What is the protective device closest to the source called?

The term that accurately describes the protective device closest to the source is "guard device." This type of device is designed to provide a barrier or safeguard against hazards by preventing access to harmful areas or conditions associated with the source of the potential danger. Guard devices are engineered to ensure safety around equipment and installations, thus minimizing risks to workers and preventing accidents.

The other terms, while related to safety and protection, do not specifically refer to the protective device nearest to the source. For instance, an isolation device generally serves to disconnect equipment from power sources, thereby creating a safe working environment but is not necessarily positioned closest to the source. A shield device typically refers to barriers that deflect harmful elements like radiation or heat, but again does not pinpoint its proximity to the source in the way that guard devices do. As for the concept of a protected device, it is more vague and doesn't specifically outline the proximity or functionality linked directly to guarding against immediate dangers.