6. Grounding, earth leakage and RCD 6.1 RCD
Electricity above 50V is dangerous therefore personal protection against electricity is needed in an AC circuit. This protection needs to prevent that metal housing around a device can become live. A dangerous situation can occur when a person touches the live housing. As soon as a person touches a “live” device, the person will complete the electrical circuit between the faulty device and earth. Current will flow through the person into earth and this can be lethal. Even a low current trough a human body can result in death. It is important to detect and disconnect when electricity is flowing into earth or potential via a person. Earth leakage detectors are incorporated into an electrical system. These are designed to disconnect quickly when a current flow to earth is detected in order to mitigate the harm caused by electric shocks.
Earth leakage detectors can be known under different names: • Residual current device (RCD) • Residual current circuit breaker (RCCB) • Ground fault circuit interrupter (GFCI) • Ground fault interrupter (GFI) • Appliance leakage current interrupter (ALCI) • Safety switch RCDs operate by measuring the current balance between two conductors. The device will open its contacts when it detects a difference in current between the phase conductor and the neutral conductor. The supply and return currents must sum to zero, otherwise there is a leakage of current to somewhere else, to earth or to another circuit. RCDs are designed to prevent electrocution by detecting this leakage current, which can be far smaller (typically 5– 30 mill amperes) than the currents needed to operate conventional circuit breakers or fuses (several Amperes). RCDs are intended to operate within 25–40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock. A safe system is a system where short-circuit and overload is prevented as well as preventing of earth leakage currents. Earth leakage detection can only take place in systems where the neutral conductor is connected to the earth conductor; like in a TN or TT system. Earth leakage detection is not possible in an IT network. Where to mount a RCD
A RCD must be mounted before the loads in an electrical installation. In reality this means that the RCDs have to be mounted before the installation is split up in groups. If an inverter or inverter/charger is used, the RCD should come after this, otherwise there will be no earth protection while the inverter is operational. Consumers that are only operational when connected to shore power will need their own RCD. Nuisance Tripping of RCDs In some installation RCD’s will trip prematurely. This can be caused by the following:
• The system has a double MEN link, this will cause the RCD to trip due to a potential difference in earth
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