|
MV-Fuses
What is a
fuse?
What is an MV-fuse
link?
What is a MV-fuse
element?
What is a striker?
What are rated
values?
What is the rated
voltage?
What is the rated
current?
What is the breaking
capacity?
What is the rated
frequency?
What is a prospective
current?
What is a cut-off
current or let-through
current?
What is the pre-arcing
time or melting
time?
What is the arcing
time?
What is the operating
time or total
clearing time?
What is the IÝt or Joule
integral?
What is the virtual
time?
What is the time-current-characteristic?
What is the cut-off
(current) characteristic or let-through
current characteristic?
What is the recovery
voltage?
What is the switching
voltage?
What is the minimum
breaking current?
What is the power
dissipation (of a MV-fuse link)?
What are temperature-rise limits?
Temperature-rise limits means that the MV-fuse link and MV-fuse
base shall be able to carry their rated current continuously
without exceeding the limits of temperature rise given in
table 6 of IEC 60282-1.
What is the rated
insulation level (of an MV-fuse base)?
What is a current-limiting
MV-fuse link?
What are the classes of MV-fuse links?
There are three classes of current-limiting MV-fuse links
defined according to the range in which they can be used:
- Back-Up fuse
- General Purpose fuses
- Full Range fuses
What is an MV-Back-Up
fuse?
What is an MV-General
Purpose fuse?
What is an MV-Full
Range fuse?
How does one select the voltage
rating?
Fuse links are voltage sensitive devices and it is important
to note that the satisfactory operation of a fuse link under
fault conditions is dependent on the system voltage. They
must not therefore be installed in circuits above their voltage
rating. They can however be used satisfactorily in circuits
at lower voltage levels. For example, to protect a 10 kV system
a 12 kV or 24 kV MV-fuse link can be used, but not a 7,2 kV
MV-fuse link.
How does one select the current
rating?
Every fuse link has a specific ampere rating. In selecting
the ampere rating of an MV-fuse link consideration must be
given to the type of load and circuit requirements.
How do high ambient temperatures affect
the current rating of a fuse link?
A fuse link is a thermal device and as such may require some
de-rating when used at elevated ambient temperatures. Fuse
links can carry rated current up to an ambient of 40 °C.
When the ambient temperature is higher than 40 °C de-rating
may be required (a simple rule is to de-rate by 0,5 % per
degree centigrade). The voltage rating is not dependent upon
ambient temperature.
Why is breaking capacity important?
A protective device must be able to withstand the destructive
energy of short circuit currents. The rating, which defines
the capability of a protective device to maintain its integrity
when reacting to fault currents, is termed its breaking capacity
or interrupting rating.
Is it useful to have a low value of
I2t?
Yes – the energy released during a short circuit, if
not limited, can severely damage part or whole installations.
Two specific parameters affect the operating IÝt:
- Power factor
The lower the power factor the higher the I2t energy let-through.
- Voltage
The higher the voltage the higher the IÝt energy let-through
MV-fuse links considerably limit this energy. For instance,
without a MV-fuse link an asymmetrical short circuit current
of 10,000 A at 10 kV a.c. could pass through the circuit during
several cycles.
For example during the first cycle, the IÝt could be as high
as 4,000,000 AÝs. In these conditions a 100 A MV-back-up fuse
link will limit the IÝt value to about 210,000 AÝs, i.e. only
5 % of the value during the first half cycle.
Does an MV-fuse link limit the peak
current under short circuit conditions in an electrical system?
Yes – current limitation depends on the short circuit
conditions (prospective value, power factor, making angle
at the beginning of the short circuit). Cut-off current characteristics,
given by the manufacturer of the MV-fuse link, show maximum
values of limited current reached in the worse conditions.
For example a prospective short circuit current, which is
the value of a current that would flow if there would be no
fuse protection in the circuit, of 10,000 A r.m.s. with a
totally asymmetrical current, the maximum value could reach
25,000 A peak. A 100 A MV-back-up fuse link limits the first
peak to 8,000 A, less than the third part of the prospective
maximum value. The destructive electro-dynamic effects are
reduced by 90 %(8,000/25,000) Ý!
Is the protection of power transformers
standardized?
IEC 60 787 gives some guidance HOW to protect, but does not
advise WHICH current ratings should be chosen. The fuse manufacturers
recommendation lists will usually be found helpful.
What do I have to consider to find
the correct fuse?
The high-voltage fuse link has to be able to withstand the
transformer inrush current. Hence, the fuse time-current-value
at 100 ms has to be at lest 10-12 times the transformer rated
current. This will normally dictate a value of fuse current
rating up to twice that of the transformer full load current.
It is also usually a requirement that there should be discrimination
between the MV-fuse and any low-voltage protective device
on the secondary side of the transformer. The interrupting
capacity of the fuse has to be at least as high as the maximum
calculated short circuit current for the system.
What is the technical background of
the manufacturers recommendation lists?
The recommendations of fuse manufacturers are generally based
on the considerations in 4.2.2. Sometimes more that one fuse
rating is quoted for a given transformer size depending upon
such factors as permissible overloads etc. Discrimination
between MV- and LV-fuses can be checked by reference to the
relevant time-current characteristics for the fuses concerned.
Why is the recommended fuse rating
more or less twice the transformer rating?
The main reason why the MV-fuse has to have a much higher
current rating than the transformer load current is due to
the need for the fuse to withstand the high transient magnetising
inrush current when the transformer is switched into service.
A second reason which may need to be taken into account is
for cases of permissible overloading of the transformer for
periods several hours and /or to allow for applications where
the fuse is mounted in an enclosure having very restricted
ventilation.
Is there a need for discrimination
distance between up- and downstream fuse-links?
Yes, discrimination between upstream MV-fuse and downstream
LV-fuse is obtained by comparison of time current curves for
the two fuses using an appropriate transformation ratio to
take account of the voltage difference of the two circuits.
What is the power loss of the fuse-link
at transformer rated current?
The power loss of the fuse at transformer rated current is
about 20 % of the value given in the manufacturers catalogues.
How warm can a fuse be at transformer
rated current?
The temperature rise of a fuse in service has to comply with
the limits given in 1EC 60282?1. This may entail de-rating
the fuse in terms of current rating where the fuse is mounted
in an enclosure or in situations of high ambient temperature.
i.e above 400C
Do MV-fuse links age or is there a
need for maintenance?
No, if the fuse is taken according to the manufacturers recommendation
list, ageing is not expected. There is no need for maintenance.
Is the protection of capacitor banks standardised?
IEC 60 549 gives some guidance HOW to protect, but does not
advise WHICH current ratings should be chosen. The fuse manufacturers
recommendation lists will usually be found helpful.
What do I have to consider to find
the proper fuse for a capacitor bank?
To select the correct fuse rating for a given application
it is necessary to consider the harmonic content of the load
current as well as the charging inrush current during switch-on.
Is there any overload protection possible
by the fuse links in capacitor circuits?
Low overload protection is not usually possible due to the
need to select a fuse of large enough current rating to withstand
harmonics and charging current.
Is the protection of MV motor circuits
standardised?
IEC 60 644 gives some guidance HOW to protect, but does not
advise WHICH current ratings should be chosen. The fuse manufacturers
recommendation lists will usually be found helpful.
What do I have to consider to find
the correct fuse for motor circuits?
For direct-on-line applications, it is necessary to allow
for the motor starting current which may typically be 6 times
full load current for a period of several seconds. This point
should be well to the left of the time-current curve for the
fuse in question. Where start up is frequent, an additional
allowance may need to be made to ensure against long term
fuse deterioration.
For both direct-on-line and assisted start applications it
is also necessary to allow for any de-rating of the fuse consequent
on confining the fuses in a small enclosure.
Is it allowed to parallel fuse-links
for bigger machines?
Yes… Where the required current rating to protect the
motor circuit is greater than the largest fuse rating, two
or more fuse links may be connected in parallel as required.
Due to proximity heating effects, the combined rating of the
combination will usually be somewhat less than n x the rating
of each fuse, where n = number of fuses in parallel. In this
case, ask the manufacturer for details.
Are there special fuses for MV motor
circuit applications?
While standard back-up fuses may be used for this application,
purpose-built types of fuse are available which have special
elements designed to cope with cyclical load conditions
Are there any standards relating to
use of MV-fuses in combination fuse switch units?
Yes, IEC 60 420 gives guidance for the combination of a fuse
link according IEC 60 282-1 and a load break switch according
IEC 60 265-1.
What is the task of IEC 60 420?
This standard describes the tests required for such combinations
and the criteria for using non-tested fuses in a given switch
unit
|
