My understanding is that "inmunizado" generally means "immunized" in the sense of vaccines against disease, but I also found this set of definitions:
inmunizado, inmunizada
adjetivo
inmune (ciencias naturales), inatacable, invulnerable.
http://es.thefreedictionary.com/inmunizado

I've never encountered inmunización/inmunizado in an electrical context as synonymous with "protection/protected," (meaning protected by circuit breakers or related protective devices against power surges, etc.), but that really seems to be how it's being used here.

differential protection: a protective relaying scheme in which the currents entering and leaving the protected line or device are compared.
http://www.google.com/search?hl=en&rlz=1C1GGLS_enUS318US318&...

What's the point of differential protection?
Because of the voltages and currents involved, input to the protective relays is supplied by current transformers (CT's) in the motor supply lines. Figure l(a) illustrates the principle. The CT ratio is chosen so that the expected primary current range-the motor line current-results in secondary amperes between O and 5. Transformer accuracy and design determine whether such a circuit is usable for relaying, or just for monitoring actual line amperes for remote display.

However, the simple scheme of Figure l(a) may not suit all motor applications. The problem is that a motor draws high current during acceleration. To avoid "nuisance trips" during starting, the protective circuit must allow that starting current to flow uninterrupted. However, a low-level fault, caused by a partial ground or short-circuit within the motor winding, may lead to a catastrophic breakdown despite initial overcurrent that is below the normal starting value.

Within one phase of a three-phase winding, as in any single-phase circuit, a ground fault results in a difference in line current between the two sides of the circuit. To sense that, large motors are sometimes provided with differential relaying. As the name implies, the system involves CT's that monitor the difference in current between the two ends of a winding phase. Because that difference is always zero when no fault exists, the relay sensing CT output can be quite sensitive; even the smallest difference indicates trouble. Figure 1(b) illustrates that principle. Normal motor overload or starting, while increasing phase and line current, will not create any difference to produce a CT and relay response.
http://findarticles.com/p/articles/mi_qa3726/is_200504/ai_n1...