Primary backup protection is the first line of defense in a power system to protect the equipment from faults and abnormal conditions. It is designed to provide fast and reliable protection in case of any abnormalities.

In the given components, the option 'D' - Equipment to be protected, can be common for primary backup protection. Here's why:

Equipment Protection:
- The primary objective of backup protection is to ensure the safety and integrity of the equipment in the power system.
- Equipment such as transformers, generators, motors, and other electrical devices are susceptible to faults and abnormal conditions that can lead to damage or failure.
- Primary backup protection is designed to detect these faults and disconnect the faulty equipment from the power supply to prevent further damage.

Importance of Equipment Protection:
- Equipment in a power system is expensive and vital for the operation of the system.
- Faults or abnormalities in equipment can lead to power outages, damage to other equipment, and even safety hazards.
- Protection systems are installed to minimize the impact of faults and ensure the reliability and longevity of the equipment.

Primary Backup Protection:
- Primary backup protection operates quickly and independently to detect faults and isolate the faulty equipment.
- It is designed to provide a reliable and fast response to faults, ensuring the safety and integrity of the equipment.
- The primary backup protection system typically includes protective relays, which are devices that monitor electrical parameters and can trip circuit breakers to isolate the faulty equipment.

Role of Other Components:
- Circuit breakers (option 'A') are essential components of the primary backup protection system as they are responsible for disconnecting the faulty equipment from the power supply.
- Current transformers (option 'B') are used to measure the current flowing through the equipment and provide input to the protective relays for fault detection.
- The battery room (option 'C') is not directly related to primary backup protection. However, it may provide backup power for the protection system in case of a power failure.

Conclusion:
While circuit breakers and current transformers are crucial components of the primary backup protection system, the equipment to be protected (option 'D') is common to all protection schemes. The primary backup protection is designed to ensure the safety and integrity of the equipment in the power system by detecting faults and isolating the faulty equipment.