• Circuit breaker operation: motors that charge breaker springs.
• Power too close or trip the solenoids that operate the breaker
• Electronic devices such as power monitors, automatic power factor correction systems, etc
• Convenience outlets
• Cabinet lighting/ indicator lamps
• Station battery charging systems

Control Power Transformers and Converting Voltage

Control power transformers in switchgear change the VOLTAGE levels of the primary voltage, to an acceptable level needed by the devices listed above. For instance, a common low voltage system (1000 volts or below) operates at 480 volts, a popular distribution voltage in factories. 480 volts is used to power 480-volt 3 phase motors, bus duct and other industrial equipment. The control power transformers may convert 480 volts to 240 or 120 volts.

Medium and high voltage systems use similar transformers that are rated at those higher voltages.

Single Phase Control Power Transformers

The control power transformer is usually single phase, is fused at the primary volt levels and at the lower volt level. Fusing may be at both or only 1 of the levels. These transformers generally provide a range of 5-30 amps and are rated in VA and they are usually single phase. For example some common sizes are 100 , 500, 750, 1000 VA . Some go higher.

Control power transformers generally are powered AHEAD of the main breaker, so be very careful when investigating and working these devices.

Finding Your Control Power System

Location? Good luck! Some OEM’s build convenient drawout cabinets to safely shut down control power systems: open a certain door/cabinet and access to fusing and the transformer may be safe and easy. However some control power systems are still powered after opening their cabinets. Still, some OEM’s hide their control power system. While others mount them in dangerous locations.

Some may have fusing with or without a disconnect switch ahead of the fusing. Some are in accessible locations. Other OEM’s work very hard to make accessibility difficult, depending upon their design Philosophy.

In ALL cases, refer to the OEM’s documentation for safety instructions regarding these important and necessary control power transformers in switchgear. Above all if in doubt, STAY OUT of control power systems.

That’s all for now. Take care

-Neil Volk, Electrical Engineer

Owner, Operator All-Test Company

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