Electrical safety issues
Cezary Worek, Łukasz Krzak
EMC 2013
Agenda
1.
What are electric shocks and how to avoid them.
2.
Electric shock severity factors.
3.
Safe current and voltage levels.
4.
First aid in case of an electric shock.
5.
Classification of electrical systems due to voltage levels.
6.
Appliance classes.
7.
Earthing arrangements in power grids.
8.
DC power networks.
2
Electric shocks
and how to avoid them
3
What are the main reasons for electric shocks to occur.
Main reasons people get electric shocks:
• lack of proper caution
• headlessness
• disregard to rules and regulations
• not knowing the rules and regulations
• lack of education (that’s why we’re here)
• mistakes
• lack of supervision
• malfunctions due to lack of proper conservation
4
Common causes of electric shocks.
An electrical shock may happed due to following events.
• Approx. 60% of all electric shocks come from operating voltages which means that
a person touches parts of an electrical circuit which are normally under voltage or
is close enough to get shocked.
• Approx. 40% of all electric shocks come from the fact that people touch
conductive parts which are not normally under voltage, but due to isolation
malfunction these parts become energized.
• Approx. 1% of all electric shocks come from stray
voltages, that is due to potential difference on
the ground surface caused by a current injected
to it.
Statistics show that approx. 80% of electric shocks
are caused by low voltage with 5% death rate, and
the other 20% by high voltage with 20% death rate.
5
What is an electric shock? What are it’s effects?
An electric shock is actually caused by the current flow through skin, muscles or hair
which causes changes and disturbances in how our body normally functions. These
changes and disturbances are mainly:
• disturbances in heart beat (in ex. ventricular fibrillation),
• disturbances in breathing,
• heat effects due to current flow,
• shock and the reaction to shock.
An electric shocks may have also indirect effects due to current flow, such as:
• electric arc burns,
• eye damage due to excessive light,
• ear damage due to excessive noise,
• mechanical damages due to in ex. falling.
6
What are the electric shock severity factors?
In the last 30 years there has been a lot of progress in research on the effects of
electric shock to human body. The conducted tests on animals and humans were
carefully analyzed by the International Electrotechnical Commision (IEC).
There are three main groups of factors that influence how severe is a shock to the
human body:
• electrical factors
• physiological factors
• external factors
7
Electrical factors.
Electrical factors include:
• Type of current: DC or AC – in case of AC currents what matters is also the
frequency.
• Current magnitude.
• Time of exposure to current flow.
• The path of current flowing through the body.
• Individual body factors (in example skin conductivity).
The most important aspects are current magnitude and exposure time. The current
magnitudes are roughly divided into 3 ranges:
Current magnitude
1 mA
Physiological effect
Threshold of feeling, tingling sensation
10-20 mA
"Can't let go!" current - onset of sustained
muscular contraction.
30-400 mA
Ventricular fibrillation, fatal if continued.
The IEC 60479-1 publication updated in 2005 defines four zones of currentmagnitude / time-duration, in each of which the pathophysiological effects are
described.
8
Electrical factors.
9
10
What are the other factors?
Physiological factors:
• How is the body developed (body mass and geometry)
• Emotional and psychological state
• Disease conditions such as coronary heart disease, asthma, tuberculosis,
diabetes, alcoholism etc.
Externals factors:
• Factors that cause human resistance to drop (humidity, temperature)
• Factors that make it easier for current to flow to the ground (bare ground places,
conductive floor)
11
First aid in case of an
electric shock
12
Important qualities during first aid.
You should:
ACT FAST
BE DETERMINED
BE CALM
It is especially important to act fast since the chances of saving the other people’s life
who is unconscious and not breathing are dropping rapidly every minute.
If the cardiopulmonary resuscitation (CPR) is undertaken after 1 minute after loosing
breath the chances of saving life are about 95%. After 5 minutes the chances drop to
25% and after 8 minutes it’s only 5%.
13
First aid in case of an electric shock.
FIRST MAKE SURE YOU ARE SAFE !
DO NOT touch the casualty with your unprotected hands. You don’t want to be the
casualty too.
1.
2.
3.
4.
Break the contact by switching off the current, removing the plug or wrenching
the cable free; or if this is not possible
stand on a dry insulating material, such as a wooden pallet or plastic mat, then
use a dry wooden or plastic implement to free the casualty from contact with
the electrical source; or
wear rubber or plastic insulating gloves to pull the casualty free; or
if dry rope is available, without touching the casualty, loop it around the feet or
under the arms and pull the casualty free.
Before you continue make sure you and the casualty are safe.
14
First aid in case of an electric shock.
CHECK IF THE CASUALTY IS RESPONSIVE
Unresponsive ?
Shout for help
Open airway
15
First aid in case of an electric shock.
CHECK IF THE CASUALTY IS BREATHING
Not breathing
normally ?
Order someone
to call 999 or do
it yourself
Breathing OK ?
Place in the
recovery position
Begin CPR
Get help
Check for
continuous
breathing
16
CPR (cardiopulmonary resuscitation)
30 chest compressions
2 rescue breaths
PROCEED UNTIL THE AMBULANCE ARRIVES!
17
First aid in case of an electric shock.
18
Safety issues in the electrical grid
19
IEC voltage ranges
The International Electrotechnical Commission (IEC) recognizes these 3 voltage
ranges.
IEC voltage range
AC
DC
Defining risk
High voltage
> 1000Vrms
> 1500V
Electrical arcing
Low voltage
50-1000Vrms
120-1500V
Electrical shock
Extra low voltage
< 50Vrms
< 120 V
Low risk
20
Extra low voltage range
The International Electrotechnical Commission (IEC) recognizes these 3 voltage
ranges.
IEC voltage range
AC
DC
Defining risk
High voltage
> 1000Vrms
> 1500V
Electrical arcing
Low voltage
50-1000Vrms
120-1500V
Electrical shock
Extra low voltage
< 50Vrms
< 120 V
Low risk
21
Voltage safety levels
One of the key elements to lower the risk of an electric shock is to lower the
operating voltage .
Generally, under normal conditions the human body has a resistance of about
1kOhm. The dangerous current magnitude is somewhere near 50mA for AC and
120mA for DC. This in turn gives that a voltage that is safe to touch is 50VAC or
120VDC. However these levels are applicable in normal conditions (dry air). When
humidity increases these safety levels are divided by two. In extremely wet
conditions (such as swimming pools or sauna) these levels are further divided.
Conditions
Safe voltage levels
Normal, dry conditions
50VAC
120VDC
Special conditions
25VAC
60VDC
Extreme conditions
12VAC
30VDC
These voltage levels are considered to have low risk of electrical shock and are part
of a so called extra low voltage range defined by IEC.
22
Special conditions
The common rule is that the normative voltage levels apply to normal conditions. In
cases when environmental or other factors increase the risk of electric shock
additional precautions are introduced.
These precautions include:
• safety zones with limited electrical equipment
• additional functional bonding conductor
• lower voltage levels up to 25 or 12 VAC and 60 or 30VDC
• residual-current devices with threshold level not higher than 30mA
• supervision of the isolation state in IT power grids
23
Basic protection
The basic protection is designed to prevent electric shock from operating voltages.
The purpose is actually to prevent touching of energized elements.
According to IEC 60364-41 the means of achieving this goal are:
• isolation of operating elements
• providing barriers (compartments)and shields (enclosures) min IP2X*
• providing fences
• putting equipment out of the reach
• using sensitive residual-current devices with threshold level not higher than 30mA
as complementary protection
* - The IP Code, Ingress Protection Rating classifies and rates the degree of
protection provided against the intrusion of solid objects (including body parts like
hands and fingers), dust, accidental contact, and water in mechanical casings and
with electrical enclosures. P2X is frequently used on electrical items to specify the
item must prevent finger access to live terminals in ex. plug sockets are IP2X
24
Additional protection
The additional protection is designed to minimize effects of electric shock. The actual
purpose is not to let human body to interact with elements under voltage higher
than considered safe (in ex. 50VAC under normal conditions) for a longer time.
This can be achieved by:
• using automatic shutdown of power supplies in TN, TT and IT power grids
• using safety class II devices or devices with equivalent degree of protection
• electrical separation
• isolation of work station
• ungrounded equipotential bonding
The following devices are considered to deliver safe voltage:
• isolation transformers and isolated power converters
• batteries and engine generators
• electronic devices
25
Appliance classes.
In the electrical appliance manufacturing industry, the following IEC protection
classes are used to differentiate between the protective-earth connection
requirements of devices: 0, I, II and III.
Class 0. These appliances have no protective-earth connection and feature only a
single level of insulation and were intended for use in dry areas. A single fault could
cause an electric shock or other dangerous occurrence.
Class I. These appliances must have their chassis connected to electrical
earth by an earth conductor (in Europe marked with green and yellow
colors). A fault in the appliance which causes a live conductor to contact
the casing will cause a current to flow in the earth conductor. This current
should trip either an overcurrent device (fuse or circuit breaker) or a
residual-current device (RCD) which will cut off the supply of electricity to
the appliance.
26
Appliance classes.
Class II. Also called double insulated electrical appliance is one which has
been designed in such a way that it does not require a safety connection to
electrical earth (ground).
The basic requirement is that no single failure can result in dangerous
voltage becoming exposed so that it might cause an electric shock and that
this is achieved without relying on an earthed metal casing. This is usually
achieved at least in part by having two layers of insulating material
surrounding live parts or by using reinforced insulation.
In Europe, a double insulated appliance must be labeled Class II, double
insulated, or bear the double insulation symbol (a square inside another
square).
Class III. An appliance that is designed to be supplied from a
separated/safety extra-low voltage (SELV) power source. The voltage from
a SELV supply is low enough that under normal conditions a person can
safely come into contact with it without risk of electrical shock. The extra
safety features built into Class I and Class II appliances are therefore not
required. For medical devices, compliance with Class III is not considered
sufficient protection, and further more-stringent regulations apply to such
equipment.
27
Extra low voltage (ELV) circuits
The International Electrotechnical Commission and its member organizations define
an ELV circuit as one in which the electrical potential of any conductor against earth
(ground) is not more than either 25 volts RMS (35 volts peak) for alternating current,
or ripple-free 60 volts for direct current under dry conditions. Lower numbers apply
in wet conditions, or when large contact areas are exposed to contact with the
human body.
The IEC defines three types of extra-low-voltage systems:
• FELV,
• PELV,
• SELV
which are distinguished by their successively more restrictive safety properties.
NOTE: Lower values noted here come from some confusion in IEC standards.
28
Separated extra low voltage (SELV) circuits
IEC defines a SELV system as "an electrical system in which the voltage cannot exceed
ELV under normal conditions, and under single-fault conditions, including earth faults
in other circuits".
A SELV circuit must have:
• protective-separation (i.e., double insulation, reinforced insulation or protective
screening) from all circuits other than SELV and PELV (i.e., all circuits that might
carry higher voltages)
• simple separation from other SELV systems, from PELV systems and from earth
(ground).
The safety of a SELV circuit is provided by
• the extra-low voltage
• the low risk of accidental contact with a higher voltage;
• the lack of a return path through earth (ground) that electric current could take in
case of contact with a human body.
The design of a SELV circuit typically involves an isolating transformer, guaranteed
minimum distances between conductors and electrical insulation barriers. The
electrical connectors of SELV circuits should be designed such that they do not mate
with connectors commonly used for non-SELV circuits.
29
SELV circuit
SELV (Separated Extra-Low Voltage) circuit is a low voltage circuit without grounding,
which is powered by a voltage that is safe in a long term. This allows robust
separation from other circuits.
Such connections deliveres both basic and additional protection.
Isolation transformer
class III device
30
Protected extra low voltage (PELV) circuits
IEC 61140 defines a PELV system as "an electrical system in which the voltage cannot
exceed ELV under normal conditions, and under single-fault conditions, except earth
faults in other circuits".
A PELV circuit only requires protective-separation from all circuits other than SELV
and PELV (i.e., all circuits that might carry higher voltages), but it may have
connections to other PELV systems and earth (ground).
In contrast to a SELV circuit, a PELV circuit can have a protective earth (ground)
connection. A PELV circuit, just as with SELV, requires a design that guarantees a low
risk of accidental contact with a higher voltage. For a transformer, this can mean that
the primary and secondary windings must be separated by an extra insulation barrier,
or by a conductive shield with a protective earth connection.
31
PELV circuit
PELV (Protected Extra-Low Voltage) circuit is a low voltage circuit with grounding,
which is powered by a voltage that is safe in a long term and is well separated from
other circuits. The main difference between SELV is that in PELV one line has to be
grounded. In addition all conductive elements of powered devices that can be
touched should also be grounded.
Such connections also deliveres both basic and additional protection.
Isolation transformer
class III device
32
Functional extra low voltage (FELV) circuits
The term functional extra-low voltage (FELV) describes any other extra-low-voltage
circuit that does not fulfill the requirements for an SELV or PELV circuit.
Although the FELV part of a circuit uses an extra-low voltage, it is not adequately
protected from accidental contact with higher voltages in other parts of the circuit.
Therefore the protection requirements for the higher voltage have to be applied to
the entire circuit.
Examples for FELV circuits include those that generate an extra low voltage through a
semiconductor device or a potentiometer.
33
FELV circuit
FELV (Functional Extra-Low Voltage) circuit is a low voltage circuit that does not
guarantee well separation from other circuits and an extra low voltage is used
because of functional reasons and not safety precautions like in SELV. This circuit can
be powered from any device galvanically separated from the power grid.
class I device
Transformer
34
Low voltage range
The International Electrotechnical Commission (IEC) recognizes these 3 voltage
ranges.
IEC voltage range
AC
DC
Defining risk
High voltage
> 1000Vrms
> 1500V
Electrical arcing
Low voltage
50-1000Vrms
120-1500V
Electrical shock
Extra low voltage
< 50Vrms
< 120 V
Low risk
The low voltage range denotes voltages above extra-low voltage range and up to
1000V rms for AC with frequency up to 60Hz and up to 1500V for DC.
Full information about power grids are published in IEC 60364-3:2008 — Electrical
installations of buildings. Part 3: Assessment of general characteristics.
35
Three phase electric power.
How to transfer electrical energy across long distances in a safe and economical way?
Let’s try a simple 2-wire (single phase) network:
In this example the maximum operating voltage is 120VAC.
However we need a copper wire that can carry 250A of current.
In this example such wire must have about 12mm diameter and will weight 760kg per km
per wire giving more than 1.5 tons of copper per km of installation.
36
Three phase electric power.
What happens if we add another wire ?
The operating voltage increases to 240VAC.
The currents drop to 125A which gives us copper wire with 6,5mm diameter which in turn
wights about 300kg per km per wire giving about 900kg for one km of installation.
37
Three phase electric power.
In a 3-phase network the maximum operating voltage drops to 208VAC. The current
drops to about 84A. We need a copper wire with diameter < 5mm which gives 186kg per
km per wire, wich in turn leads to 744kg of copper per one km of installation.
38
Skin effect.
Skin effect is the tendency of an
alternating electric current (AC) to
become distributed within a
conductor such that the current
density is largest near the surface
of the conductor, and decreases
with greater depths in the
conductor. The electric current
flows mainly at the "skin" of the
conductor, between the outer
surface and a level called the skin
depth.
39
Three phase electric power.
Advantages of a 3-phase power systems:
• Good compromise between voltage and
current magnitudes.
• The phase currents tend to cancel out one
another, summing to zero in the case of a
linear balanced load. This makes it possible
to reduce the size of the neutral conductor;
all the phase conductors carry the same
current and so can be the same size, for a
balanced load.
• Power transfer into a linear balanced load
is constant, which helps to reduce
generator and motor vibrations
• Three-phase systems can produce a
magnetic field that rotates in a specified
direction, which simplifies the design of
electric motors.
40
Three phase electric power. 230/400V (Europe)
41
Three phase electric power – load configurations.
There are two basic three phase configurations: delta
(triangle) and wye(star). Either type can be wired for three
or four wires. The fourth wire is called a neutral. The '3wire' and '4-wire' designations do not count the ground
wire used on many transmission lines which is solely for
fault protection and does not deliver power.
High-leg delta
connection
42
Earthing arrangements.
There are several types of earthing arrangements. International standard IEC 60364
distinguishes three families of earthing arrangements, using the two-letter codes:
TN, TT, and IT.
The first letter indicates the connection between earth and the power-supply
equipment (generator or transformer):
T - direct connection of a point with earth (Latin: terra);
I - no point is connected with earth (isolation), except perhaps via a high impedance.
The second letter indicates the connection between earth and the electrical device
being supplied:
T - direct connection of a point with earth
N - direct connection to neutral at the origin of installation, which is connected to the
earth
Optional following letters denote whether the PE and N are separated:
C – PE and N are combined in a single wire
S – PE and N are separated
43
Common symbols in power networks.
color codes used in European Union (EU) defined in IEC 60446
These are common symbols used in electrical power network
schematics.
L1, L2, L3 – phase wires
N – neutral wire, is a circuit conductor that carries current in
normal operation and is connected to the neutral point of the
electrical network.
PE – protective earth, (known as an equipment grounding conductor in the US) is a low
impedance connection to an earth electrode or an equipotential bonding wire. To avoid possible
voltage drop no current is allowed to flow in this conductor under normal circumstances, but
fault currents will usually trip or blow the fuse or circuit breaker protecting the circuit.
PEN – protective earth and neutral, is a conductor that acts both as protective earth and neutral.
44
Common symbols in power networks.
color codes used across different countries
L2
L1
L3
Neutral
Ground/
protective earth
Green/yellow striped
(green on very old
installations)
White (or black)1(prev.
Dark blue (or grey)1
yellow)
Black (or
Red
Black
Blue
White or Grey
Green or bare copper
Canada (isolated
installations)
Orange
Brown
Yellow
White
Green
European Union
Black
Brown
Grey
Blue
Green/yellow striped
Older European
Black or brown
Black or brown
Black or brown
Blue
Green/yellow striped
UK
Red
Yellow
Blue
Black
Green/yellow striped
Republic of
India and Pakistan
Red
Yellow
Blue
Black
Green
Russia, Ukraine,
Kazakhstan, China
Yellow
Green
Red
Light blue
Green/yellow striped
Norway
Black
White/Grey
Brown
Blue
Yellow/green striped
Australia and New
Zealand
Canada
Red (or
brown)1
blue)1
United States
Black
Red
Blue
White, or grey
Green, green/yellow
striped,7 or a bare
copper wire
United
States (alternative
practice)5
Brown
Orange (delta), violet
(wye)
Yellow
Grey, or white
Green
45
TN networks.
picture presents a TN-S network
• Neutral point of the voltage source
should be grounded.
• All touchable conductive elements
that in normal operation are not
under voltage should be grounded
through PE or PEN wire.
• PE and PEN wires should be
connected to the grounding electrode
• It is advised to ground the point at
which the the PE is inserted into the
building.
• It is advised that the point at which
the PEN wire is separated to PE and N
was grounded through artificial or
natural grounding electrode.
• Every building should have main
equipotential bonding.
• Additional bondings of touchable
parts should present in places with
greater risk of electrick shock.
46
TN-S networks.
TN-S earthing arrangement allows to use the most effective protection among all TN
networks.
In a TN-S network, N and PE are separated along the whole installation and are connected
to the main grounding electrode. This means that during normal operation the current
flows only through phase and neutral wires.
The advantage is that PE can be connected with many equipotential bondings along its
way. In each place it is also possible to use residual-current devices (RCD)
47
Short circuit in a TN-S network.
What happens if
somebody touches the
phase wire.
The dashed line shows the short circuit current loop when (due to malfunction) one of
the phase wires is connected to PE (in ex. enclosure).
48
Additional protection in a TN-S network.
What happens if somebody
touches the phase wire?
The current will flow through the human body closing via earth. This current may
have significant and dangerous magnitude, but usually will not trip the overcurrent
protection which must be rated at even higher current for functional reasons.
49
Residual-current devices.
One of the most effective protection devices used in electrical networks are residual
current devices (RCDs). Depending on the type of a RCD it can serve several purposes:
• protection from electrical shock caused by indirect touching of energized elements
• protection from electrical shock caused by direct touching of energized elements,
when nominal trip current is below 30mA
• as a mean of automatic power shutdown
• protection
from
fire
caused
by
currents
flowing
to
the
ground
when nominal trip current is below 500mA
50
RCD. Principle of operation.
RCDs operate by measuring the current balance
between two conductors using a differential current
transformer. This measures the difference between
the current flowing through the live conductor and
that returning through the neutral conductor. If these
do not sum to zero, there is a leakage of current to
somewhere else (to earth/ground, or to another
circuit), and the device will open its contacts.
Residual current detection is complementary to overcurrent detection. Residual current detection cannot
provide protection for overload or short-circuit
currents, except for the special case of a short circuit
from live to ground (not live to neutral).
51
RCD. Principle of operation.
Human resistance
Site resistance through
ground
When a human body touches a wire, some I current will flow potentially causing an
electric shock. This current should trip over the RCD causing it to shut off the voltage
from the protected circuit in a very short time (typically < 0.2s)
52
RCD. Principle of operation.
Here’s how it may actually look like in a single phase RCD.
53
RCD. Principle of operation.
Here’s how it may actually look like in a three phase RCD.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Transformer core
Sensing winding
Trip winding
Connector
Magnet/Electromagnet
Spring
Switch lock
On/Off switch
Test switch
Current
limiting
resistor
(for testing)
54
RCD. Basic parameters.
• In – nominal current that is allowed to flow through the RCD in a normal operation
(current rating)
• IΔn – nominal trip current that is the value of the current imbalance that will cause
the RCD to activate
• t – switch off time
Additional parameters may include: temperature range, max. short-circuit power that
can be dissipated, frequency characteristics.
Due to the value of trip current 3 types of RCDs may by distinguished:
• IΔn ≤ 30 mA – these devices are designed to protect people from getting a
dangerous electrical shock
• IΔn ≤ 500 mA – these devices mainly protect sites from flames causing by currents
flowing through grounded elements (these should trip over in 0,2s or less)
• Some RCDs are designed for IΔn of up to 10A, but these protect mainly the
equipment – usually in high power applications.
55
TN-C networks.
In a TN-C earthing arrangement the PE and N are combined in a single wire. This is a
major drawback. In a single phase circuit this wire carries full operating current. In 3phase circuits this wire may also conduct current due to phase asymmetry.
56
Protection in TN-C networks.
The schematic above shows the principle of automatic power shutdown (so called
zeroing) in case of malfunction in a TN-C network.
A TN-C network should be protected by overcurrent protective devices (fuses, circuit
breakers).
57
TN-C networks.
The big disadvantage of TN-C is when due to malfunction PEN wire is broken, in class I
devices dangerous voltage becomes present at the device enclosure.
In single phase devices this voltage may be close to the nominal network voltage.
In 3-phase devices this voltage may be close to the nominal network phase-to-phase
voltage, depending on the load of particular phases at the time of malfunction.
The risk of electric shock or device damage is even higher, because the overcurrent
protection devices will not be activated until their trip current is reached.
58
TN-C networks.
Another disadvantage of TN-C arrangement is that due to asymmetry of loads some
voltage will be generated between PEN and ground in a place of device connection. The
actual value will vary depending on the level of asymmetry.
In example in light sources powered by a TN-C network part of the load current flows
through the PEN wire and another part flows through the conductive truss
(construction). When PEN is broken due to malfunction the light source will still work
and the whole current will flow through the construction.
59
TN-C networks.
It is also worth to note that in TN-C arrangements we cannot use residual-current
protective devices.
The PEN wire and all elements attached to it do not guarantee good isolation from earth.
It seams that the only advantage of the TN-C arrangement is cost of installation (no
additional wire). The only question is whether we should look for savings in the area of
our safety.
60
TN-C-S networks.
TN-C-S are now most often used in new or modernized buildings. This arrangement can
be regarded as a TN-C network that powers a TN-S network. It is considered to be more
safe than TN-C arrangement but carries some of it’s flaws.
In a point of PEN separation it is advised to have additional grounding that is compliant
to regulations. However this grounding does not provide sufficient protection in case
when PEN wire is broken.
This arrangement allows to use residual-current devices as additional protective
measures.
61
Protection in TN-C-S networks.
The schematic above shows how fuses and RCDs are used in TN-C-S network for
protection.
The PE wire must be a solid connection. No circuit breakers, fuses, switches or
connectors are allowed.
62
How fast the overcurrent protection should activate?
According to regulations in order to determine the shutdown current we have to use the
characteristics provided by the supplier of the protection device, that show both current
and time.
Nominal grid
voltage
Normal conditions
UL < 50VAC and 120VDC
Special conditions
UL < 25VAC and 60VDC
120V
0,8 s
0,35 s
230V
0,4 s
0,2 s
277V
0,4 s
0,2 s
400V
0,2 s
0,05 s
480V
0,1 s
0,05 s
580V
0,1 s
0,02 s
At substation level the activation time may reach even 5s.
63
TT networks.
In TT arrangements the neutral point is grounded in the transformer. Each device has it’s
own grounding, usually provided in the exact place of installation. All touchable
conductive elements of these devices are locally grounded, separately for each device.
These arrangements are often used for high power devices or high in-rush currents,
when requirements for a regular PE wire are not economically effective.
The TT arrangements must be supervised. The state of grounding must be checked
periodically.
64
TT networks.
In
arrangements
with
grounded middle star point
the current that flows when
due to malfunction one of the
phase wires is shorted to the
grounded enclosure should
force the protective devices
to:
• either shut down the
power, or
• cause the voltage to drop
to a safe level (in ex.
50VAC)
Safe level
65
Protection in TT networks.
As a protection measure in TT
networks,
the
following
devices can be used:
• overcurrent
protection
devices (fuses, circuit
breakers)
• residual current devices
• earth
leakage
circuit
breakers
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IT networks.
In IT arrangements the neutral point of the transformer is isolated. The PE points of each
load are connected directly and separately to the ground usually in the exact place of
installation. All touchable conductive elements of these devices are locally grounded,
separately for each device.
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IT networks.
Spark gap
In addition, in IT arrangements the neutral point of the transformer should be
protected by a spark gap or high impedance, which main purpose is no to let the
voltage between N and PE to go too high.
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IT networks.
In case of an insulation break
in a protected device some
current will flow due to line
capacitance to ground.
This current usually will not
trip
the
overcurrent
protection, but will cause the
operating voltage to drop
below 50VAC.
Safe level
However there is still the
possibility of double isolation
failure which may lead to a
current loop through ground.
This is why in IT arrangements
it is obligatory to control the
state of insulation.
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IT networks. Single insulation failure example.
Before the malfunction. The capacitive
currents depend on the actual capacity
between the lines and ground. This in turn
depends on the length of the wires.
Insulation break. The short circuit current to
ground does not reach high magnitude. The
voltages between „healthy” phases and
ground increses 3 times. The Iz current may be
too low to trip the overcurrent protection.
This is why additional protection is needed:
• grounding of all exposed conductive parts
• controling the earthing resistance to make
sure that the voltage between PE and
ground is not exceeding safety levels
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IT networks. Double insulation failure example.
The overcurrent protection will trip
when there is a double insulation
failure.
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TODO: DC power networks
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