When RCDs are connected in series, what must be applied to ensure correct operation?

In the context of Residual Current Devices (RCDs) connected in series, the concept of discrimination refers to the capability of the RCDs to operate sequentially without nuisance tripping. This means that in the event of a fault, only the downstream RCD should trip, while the upstream RCD remains operational. This is crucial for maintaining the circuit's overall functionality and safety, especially in complex installations where multiple RCDs might be utilized for different sections of the electrical system.

When RCDs are connected in series, they need to be adequately discriminated to ensure that the correct RCD responds to a fault. If the RCDs are not properly discriminated, a fault could potentially cause more than one RCD to trip, leading to unnecessary power loss and complicating diagnostics.

While test current, equal impedance, and uniform voltage are all important in an electrical context, they do not address the specific requirement of ensuring that only the appropriate RCD trips during a fault condition in a series configuration. Thus, discrimination is the necessary condition for the efficient operation of series-connected RCDs.