Basic concept of drop out fuse and operation attention of drop

April 11,2021

Drop out fuse is the most commonly used short circuit protection switch for 10kV distribution line branch line and distribution transformer. It has the characteristics of economy, easy operation and strong adaptability to outdoor environment. It is widely used in the primary side of 10kV distribution line and distribution transformer as protection and equipment switching operation.





Structural composition

Drop out fuse is mainly composed of the following components


Nut、Long Bolts、Mounting BracketⅠ、Mounting BracketⅡ、Porcelain Insulator、Spring Washer、Onepiece channel、Fuse Tube、Lower Socket、Lower contacts、Connector、Upper Contacts、Attachment Hooks、 Operation Ring


Working principle of drop out fuse

The moving contacts at both ends of the fuse tube are fastened by the fuse (melt). After the upper moving contact is pushed into the protruding part of the “duck’s beak”, the upper static contact made of phosphor copper sheet is against the upper moving contact, so the fuse tube is firmly stuck in the “duck beak”. When the short-circuit current is fused through the fuse, an arc will be generated, and a large amount of gas will be produced by the steel paper tube lined with the fuse tube under the action of the arc. Because the upper end of the fuse tube is sealed, the gas will eject to the lower end to blow out the arc. Due to the fuse fusing, the upper and lower moving contacts of the fuse tube lose the binding force of the fuse. Under the action of the fuse tube’s own gravity and the upper and lower static contact spring pieces, the fuse tube falls down rapidly, so that the circuit is disconnected and the fault section line or fault equipment is cut off.


In the practical 10kV line system and the fuse on the distribution transformer can’t act correctly. One of the reasons is that the electrician’s quality is poor and the sense of responsibility is not strong. The maintenance and repair of drop out fuse is not carried out all the year round; the second reason is that the product quality of drop out fuse is poor and can not be operated flexibly. Two reasons reduce the function of drop out fuse. In reality, it is often found that there is lack of melting tube, lack of melt, or copper wire, aluminum wire or even iron wire hanging instead of melt. The trip out rate of the line and the fault rate of the distribution transformer remain high.


Main functions of drop out fuse

Drop out fuse is installed on the branch line of 10kV distribution line, which can reduce the scope of power failure. Because it has an obvious breaking point and has the function of isolating switch, it creates a safe working environment for the line and equipment in the maintenance section, and increases the safety sense of maintenance personnel. Installed on the distribution transformer, it can be used as the main protection of the distribution transformer, so it has been widely used in 10kV distribution lines and distribution transformers.


Model of outdoor drop out fuse

Outdoor drop out fuse is suitable for the power system with AC 50 Hz and rated voltage of 10kV. It is used for overload and short circuit protection of power transmission and distribution lines and power transformers, as well as the opening and closing of rated load current.


There are three types of outdoor drop out fuse widely used: rw7, RW11 and rw12.


Applicable environment

10kV drop out fuse is suitable for outdoor places where the ambient air is free of conductive dust, corrosive gas, inflammable and explosive environment, and the annual temperature difference ratio is within ± 40 ℃. Its selection is based on the rated voltage and rated current parameters, that is, the rated voltage of fuse must match the rated voltage of the protected equipment (line).


Environmental conditions for use

1. Normal operation conditions of the product: the ambient temperature is not higher than + 40 ℃ and not lower than – 40 ℃; the altitude is not more than 1000m; the maximum wind speed is not more than 35m / S; the earthquake intensity is not more than 8 ℃.


2. The product is not suitable for the following places: places with combustion or explosion risk; places with severe vibration or impact; areas with conductive and chemical gas and seriously polluted salt fog.


Installation tips of drop out fuse

(1) During installation, the melt should be tightened (the melt is subject to about 24.5n tension), otherwise it is easy to cause contact heating.


(2) The fuse shall be installed on the cross arm (frame) firmly and reliably without any shaking or shaking.


(3) The melting tube should have an inclination angle of 25 ± 2 ° downward to facilitate the rapid drop of the melting tube depending on its own weight when the melt is melted.


(4) The fuse should be installed on the cross arm (frame) with a vertical distance of not less than 4m from the ground. If the fuse is installed above the distribution transformer, it should keep a horizontal distance of more than 0.5m from the outermost contour boundary of the distribution transformer, in case of other accidents caused by the falling of the fuse tube.


(5) The length of the melting tube should be adjusted appropriately. It is required that the duck’s beak tongue can buckle more than two thirds of the contact length after switching on, so as to avoid the misoperation of self falling during operation. The melting tube should not be jacked to prevent the melting tube from falling down in time after the melt is fused.


(6) The melt used must be the standard product of regular manufacturer and have certain mechanical strength. Generally, it is required that the melt can bear at least 147n tensile force.


(7) 10kV drop out fuse is installed outdoors, and the phase to phase distance is required to be greater than 70cm.


Attention to the operation of drop out fuse


In general, it is not allowed to operate drop out fuse with load, only allow it to operate no-load equipment (line). However, branch lines of 10kV distribution lines and distribution transformers with rated capacity less than 200KVA are allowed to operate with load according to the following requirements:


(1) The operation shall be carried out by two persons (one for supervision and one for operation), but the qualified insulating gloves, insulating boots and eye protection glasses must be worn, and the qualified insulating rod with matching voltage grade shall be used for operation. The operation is prohibited in the climate of thunder and lightning or heavy rain.


(2) When the gate is opened, the intermediate phase is first pulled off, then the leeward side phase is pulled, and finally the windward side phase is pulled off. This is because the distribution transformer is changed from three-phase operation to two-phase operation, and the arc spark generated by breaking the intermediate phase is the minimum, which will not cause phase to phase short-circuit. The second is to break the leeward side phase, because the middle phase has been opened, the distance between the leeward side phase and the windward side phase is doubled. Even if there is overvoltage, the possibility of phase to phase short circuit is very small. Finally, when the windward side phase is pulled off, there is only capacitive current to the ground, and the generated spark is very slight.


(3) When closing, the sequence of operation is opposite, the windward side phase is closed first, then the leeward side phase is closed, and finally the intermediate phase is closed.


(4) The operation of melting tube is a frequent item. If you don’t pay attention to it, the contact will be burned, and the contact will be overheated and the spring will be annealed, which will make the contact worse and form a vicious circle. Therefore, when pulling and closing the melting tube, the force should be moderate. After closing, carefully check that the duck’s beak tongue can tightly buckle more than two-thirds of the length of the tongue. You can hook the upper duck’s beak with a brake lever, press it down a few times, and then try to pull it gently to check whether it is closed properly. When the switch is not in place or not firmly closed, the pressure of the static contact on the fuse is insufficient, which will easily cause the contact burn or the fuse tube to fall off.


Operation and maintenance of drop out fuse


(1) In order to ensure more reliable and safe operation of fuses, in addition to strictly selecting qualified products and accessories (including fusible parts) produced by regular manufacturers according to the requirements of regulations, the following matters shall be paid special attention to in operation and maintenance management:


① Whether the rated current of fuse matches with the current value of melt and load properly, if not, it must be adjusted.


② Each operation of the fuse must be careful and careful, and should not be careless, especially the closing operation, the dynamic and static contacts must be in good contact.


③ The standard melt must be used in the melting tube. It is forbidden to replace the melt with copper wire and aluminum wire, and it is not allowed to bind the contact with copper wire, aluminum wire and iron wire.


④ For the newly installed or replaced fuse, the process of acceptance must be strict, and the quality requirements of the regulations must be met. The installation angle of the fuse tube should reach about 25 ° inclination angle.


⑤ After the melt is fused, it should be replaced with a new one of the same specification. It is not allowed to connect the fused melt together and then put it into the melting tube for further use.


⑥ The fuse should be inspected regularly, at least once a month at night, to check whether there are discharge sparks and poor contact phenomenon. If there is discharge, there will be hissing sound. It is necessary to arrange treatment as soon as possible.


(2) In spring inspection, the following inspection shall be carried out for fuse during power outage maintenance:


① Whether the contact between static and moving contacts is consistent, tight and intact, and whether there are burn marks.


② Whether the rotating part of the fuse is flexible, whether there is corrosion and inflexible rotation, whether the parts are damaged and whether the spring is corroded.


③ Whether the melt itself has been damaged, whether there is heating after long-term power on, and whether the elongation is too much and becomes slack and weak.


④ Whether the arc suppression tube for gas production in the tube is burned, damaged or deformed after exposure to the sun and rain, and whether the length is shortened.


⑤ Clean the insulator and check whether there is any damage, crack or discharge trace. After the upper and lower leads are disconnected, the insulation resistance shall be greater than 300m Ω by 2500V megger.


⑥ Check whether the upper and lower connecting leads of the fuse are loose, discharged or overheated.



Fault analysis of drop out fuse

Cause analysis

In theory, the fuse protection plays a role. However, it can be clearly seen from the statistical charts of previous years that there are time and climate laws for abnormal fusions, which are reflected in the high temperature, large power load, rapid rise of distribution transformer load, and concentrated and frequent failures of fuse fusing and tube dropping. The reasons are as follows: (1) improper configuration of fuse capacity and distribution transformer capacity, which can not meet the technical standard of fuse configuration. (2) The quality of fuse is not up to standard, and the fusing characteristic is poor.


It can be seen from the statistical charts of previous years that there is no special rule of time and climate when the fuse falls off due to rolling damage. However, from the analysis of the broken parts of the fuse body, it is found that one is the bolt fixed at both ends of the fuse, and the other is at the corner of the metal casting at both ends of the fuse tube. The reasons for the broken fuse are as follows:


(1) When tightening the bolt, the end of the fuse turns and breaks with the rotation of the bolt.


(2) Due to the sharp edge of concave convex edge at the corner of the metal casting at both ends of the fuse tube, the fuse is cut and broken after running for a period of time after being fixed and tightened.


When the fuse is used on the drop out fuse, it is in the stress state for a long time. When replacing the fuse, if it is too tight or too loose, after a period of operation, due to the influence of natural environment, mechanical vibration and long-term stress, the fuse will pull out in the over tight state, or the fuse will be stretched and loose than the original replacement, resulting in tube dropping failure. The reasons are as follows:


(1) When replacing the fuse, the adjustment force is not appropriate.


(2) The loose pulling of fuse mainly refers to that the fuse body is pulled out from the crimping joint with multi strand tail wires, which belongs to the product quality problem of the manufacturer.


(3) Drop out fuse has been running for a long time, especially for the distribution transformer with small load for a long time, there is a phenomenon that the fuse is broken due to water and moisture in the fuse tube.


Installation defects

(1) The product process is rough, the manufacturing quality is poor, and the contact spring is not elastic enough, resulting in poor contact and spark overheating.


(2) The rough and inflexible manufacturing of the rotating shaft of the melting tube makes the angle of the melting tube fail to meet the requirements of the regulation, especially the size of the fuse tube equipped can not meet the requirements of the regulation. If the melting tube is too long, the duck’s beak will die, which will cause the melting tube not to fall down quickly after fusing, and the electric arc will be cut off and extinguished in time, resulting in the burning or explosion of the melting tube; the size of the melting tube is short, the closing is difficult, and the contact is not in good contact, resulting in electric fire FLOWER.


(3) The rated breaking capacity of fuse is small, and its lower limit is less than the three-phase short-circuit capacity of the protected system. 10kV outdoor drop out fuse is divided into three types, namely 50a, 100A and 200A. The upper limit of breaking capacity of 200A drop out fuse is 200mva and the lower limit is 20MVA. According to the capacity of interrupting capacity, it is not difficult to see that the arc can not be extinguished in time after the melt is fused in case of short circuit fault, and the melting tube is easy to burn or explode.


(4) Some new switches fuse tube size and fuse fixed contact part size matching is not suitable, easy to loose, in the operation once encountered external force, vibration or gale weather, it will automatically misoperation and fall, not only reduce the electricity sales, but also difficult to ensure the power supply reliability index.


The above defects not only increase the workload of maintenance personnel, but also promote the improper use of maintenance personnel (especially agricultural electricians), and completely lose the protection function of drop out fuse. Short circuit in the line can only expand the scope of power failure, skip to the main circuit breaker of 10kV outgoing line of Substation to trip, resulting in power failure of the whole line.