Which devices reduce voltages and amps to usable levels in the primary circuits of substations?

The correct choice, which identifies the devices that reduce voltages and amps to usable levels in the primary circuits of substations, is instrument transformers. These devices, which consist of current transformers and voltage transformers, are specifically designed to scale down higher voltages and currents in primary circuits to levels that can be safely measured and used for monitoring, protection, and control purposes.

Instrument transformers ensure that the actual high voltage and high current levels in the substations do not have to be directly handled, which enhances safety and allows for the use of standard measuring equipment. This makes it easier to integrate readings into control systems without compromising safety.

Current transformers measure and convert current levels, while voltage transformers serve a similar purpose for voltage, combining their functionalities under the umbrella of instrument transformers. This provides a comprehensive solution for managing the high electrical energy found in substations, ensuring that operators can work effectively with lower, safer levels of the electrical parameters.

The other options, while related to the management of electrical systems, do not encompass the complete function of scaling down voltages and currents specifically for measurement purposes as accurately as instrument transformers do.