Reverse Polarity Protection
This is the protection method that prevents damage to the circuit if the positive and negative terminals are connected the wrong way round. It is built with a series diode, a reverse-connected diode together with a fuse, or MOSFET-based ideal diode circuits; each method has a different cost in terms of voltage loss and current limit.
The simplest method is to put a diode in series with the supply line. On a reversed connection the diode does not conduct and the circuit is protected; on the other hand, a loss of typically 0,3–0,7 V occurs across it in normal operation. In a 12 V system this loss is significant and generates serious heat at high current.
In the second method the diode is connected in parallel and reversed across the supply line, with a fuse ahead of it. On a correct connection the diode does nothing, while on a reversed connection it short-circuits and blows the fuse. There is no loss, but protection happens by the fuse blowing.
The third and modern method is a MOSFET placed so that its body diode faces the reverse direction. Since its conduction resistance is in the milliohm range, the loss is very low and it suits high current.
In RV installations, reversed connection happens most often during battery replacement and when dismantling the wiring. Even if electronic boards such as the levelling control unit have protection, measuring the polarity before connecting and labelling the terminals according to the colour codes is the most reliable precaution.
Context in which this term is used: Guide to automatic RV levelling