We have investigated nonlinear electrical characteristics in low bandwidth manganites R1-xCaxMnO3 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-br 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. Enhancement of conductivity is proposed to arise from double exchange induced by local magnetic field.