At the end of the course, the electrician (o going to be) must be able to
1.Define types of protections in electrical distribution
2.Identify physically any electrical device or material having protection functions,
within a switchgear or MCC
3.Select the appropriate electrical protection according to load
4.Differentiate the different types of fuses
5.Choose, determine, the required protection breakers and/or thermal relays
6.Explain the use and functions of any type of protection relays used in electrical
distribution
7.Differentiate isolator, switch, contactor, relay, breaker and associate them appropriately in a schematic diagram
8.Expose the different principles in earthing protection (RCD, PIM,…)
9. Identify and select the proper lightning protection in an electrical (and low power
domain) distribution
10.Use this document as a reference booklet, coming back to it when required, adding
notes, and…correcting it, if needed…
2. INTRODUCTION
2.1. THE ELECTRICAL PROTECTION ?
Subject to standardisation, in France with the C15-100 of UTE (Union Technique de
l’Electricité), last edition 05.02.2002. This standard clearly identify and separate protectionof personnel and protection of materials.
The electrical risk is at first physical: the human body accidentally in contact with voltage is conducting electrical current which can produce:
Internal or external burns
Muscular tetanization
Then, for the materials, the risks and consequences are mainly thermal: all electrical
cables, features, apparatus…etc, allow a rated (and maximum) current. Excess current
gives overheating (by Joule Law) which can cause deterioration and/or fire; short-circuit
can be classified as a “speedy” overheating.
2.2. PROTECTION OF PERSONNEL
Risk is to be in contact with an electrical conductor either by “direct contact” or “indirect
contact” through a defective insulation. Serious (and fatal) danger starts from a current of 500 mA during 10 ms through the human body. Intensity of current is related to skin surface and pressure in contact, health of victim, status of skin (callous or soft hands), wet or dry conditions,…etc.
Value of human body (ohmic) resistance is generally taken at (lowest) 2000 ohms. VLVS
(Very Low Voltage Safety – TBTS in French) maximum value is limited at 50 volts (in AC,
50 Hz). This gives a current through the body (in case of direct contact) of 50 / 2000 = 25
mA under the value of risk.
Protection of personnel in electrical distribution is performed by using the RCD (Residual Current Detector) or differential method; system detecting a faulty current towards the ground and making and associated switch or breaker to trip (threshold being 30 mA). As complementary protections, all mobile (portable) tools and equipment’s must be double insulation type, an equipotential ground wire has to be included in all distributions.
In this course, we shall see the RCD system but see as well the specific courses on this
subject: SE070 Ground and neutral systems and SE080 Electrical Safety
