We have investigated non-linear electrical characteristics in low bandwidth manganites R<sub>1-x</sub>Ca<sub>x</sub>MnO<sub>3</sub> with R=Pr, Nd, Ho, Er and x=0.3-0.5. In all these materials we observe strong nonlinear I-V characteristic that is manifested in negative differential resistance (NDR) and breakdown voltages V<sub>br</sub> as low as few volts. These effects are accompanied by intense Joule heating that seems to be inseparable from the effect itself. We present different types of measurements with the aim to resolve the origin of the phenomena. Melting of insulating state is observed regardless of its origin - antiferromagnetic, charge ordered or paramagnetic. This nonlinearity is found to have microscopic origin, indicating that it is some sort of percolation effect. Hysteretic effects also indicate that the heating is not the only cause of NDR. Theory of phase separation gives a plausible explanation for these nonlinearities, although it appears to be diffcult to prove. Pulsed and relaxation measurements show that the effect is time-dependent, similarly to nonlinearities induced by magnetic field.