Which method allows downstream circuit protection devices to be rated at lower values than the main device?

The series rating method is designed to allow downstream circuit protection devices, such as circuit breakers or fuses, to be rated at lower values than the main circuit protection device. This method works by employing the main device to provide overcurrent protection while allowing the downstream devices—connected in series—to have lower ratings because they are protected by the upstream device.

In assessing the series rating, the upstream device has to interrupt any fault current that may occur, thus ensuring that the downstream devices will not see the entire fault current. This approach allows for the use of smaller, less expensive, and more specialized devices downstream while still complying with safety standards and ensuring proper protection in the system.

The other options do not provide this capability. The full rating method requires both the main and downstream devices to be rated for the full current they may experience, which can lead to higher costs and may require larger devices. The individual rating method typically evaluates each device on its own merits without the consideration for series connections, which does not support the idea of reducing ratings. The single device method involves only one protective device, not allowing for any downstream protections that are rated lower than the main protection.