PCBs have been used in which systems or devices?

PCBs, or polychlorinated biphenyls, are synthetic organic chemicals that were widely used in various industrial applications due to their chemical stability and insulating properties. They have been utilized in transformers, which require dielectric fluids that can handle high voltage without breaking down, making PCBs an effective choice in these systems.

In addition to transformers, PCBs have also been used in heat transfer systems. Their ability to remain stable at high temperatures and resist chemical breakdown made them appropriate for use in systems where heat management is critical.

Lubrication systems also utilized PCBs due to their good lubricating properties and resistance to heat and oxidation, which are necessary for maintaining performance in various mechanical devices.

Given that PCBs have found applications across all these systems, the most comprehensive answer reflects that they are used in lubrication systems, transformers, and heat transfer systems collectively. This highlights the widespread industrial reliance on PCBs prior to their regulation and eventual phase-out due to environmental and health concerns.