D R I V E S
&
S W I T C H G E A R
his article discusses the application of fuse
technology in low voltage distribution and motor
control circuits. The effects of short circuit current on
electrical equipment can be dramatic if no appropriate
protection is provided.
T
Is fuse technology in electrical
installations alive or blown?
Some of the effects include:
Melting of conductors or busbars;
Vaporization of metal;
Ionization of gases;
Arcing, fire and explosion;
Insulation damage.
Apart from being hazardous to
personnel, significant economic losses
can result from downtime and the
repairs required to restore damaged
equipment. These downtimes can
be very significant depending on the
installation concerned. In some cases
they can cause production stops for a
long period.
If we look at the thermal stress in
switchgear, we can say that this is not a
major issue due to the short period of
a short circuit. This short period would
not probably increase the temperature
of bus-bar systems above 200 °C, which
does not create much damage on the
busbar system. However the dynamic
stresses are far more important. This
stress causes vibrations, magnetic
effects, etc. which stretch the busbars
and supports in different directions and
could potentially cause another massive
short of the busbar systems. This is why
the highest short circuit peak current
needs to be considered when calculating
effects of dynamic stress.
According to IEC 60364, each circuit
needs to be provided with over-current
protection (over-load and short circuit),
which breaks the over-current of the
circuit before it causes danger due to
by Hakan Bergqvist, ABB Oy, Finland
temperature and mechanical
effects. These protective devices
can be of the following types:
Protection against both
overload and short-circuits
Only short circuit protection
Only overload protection
Complete protection by fuse
technology
The current-limiting fuse provides
complete protection against the
effects of over-currents (over
5. Indicator wire
loads and short circuits), by 1. Blade contact
protecting both electric circuits 2. Fuse-element
6. M-effect material
and their components. The fuse
3. Fuse body
7. Quartz sand
technology offers a combination
8. Indicator
of exceptional features, such 4. Endplate (with gripping lug)
as high breaking capacity: a
Fig. 1: Typical LV fuse link.
fuse has a breaking capacity of
minimum 50 kA, and normally the
industrial fuses available on the market
not consist of any moving parts to wear
today have a breaking capacity >100
out or become contaminated by dust, oil
kA. Thanks to this high breaking capacity
or corrosion. Fuse replacement ensures
there is no need for complex short-circuit
protection is restored to its original state
calculations to design the electrical
of integrity. This increases the reliability
system. Also, the breaking capacity
of the protection for the electrical
allows an easy and inexpensive system
expansion involving increased fault
installation.
currents in the electrical installation.
Fuse technology is very compact
in design, which offers low cost
Unlike other short-circuit protective
over- current protection at high
devices (SCPD), fuses cannot be reset,
short-circuit levels. This can easily
thus forcing the user to identify and
be seen in motor protection (type
correct the over-current condition before
2 coordination) according to
re-energizing the circuit. The fuse does
May 2005 - Vector - Page 27
Fuse type
Rated
voltage
Maximum operational
voltage
gG, gM, aM,
230V
253V
aR, gR, gS
400V
500V
690V
Fuse type
Application
Breaking range
Gg
General purpose mainly for conductor protection
Full range
gL, gF, gl, gll
Former type of fuse for conductor protection (replaced by gG type)
Full range
440V
aM
Motor circuit protection
Full range
550V
aM
Short-circiut protection of motor circuits
Partial range (back up)
725V
aR
Semiconductor protection
Partial range (back up)
gR, gS
Full range
Table 1.
Table 2.
IEC 60947-4-1. By limiting the shortcircuit energy and peak currents to
extremely low levels, other equipment
can be used to its maximum rating,
and therefore the complete switchgear
will be more compact and cost
effective. To this can be added that the
fuse is completely safe in operation.
There is no emission of gas, flames,
arcs or other materials when the
fuse is clearing the highest levels of
short-circuit currents. In addition, the
speed of operation at high short-circuit
currents significantly limits the arc flash
hazard at the fault location. This will
also reduce the total space necessary
for the equipment.
Fuse-links are standardised according
to IEC 60269, which ensures availability
of replacements with standardized
characteristics throughout the world.
Standardized fuse characteristics and a
high degree of current limitation ensure
effective coordination between fuses
and other devices.
Design and different fuse types
The design of typical low voltage fuselinks for industrial application is shown in
Fig.1 and consists of different elements.
The main elements are the internal fuse
element, the blade contacts and the fuse
body. Such fuse-links are commonly called
current-limiting or high breaking capacity
fuse-links.
The most common fuses are DIN, BS,
NFC and UL. DIN types are widely
used in Europe and German influenced
countries. The BS types are used widely
in UK, Australia and other UK influenced
countries. The NFC type of fuse is used in
France & North Africa, due to its French
origin. UL types of fuses are used in
North America, but can also be found in
other parts of the world where there are
requirements for fuses according to UL
standards.
Fuse markings
All fuses are marked with two letters
that describe the characteristics of the
fuse. Fuses for both overload and short
circuit protection or “Full range” means
that the fuse can break any current able
to melt the fuse-element up to the rated
breaking capacity. These have the first
letter “g”. Full range fuses can be used
as stand-alone protection devices.
Fuses for only short circuit protection
or partial range, or back-up fuses,
are designed to interrupt short-circuit
currents only. These have the first letter
“a”. They are generally used to increase
the breaking capacity of other overcurrent protection devices. The second
letter expresses the main application
for the fuse: “G” for cable protection,
“M” for motor protection, and “R” for
semiconductor protection.
Fuse operation
total operation phase of the fuse.
The rapid current limitation minimises
the stresses caused by the short-circuit
and therefore there is no need for over
dimensioning of other devices e.g. in
motor starters.
Fuse selection
Fuse selection needs to consider
the equipment to be protected and
the power supply that has to be
interrupted. Fuse selection for a specific
application involves consideration of
the time-current characteristics and
the breaking range. The time-current
characteristics determine the field of
application, while the breaking range
indicates whether fuses are to be used
together with additional over-current
protection devices:
g - Both overload and short
circuit protection
“Full range” means that the fuse
can break any current able to
melt the fuse-element up to the
rated breaking capacity. Full range
fuses can be used as stand-alone
protection devices.
a - Only short circuit protection
“Partial range”, or back-up fuses,
are designed to interrupt short-circuit
currents only. They are generally used
to increase the breaking capacity
of other over- current protection
devices.
The maximum operational voltage of
the fuses is presented in Table 1:
The intended applications for the fuses
are presented in Table 2:
During a short-circuit, the restrictions
in the fuse element melt simultaneously
forming a series of arcs equal to the
number of restrictions in the fuse
element. The resulting arc voltage
ensures rapid current limitation and
forces the fault current to zero. The
fuse operation occurs in two stages.
The first is called the pre-arcing stage.
This is the heating of the restrictions to
the melting point and vaporization of the
material. The second stage is the arcing
stage, where the arc begins and is then
extinguished by the filler (usually quartz
Fig. 2: The co-ordination between the
sand). Both stages together make up the
different devices in the motor starter.
May 2005 - Vector - Page 28
Fuse selection for cable
protection
The rated current In of the fuse-link is
selected to:
IB ≤ In ≤Iz
Fuse-links of type gG are able to break
over-currents in the conductors before
such currents can cause a temperature
rise detrimental to insulation. The fuselink selection can be easily made, taking
the following steps:
The maximum operational voltage of
the fuse-link is selected to be greater
or equal to the maximum system
voltage.
The operational current IB of the
circuit is calculated.
T h e c o n t i n u o u s c u r r e n tcarrying capacity of the
conductor I z is selected from
IEC 60364-5-52.
Fuse selection for motor
protection
According to the standard IEC 947-41, the co-ordination classes for motor
protection are:
Type 1: where the contactor or starter
should not cause harm to personnel,
and after the short circuit, a repair or
change of the components must be
done to achieve full integrity again.
Type 2: where the contactor or starter
should not cause harm for personnel
or the equipment, and after the short
circuit, full integrity of components
should be available again. Possible
contact welding should be considered
and manufacturer must inform
necessary maintenance procedure.
This means that Type 1 co-ordination
allows the contactor and thermal
overload relay to be damaged by the
short circuit. It is the responsibility of
the end-user to inspect the starter after
the short, and to replace damaged
components. If inspection is not done,
the circuit is potentially damaged and
without overload protection. This has
the potential to destroy the motor.
The Type 2 co-ordination guarantees
functionality of starter components even
after the short circuit. This means that
the overload protection is functional
after the short circuit and will protect
the motor from overload.
To assure the co-ordination between
the different components in the motor
starter, each manufacturer publishes
co-ordination tables. Their tables are
based on tests between the different
components.
Selectivity or discrimination
Discrimination of protective devices is
a major point to be considered when
designing low voltage installations.
The aim of discrimination is to minimize
the effects of a fault. Only the faulted
circuit shall be disconnected while
the others shall remain in service. The
discrimination is achieved if a fault
is cleared by the protective device
situated immediately upstream of
the fault without operation of other
protective devices. The essential tools
to investigate discrimination between
protective devices are the time-current
characteristics and I2t values.
Discrimination between fuses
The discrimination between fuselinks is verified by means of the timecurrent characteristics for operating
times ≥ 0,1 s and the pre-arcing and
operating I2t values for operating
times < 0,1 s. These verifications
are made by examination of the
time-current characteristics, and I2t
values shall be met to achieve total
discrimination between fuses.
Fuses according to IEC
60269-2-1 of the same type, with
rated currents ≥ 16 A, meet these
total discrimination requirements by
definition if the ratio of rated currents
is 1,6:1 or higher. No additional
verification by the user is therefore
needed.
Discrimination of circuit
breakers upstream of a fuse
The discrimination is verified by using
time-current characteristics and I2t
values.
May 2005 - Vector - Page 30
Verification of discrimination for
operating time ≥ 0,1 s: the maximum
operating time of the fuse shall be
lower than the minimum unlatching
time of the circuit breaker.
Verification of discrimination
for operating time < 0,1 s: the
discrimination may be determined
using the I2t values
Verification of total discrimination: the
requirements of both shall be fulfilled
to obtain total discrimination.
In practice, circuit-breaker manufacturers
give discrimination tables between circuit
breakers and selected fuses.
Discrimination of a fuse
upstream of circuit breaker
The discrimination is verified by using
time-current characteristics and I2t values
of the fuse.
Verification of discrimination for operating
time < 0,1 s: the discrimination may
be determined using the I2t values. The
maximum operating I2t values of the circuit
breaker shall be lower than the minimum
pre-arcing I2t values of the fuse.
Contact Andre van der Elst,
Electromechanica,
Tel (011) 249-5000,
andre@em.co.za ∆
May 2005 - Vector - Page 31