage Transformer or Potential Transformer

If you read our post on “Medium Voltage Fuses for Power Distribution Transformers, you know that power transformers don’t generate electrical power; They transform and transfer power safely between circuits. In this article we’ll discuss Instrument Transformer types and fuses for instrument transformer and switchgear circuit protection.

Current Limiting Fuses for Instrument Transformers

Instrument Transformer Fuses are current-limiting fuses with high interrupting ratings used for primary winding protection since it’s receiving the high power input. See Cooper Power ELSG Fuses and Eaton Helix Fuse current limiting fuse discussions.

Potential Transformer fuses are relatively inexpensive but can cause an operation to experience costly downtime when they blow. There’s a big payback when you immediately have spare fuses and parts from an independent electrical fuse and parts supplier available for replacement after determining and rectifying the cause.

Effective transformer overcurrent protection is critical to protecting personnel and equipment. Other than motors, transformer applications are next in line for demanding superior overcurrent protection.

600V-38kV Fuses for Instrument Transformer Circuit Protection

Monster Fuses provides 600V-38kV Fuses for Instrument Transformers. Providing medium voltage fuses from stock is our primary focus. Understanding the equipment protected by the fuses is an important part of the service we provide.

Instrument Transformers Isolate High Power Voltage or Current

Now let’s discuss a particular type of transformer called an Instrument transformer. Instrument transformers are high accuracy class electrical devices used to transform and isolate voltage or current levels for safe operation and maintenance. Instrument Transformers are simple devices that play a critical role in power systems. Let’s tear some apart and look inside.

An Instrument transformer steps down and “transforms” system voltage or current to a lower, safer value to enable accurate secondary metering and measurement. Commercial relays and meters (e.g., ohm meters, voltmeters) used for protection and metering are designed for lower voltages and amperage.

Let’s say we want to measure voltage or current of a common high power distribution system. This couldn’t be done practically or safely with a conventional test meter. An Instrument Transformer is used to step-down the voltage or current and isolate it from instruments and operators and allow for safe, accurate voltage measurement. Instrument transformers increase safety, accuracy and convenience. This is the basic definition of an instrument transformer.

[pullquote align=”full” cite=”” link=”” color=”” class=”” size=””]Two types of transformers fit into the classification of Instrument transformers. These are Current Transformers (CT) and Potential Transformers (PT)[/pullquote]

CT’s for Safe Power Distribution Metering, Measurement

Current transformers (CT) are a series connected type of instrument transformer.  CT’s step down current to a known ratio among the primary and secondary winding. An important distinction with CT’s is that the primary and secondary windings are separate and different from one another, facilitating the precise, known current ratio. CT’s are commonly used in power distribution systems to step down high currents for metering and measurement. Like other transformers discussed here, current transformers allow standard instruments to be used in measuring electricity for safe operator maintenance. In other words, they safely isolate the high current for the distribution, measurement and management of power.

PT’s Connect High Voltage Power Systems

A Potential Transformer (a.k.a. voltage transformer) is a parallel connected type of instrument transformer. A PT is a conventional transformer with primary and secondary windings. Windings are wire coils layered around a transformer core. The primary winding receives the energy and is called the input. The secondary winding discharges the


“Potential ransformer” precisely scales dangerous high voltage to a safe value applicable to a conventional voltmeter

energy and is called the output.  The two coils work in concert to step-up or step-down voltage.

This capability provides electrical isolation which allows PT’s to serve as a way of connecting electrical instrumentation to high voltage, high current power systems. By stepping down the voltage to feed instruments and metering, PT’s add an additional level of safety and make testing with standard test equipment possible. In many cases fuses are connected in series with the PT’s primary winding for safety and ease of disconnecting the PT from the circuit.

Potential Transformer Fuse Solutions

Monster Fuses provides potential transformer fuse solutions from Mersen, GE, Ferraz, Eaton Cutler-Hammer, Bussmann, Westinghouse & S&C. Fuse voltages for Potential Transformers Monster Fuses stocks include 600V, 2.4kV, 4.8kV, 5.5kV, 6.6kV, 8.25kV, 15kV, 25.8kV, 38kV.

Monster Fuses and sister division, Monster Controls can also help you identify and source medium voltage motor-starter contactors, switches and parts, including CT’s and PT’s for equipment built in the 1950’s through current models.  Having spare fuses immediately available for replacement after transformer overcurrents will quickly get you back online.

[pullquote align=”full” cite=”” link=”” color=”” class=”” size=””]Need it Now?  Fast turnaround, immediate shipment. Call 888.444.4335 or visit monsterfuses.com[/pullquote]

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