What are the defects in the quality of the drop out fuse?

What are the defects in the quality of the drop out fuse?
Falling drop out fuse  is the most commonly used in 10kV distribution lines branch lines and distribution transformers.
The working principle is: the movable contacts at both ends of the fuse tube rely on the fuse (melt) tied tightly, and the upper movable contact is pushed into the “duckbill” projection, and the upper static contact made of phosphor bronze sheet is pressed against the upper movable contact, so that the fuse tube is firmly stuck in the “duckbill”. When the short-circuit current through the fuse fuse, the arc is generated, the fuse tube lined with steel paper tube in the arc under the action of a large amount of gas because the upper end of the fuse tube is sealed, the gas to the lower end of the spray, blowing out the arc. As the fuse fuses, the upper and lower movable contacts of the fuse tube lose the tethering force of the fuse, and under the action of the gravity of the fuse tube itself and the spring plates of the upper and lower static contacts, the fuse tube falls down rapidly, disconnecting the circuit and removing the faulty section of the line or the faulty equipment.

With the development of the electric power industry, the production of high-voltage drop out fuse is becoming a bigger and bigger problem, the main problems are as follows.
(1) product process is rough, poor manufacturing quality, contact spring elasticity is not enough, resulting in poor contact and spark overheating.
(2) the fuse tube rotation axis manufacturing rough inflexible, so that the fuse tube angle does not meet the requirements of the specification, especially with the majority of the fuse tube size does not meet the requirements of the specification, the fuse tube is too long will duck mouth top dead, resulting in the fuse body fused after the fuse tube can not quickly fall, in time to cut off the arc, extinguished, resulting in the fuse tube burned or exploded; fuse tube size is short, difficult to close, poor contact contact, resulting in electrical sparks. Guangling Bureau of Electricity “6.7” accident belongs to this kind of reason, resulting in staff impatience, flashing out of the fuse tube with hands to close the dynamic, electrocution death.
(3) The rated breaking capacity of the fuse is small, and its lower limit is less than the capacity of the three-phase short circuit of the protected system. At present, the 10kV outdoor drop fuse is divided into three models, namely 50A, 100A, 200A. 200A drop fuse has an upper limit of 200MVA and a lower limit of 20MVA. According to its ability to cover capacity, it is easy to see that the fuse cannot be interrupted in time after the short circuit fault, and it is easy to make the fuse burn or explode.
(4) Some new switch fuses do not match the size of the fixed contact part of the fuse, so they are easy to be loosened, and once they encounter external force, vibration or windy weather, they will fall down by mistake.

The existence of the above-mentioned defects not only increases the workload of the maintenance personnel, but also promotes the improper use of maintenance personnel (especially agricultural electricians), completely losing the protection function of the fallen fuse, which can only expand the scope of power outage in the event of a short circuit in the line, crossing the level to the total circuit breaker of the 10kV outgoing line of the substation and causing a power outage in the whole line.

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