If this is the first time to learning about Fuse for you then I request you to read my previous post about Fuse Operation Principle with Design. You will get some of idea to understand this article. In this article I will discuss about Fuse Properties with Rating Principle.
Fuse Voltage Ratings:
Rated voltage of a fuse is the nominal voltage level for which it was designed. Fuse-links must perform with satisfactorily level at lower voltages but at much lower voltages the reduction in current caused by the resistance of the fuse-link should be considered.
In Europe Country, the nominal voltage level is 230 V and the permitted variations will allow supplies to remain at 240 V and 220 V. Fuse-links will marked 230 V may have been designed originally for use with higher or lower level voltages and any problems may therefore arise when replacing fuse-links because a device manufactured for use at 220 V level would not be safe to use on a level of 240 V system.
If fuse-link designed for 240 V could safely be used at 220 V level systems. This type of considerations can apply where the voltage is changed from 415 V to 400 V or 660 V to 690 V systems.
Fuse Current Ratings:
Generally the rated current for a Fuse is the maximum current that a fuse-link can carry out indefinitely without any deterioration. Current rating must printed on a fuse-link applies only at the temperatures below a particular value. Sometime de-rating may be necessary at high the ambient temperatures and where fuses are mounted in a hot locations like an enclosure with other heat generating equipment.
It is undesirable to have a proliferation of current ratings and therefore particular ratings are specified in standards. In a general tendency, particularly in the IEC standards to follow the R10 series or if necessary the R20 series may be used.
Example like using the R10 series fuse-links with the ratings from 10 A to 100 A are produced for 10 A, 12 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A and 100 A. In the United State of America there are more traditional ratings including 15 A, 30 A and 60 A fuse are used.
Fuse Frequency Ratings:
Fuses are commonly used in AC circuits system with frequencies of 50 Hz or 60 Hz and a fuse use to design for one of these frequencies will generally operate satisfactorily at the other. In the fault time if arc extinguishes at current zero then the maximum arcing time on a symmetrical fault will be 10 ms at 50 Hz and the 8 ms at 60 Hz.
Fuse manufacturers will be consulted about the suitability of manufactured fuses for other frequencies which may include 17.67 Hz for some railway supplies system and 400 Hz for aircraft with higher level frequencies for some special type electronic circuits.
But in DC circuits system there is no current zero in the normal waveform and the fuse-links designed for AC may not operate at satisfactorily level. Generally separate current and voltage ratings are given for fuse-links tested for use in DC circuits system.
But sometime DC circuits can be more inductive for a given current than AC circuit systems and since the energy in the inductance is dissipated in the fuse then it is necessary for the DC voltage rating to be reduced as the time constant (L/R) of a circuit increases.
Variations in Rating Principles:
The IEC rating principles for fuse are used worldwide but in North America where UL (Underwriters Laboratory) standards apply. In UL standard the rated current is the minimum current required to operate the fuse after many hours and the current that it will carry indefinitely is approximately about 80% of this rating.
Voltage rating use to mark on a UL fuse-link is the maximum voltage level at which it can be used whereas that marked on an IEC fuse-link is the nominal voltage. These kinds of differences must be considered when replacing fuse-links particularly in the case of miniature cartridge fuse-links which is interchangeable.
But in general it is preferable to replace a fuse-link with one of the same rating from the same manufacturer and this ensures that its characteristics are as similar as possible to those of the previous fuse-link that was used. The IEC Standard and the UL Standard ratings of fuse holders is also differ.
Time-Current Characteristics for Fuse:
Normally the total operating time of a fuse use to consists of the pre-arcing time and the arcing time. When pre-arcing times is longer than 100 ms and the arc is then extinguished at its first current zero, then the time-current characteristic can be taken represent to the total operating time of fuse.
All fuses must be operating within the conventional time when carrying the conventional fusing current and when carrying the conventional non-fusing current they must not operate within the conventional time. Now the modern trend is to specify a number of points which form gates through which the actual time–current characteristics of all manufacturers’ fuse-links and it must pass if they are to comply with the appropriate standard.
Fuse Breaking Capacity:
Generally the breaking capacity of a fuse is the current, which can be interrupted at the rated voltage. But the required breaking capacity will depend on the position of the fuse in the supply system. 6 kA fuses may be suitable for domestic and commercial applications but 80 kA fuses is necessary at the secondary of a distribution transformer.
Resistance of a fuse will result in dissipation of power in the protected circuit system when normal currents are flowing. It should be considered when designing the layout of a protection system.
Generally I2t is defined as the integral of the square of the current that goes through by a fuse over a period of operation time. This values use to given by manufacturers for pre-arcing I2t and total let-through I2t. Normally heat generated in a circuit in a short circuit or fault condition before the fuse disconnects is given by the product of I2t and the circuit resistance.
As the let-through I2t becomes a constant above a particular level of fault current and the heat generated during fault does not increase for prospective currents above this value unless the breaking capacity is exceeded.
Fuse Cut-off Current:
Normally a current limiting fuse prevents a fault current from rising above a level known as the cut-off current. The cut-off current is approximately proportional to the cube root of the prospective current and the maximum current is therefore very much lower than it would be if a non-current-limiting protection device were used in system.
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