Control Power Transformers in Switchgear II

Control power systems change the VOLTAGE levels of the primary voltage, to a usable level needed by many electrical devices. For instance, a common low voltage system (one thousand volts or below) operates at 480 volts, a popular distribution voltage in factories. 480 volt is used to power 480-volt three phase motors, bus duct and other industrial equipment. The control power transformers may convert 480 volts to 240 or 120 volts to control other internal systems.

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

Single Phase Control Power Units

These control power transformers are usually single phase in nature and are fused. Fusing may be at both or only one of the levels.

These transformers provide a range of current in the 1–50-amp levels. They are rated in VA. POTENTIAL transformers, on the other hand may be 3 phase in nature, and a topic for another blog. For now, let us stick with single phase control power units.

Common sizes are 100, 500, 750, 1000 VA; and higher.

Control power transformers are (usually) powered AHEAD of the main breaker be very careful when investigating and working these devices.

Location of these devices? Good luck! Some OEM’s build convenient draw out 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. Still, test it first.

Some are still powered after opening their cabinets. Still, some OEM’s hide their control power system. Some 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 systems.

If in doubt, STAY OUT of control power systems.