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!
