172 / p.4
c Or of common mode (between live conductors and frame or
LV system earthings are mainly concerned by common mode
earth), a fault current -said to be common mode or zero sequence
faults which mainly occur in loads and cables.
(MV)- then flows in the protective conductor (PE) and/or in the earth
1.3 Hazards linked to
An insulation fault, irrespective of its cause, presents hazards for:
ms
Time during which
insulation faults
c Human life
the human body is exposed
c Preservation of property
10 000
a
b
c1
c2 c3
c Availability of electrical power;
5 000
the above all depending on dependability
2 000
Electric Shock of persons
1 000
A person (or animal) subjected to an electrical voltage is
500
electrified. According to the gravity of the Electric Shock, this
1
2
3
4
200
person may experience:
100
c Discomfort
50
c A muscular contraction
20
c A burn
mA
10
c Cardiac arrest (this is Electrocution) (see Fig. 1 )
0.1 0.2
0.5 1
2
5 10 20
50 100 200 500 1000 2000 500010000
Since protection of persons against the dangerous effects of
Threshold = 30 mA
Current passing through
the human body
electric current takes priority, Electric Shock is thus the first
hazard to be considered.
Zone 1: Perception
c2: Likelyhood < 5 %
Zone 2: Considerable discomfort
c3: Likelyhood u 50 %
The current strength I -in value and time-, passing through the
Zone 3: Muscular contractions
human body (in particular the heart) is the dangerous aspect. In
Zone 4: Risk of ventricular fibrillation (cardiac arrest)
LV, the impedance value of the body (an important aspect of
Fig. 1 : Time/current zones of ac effects (15 Hz to 100 Hz) on persons as in
which is skin resistance) virtually changes only according to
IEC 60449-1
environment (dry and wet premises and damp premises).
In each case, a safety voltage (maximum acceptable contact
c Dry or humid premises and places: UL i 50 V
voltage for at least 5 s) has been set at 50 V. This voltage was
Presumed contact
former called U (IEC 60449 standard).
L
voltage (V)
< 50 50
75
90
120 150 220 280 350 500
IEC 60364 paragraph 413.1.1.1 (and NF C 15-100) state that if
Maximum breaking
AC
5
5
0.60 0.45 0.34 0.27 0.17 0.12 0.08 0.04
there is a risk of contact voltage Uc exceeding this 50 V voltage,
time of the protection
DC 5
5
5
5
5
1
0.40 0.30 0.20 0.10
device (s)
the application time of the fault voltage must be limited by the use
of protection devices (see Fig. 2 ).
c Wet premises and places: UL i 25 V
Fire
Presumed contact
This hazard, when it occurs, can have dramatic consequences for
voltage (V)
25
50
75
90
110 150 220 280
Maximum breaking
both persons and property. A large number of fires are caused by
AC
5
0.48 0.30 0.25 0.18 0.10 0.05 0.02
time of the protection
important and localised temperature rises or an electric arc
DC 5
5
2
0.80 0.50 0.25 0.06 0.02
device (s)
generated by an insulation fault. The hazard increases as the fault
Fig. 2 : Maximum time for maintenance of contact voltage as in standard
current rises, and also depends on the risk of fire or explosion
IEC 60364
occurring in the premises.
Cahier Technique Schneider Electric no. 172 / p.5
Unavailability of electrical power
system earthing downstream is not mastered. Consequently,
It is increasingly vital to master this hazard. In actual fact if the
some countries make this measure a requirement:
faulty part is automatically disconnected to eliminate the fault, the
v For sockets of rating i 32 A
result is:
v In some types of installations (temporary, worksite, etc.)
c A risk for persons, for example
c Indirect contact, protection and prevention measures
v Sudden absence of lighting
Contact of a person with accidentally energised metal frames is
v Placing out of operation of equipment required for safety
known as indirect contact
purposes
(see Fig. 3b ).
c An economic risk due to production loss. This risk must be
This accidental energising is the result of an insulation fault. A
mastered in particular in process industries, which are lengthy
fault current flows and creates a potential rise between the frame
and costly to restart
and the earth, thus causing a fault voltage to appear which is
dangerous if it exceeds voltage UL.
Moreover, if the fault current is high:
c Damage, in the installation or the loads, may be considerable
As regards this hazard, the installation standards (IEC 364 at
and increase repair costs and times
international level) have given official status to three system
earthings and defined the corresponding installation and
c Circulation of high fault currents in the common mode (between
protection rules.
network and earth) may also disturb sensitive equipment, in
particular if these are part of a "low current" system
The protection measures against indirect contacts are based on
geographically distributed with galvanic links
three basic principles:
v Earthing of the frames of loads and electrical equipment to
Finally, on de-energising, the occurrence of overvoltages and/or
prevent an insulation fault representing a risk equivalent of a
electromagnetic radiation phenomena may lead to malfunctioning
direct contact
or even damage of sensitive equipment.
Direct and indirect contacts
Before beginning to study the system earthings, a review of
a) Direct contact
Electric Shock by direct and indirect contacts will certainly be
ph
useful.
c Direct contact and protection measures
This is accidental contact of persons with a live conductor (phase
or neutral) or a normally live conductive element (see Fig. 3a )
Uc
In cases where the risk is very great, the common solution
consists in distributing electricity using a non-dangerous voltage,
i.e. less than or equal to safety voltage. This is
b) Indirect contact
safety by extra-low voltage (SELV or PELV).
3
In LV (230/400 V), protection measures consist in placing these
live parts out of reach or in insulating them by means of