Automotive Fuse Link Definition and Specifications
September 30, 2018
Automotive Fuse Link Definition and Specifications
Automotive use links are self-acting break appliances for the protection of electrical devices against unsuitable current loads. The current flow is interrupted by the melting of the fuse wire in which the current flows.
The following international regulations and recommendations in their currently valid version are valid for fuse links:
- DIN 72581
- DIN 43560
- ISO 8820
- UL 275
(Further the level of technology, the details of the actually valid implementation provisions, the safety-principle "human beings, animals and material assets must be protected against danger," as well as the qualification of the installed components should be taken into account - self-responsibility of the manufacturer of electrical devices.)
Selection Explanations and Recommendations
The rated voltage (UN) of a fuse link has to be at least equal or higher than the operating voltage of the device or assembly unit which is to be protected with the fuse link. If the operating voltage is very low, the fuse link's natural resistance (voltage drop) must possibly be taken into consideration.
The voltage drop (UN) is measured according to standards e.g. DIN, ISO, JASO, partially maximum values common for are stated as well.
The rated current (Irat) of a fuse link should approximately correspond to the operating current of the device or assembly unit which is to be protected (in accordance with the ambient temperature and rated current-definition, which means the permitted continuous currents).
Higher ambient temperatures (Tumg) mean additional load for the fuse links. The heating conditions of the maximum occurring ambient temperature have to be checked, in particular with high rated currents of the fuses, and a strong thermal radiation of components nearby. For such applications, the fuse should be derated in accordance with the following diagram, resp. table (see factor FT):
Due to different specifications of rated current the recommended continuous current of the fuse links is max. 80% of their rated current (with an ambient temperature of 23°C), see also the fuse-specific current-carrying capacity (F) on the individual pages of the catalog.
The pre-arcing time limits indicate the relation of fusing time to current. (They are presented as an envelope curve for all mentioned rated currents.)
The melting integral (I2t) results from the squared melting current and the corresponding melting time. At excess current with melting times < 5 ms the melting integral remains constant. The data in this catalog are based on 6 or 10 x lrat. The melting integral is an index for the time-current characteristic and informs about the pulse consistency of a fuse link. The mentioned melting integrals are typical values.
The breaking capacity (IB) should be sufficient for any operating and error conditions. The short-circuit current (maximum fault current) to be interrupted by the fuse links at the rated voltage under default conditions must not be higher than the current corresponding to the breaking capacity of the fuse link.
The maximum power dissipation (PV) is determined at a load with rated current, after having obtained temperature equilibrum. In operation, these values can occur for some time.
Typical values are indicated and in addition the standard values for fuses that comply with standards.
Automotive Fuse Link Selection
With regard to the product-safety of the device and the life/reliability of the fuse links, a correct choice is important. Only when choosing correctly and when using as agreed (that means corresponding to the level of technology and the valid recommendations, as well as the specified characteristics shown in the data sheets) under consideration of the safety-principle (that is to say "human beings, animals and intrinsic values must be protected against danger") can a definite function of the fuse links as a protection component (rated breakpoint) be possible. The personal responsibility of manufacturers of electrical devices is applicable here:
"Any person involved in the production of electrical systems or the manufacturing of electrical equipment, included those dealing with the operation of such systems or equipment is, according to the present interpretation of the law individually responsible for every aspect of compliance to the recognized rules and procedures of electrical engineering."
- The necessary voltage rating of a fuse link is established by its required operating voltage (taking into consideration the voltage drop of the fuse link).
- The current rating of a fuse link (IN Fuse) is established by the max. effective current load (Ioperating max) taking into consideration the ambient temperature (Factor FT) and the varying rated current definitions ("constant current" definition) (see Faktor FI). The following applies: IN Fuse3 Ioperating max. x FI x FT
- t-value (current-time-integral).2In case of pulse loading and for semiconductor protection a suitable current rating can also be ascertained with the help of the I
- The above mentioned two points will help you to ascertain the most suitable current rating of a fuse link and its pre-arcing time limits (if necessary verify experimentally).
- The necessary breaking capacity of the fuse link is determined by the max. possible fault current which can occur.
- In addition to the above-mentioned points, the method of installation is also important for correct fuse link selection (taking into consideration possible approvals).
With regard to specific conditions of any particular application (product-safety) it is generally necessary to check the fuse link and/or the thermal circuit breaker or holder in the device which is to be protected under normal and fault conditions!