According to IEC 60479, considered the most complete study on the effects of electric current in the human body, there are two different situations related to electric shock: the ones related to direct contact and the ones related to indirect contact. Brazilian standard NBR 5410 states that “direct contact refers to contact with live parts, this being parts which have voltage applied to them during operation” for instance if a person accidentally touches the pins of an electric plug while removing it from the outlet; or someone that touches the feeders on a distribution panel. The standard defines indirect contact as “contact with exposed conductive parts that are live under fault conditions, including contact by any thing being carried or worn by that person”.
The residual current circuit-breakers RDs are devices used to protect people and installations against direct and indirect contact. Protects against the effects of earth-leakage that may exist on electric circuits.

The sensitivity or operational residual current (In) is the first characteristic that establishes if a RD may be used to protect only against indirect contact or may be used to offer additional protection against direct contact.

The 30mA RD is considered of high sensitivity and may be used for both protections, assuring full protection to people.

The 300mA RD is considered of low sensitivity and is used to protect installations against direct contact or fire hazard (according to installation standards), limiting earth-leakage currents on places that process or storage flammable materials, such as paper, straw, pieces of wood, plastic, etc.

The RDs measure, at all times, the current flowing through the phase conductor and the current returning through the neutral conductor. The sum of these two currents should be equal, meaning that there is no leakage out of the system. Under fault conditions (insulation problem on one of the equipments in the circuit), these currents are unequal, which means that current is being lost through an alternative return circuit to earth (earth-leakage current). If a person gets in contact with a live part of the circuit, current will flow through one’s body which will also cause an imbalance that will be sensed by the RDs.