Residual current devices in LV

Protecting against electrical hazards

Today, residual current devices (RCD) are recognised as the most effective means of protecting life and property against electrical hazards in low voltage systems. Their selection and optimum use require sound knowledge of the principles and rules governing electrical installations and in particular system earthing arrangements as well as existing technologies and their performance levels.

All these aspects are dealt with in this “Cahier Technique”, with in addition numerous answers provided by Schneider Electric technical and maintenance departments to frequently asked questions.

Introduction

Compared to other energy sources, electricity has many advantages, but also many risks. It is used on a daily basis by the general public and many accidents still occur, resulting in burns, fires and electrocution. Strict installation rules have been set up by international (IEC, CENELEC) and national (e.g. NFPA in the USA and UTE in France,) organisations.

Dependable protective devices have been designed by carefully analysing the risks and consequences of equipment failures or incorrect use. Among these devices, RCDs (residual current devices) are recognised by international standardisation organisations as an effective means to protect life and property.

This document will present the subject in three steps:

• a description of the risks related to electrical currents,
• an overview of the protection techniques employed to limit those risks,
• an in-depth presentation of how RCDs operate.

Electrisation of persons

A person subjected to an electrical voltage is electrised. Depending on the level of electrisation, the person may be subjected to different pathophysiological effects:

• disagreeable sensation,
• involuntary muscular contraction,
• burns,
• cardiac arrest (electrocution).

These effects depend on various factors, including the physiological characteristics of the person, the environment (e.g. wet or dry conditions) and the characteristics of the current flowing through the body. A person may be subjected to an electrical shock in two manners:

• direct contact, e.g. the person touches an energised, bare conductor,
• indirect contact, e.g. the person touches a metal part of an electrical machine or device with an insulation fault.

The dangerous aspect is the current (magnitude and duration) flowing through the human body and particularly near the heart.

AUTHOR: Schneider Electric expert | Jacques SCHONEK