A base load is always only needed when a consumer in the isolated circuit can no longer be switched on. Many electronic devices and lamps need the full mains voltage simply in order to constitute a quantifiable load.
These consumers cannot be detected in an isolated circuit with its very low monitoring voltage of roughly 2.5 V DC and therefore cannot trigger the "net-free" circuit-breaker to restore power. In such cases, a base load of type GK 5 or GW 6 connected in parallel with the consumer will ensure a sufficiently high making current to permit reliable re-energization.
Correct connection of the base load is described in the NFA data sheet. NOTE: The base load is never connected directly to the NFA output, otherwise the NFA will invariably start switching with PTC characteristic on account of the continuous load applied (i.e. it will switch on and off continuously and uncontrollably).