What is the purpose of a supplementary overcurrent protective device?

The purpose of a supplementary overcurrent protective device is to provide limited overcurrent protection in specific applications where additional protection is needed beyond what standard circuit breakers or fuses offer. These devices are typically applied as a means to safeguard specific equipment or circuits that may be susceptible to overloads or short circuits that aren’t adequately covered by the primary protection methods.

In various scenarios, such as in conjunction with motor circuits or specialized equipment, supplementary overcurrent protective devices are designed to limit the current to a safe level to prevent potential damage. They can enhance the overall safety of an installation by providing targeted protection that complements the existing system's primary overcurrent devices.

The other options do not accurately define the role of supplementary overcurrent protective devices. They do not replace standard circuit breakers, as their function is supplementary rather than substitutive. They also do not protect against voltage spikes, which is typically the role of surge protection devices. Additionally, while they may contribute to the isolation of electrical faults indirectly by preventing driven equipment damage, their primary purpose is focused on overcurrent conditions rather than fault isolation.