MV ‘Backup’ Fuses (Current Limiting Type) Are Not Rated For ‘Overload’ Current Interruption

Published 30 Mar 2021

MV ‘Backup’ Fuses

From IEEE Std C.37.48.1
“MV backup fuses can interrupt any current between its rated minimum interrupting current and its rated maximum interrupting current. If a backup fuse is melted open at a current level less than its minimum interrupting rating, the fuse is not required to, and may not successfully, interrupt the circuit. In this case, the fuse melts open at only a few places, often a single point, along the length of the element. Burn back of the element proceeds very slowly, as the arc heat causes the sand to melt and form a resistive fulgurite. During arcing conditions, a large amount of heat is released in a limited area within the fuse. This heat can prevent the sand and the resultant fulgurite formed in the sand from developing dielectric properties needed to withstand circuit voltage. As a result, the fuse may fail to interrupt the current and be destroyed as it attempts to clear.”

Solution(s): Include a protective device in series which is rated for overload operation (increase back-up fuse size to get out of the zone), employ full range fuses.

In this case, an upstream SF6 breaker was able to be coordinated with the needs of the CL fuse – here the fuses are immediately ahead of the transformer primary.

Guest Post by Matthew Denis | Principal Engineer at Rockwood Engineering (Ontario Canada)

Services | Medium Voltage Electrical Power System Design & Analysis | Power System Analysis including: Protection Coordination, Arc Flash, Short Circuit, Harmonic & Load Flow | Power Quality Assessment/Mitigation | Power Distribution Design (Industrial, Utility, Commercial, Agricultural), Medium Voltage Power Systems Design

MV Fuses | Thorne & Derrick distribute the complete range of ABB Medium & High Voltage Fuses (MV-HV) for the protection of 11kV & 33kV switchgear, transformers, cables and substations.

Definition: MV Back-up Fuses by ABB

Current limiting back-up fuses. The current limiting fuse family is generally composed of three different fuse groups: back-up fuses, general purpose fuses and full range fuses. All of them limit the value of prospective short-circuit currents during the interruption process, thereby extending the life time of nearby installed electrical equipment. The main difference is in the minimum breaking current that characterizes the lowest fault current that the fuses are capable of interrupting. This value is generally highest for back-up fuses, slightly smaller for general purpose fuses and smallest, with the value close to the minimum melting current, for full range fuses. But reaction time is critical for the protection function. That is why back-up fuses, with an interruption time for the minimum breaking current in the range of a few seconds down to a few tense of milliseconds, are the most commonly used. The total clearing time in cases of high short-circuit currents is even shorter i.e. only a few milliseconds. That is why back-up fuses can be used as typical overload protection elements. General purpose and full range fuses capable of interrupting even the smallest values of currents can only be considered as over current devices since the interruption time is greater than one hour. Therefore, these types are used rarely and are usually recognized as a separate element of protection, without any linkage to the opening function of load break switch. ABB current limiting fuses have low minimum breaking currents, i.e. close to three times the rated current In.

Further Reading

• ABB MV HV Fuses – Catalogue Size: 2.23 MB