Varied high-temperature and low-frequency dielectric characteristics are commonly detected in deteriorated engineering dielectrics

including increased dielectric permittivity

loss factor

etc. Consequently

a clear insight into those dielectric responses is of great significance to further degradation investigation and condition assessment. However

the relaxations are usually coupled with DC conductance so that they are easily covered by conductance component in low-frequency range

thus further studies on them are hardly to be carried out. What is worse

DC conductance is sometimes mistaken for a relaxation process in some dielectric spectra due to similar temperature and frequency dependence. Therefore

we proposed an extended derivative spectroscopy which only remains relaxations but excludes DC conductance. Taking three typical engineering dielectrics

such as poxy

oil-impregnated paper

and ZnO ceramics

as examples

we successfully characterized new low-frequency relaxations from the covered DC conductance via the improved spectra. Moreover

we discussed the physical mechanism of the relaxations in the three different dielectrics respectively. The results provide a potential approach for further manipulating and assessing properties of engineering dielectrics.