Semi-enclosed fuse

Semi-enclosed fuses to BS 3036. These were previously called rewirable fuses and are used mainly on domestic installations having a maximum fault capacity of about 4 kA. 

The semi-enclosed fuse consists of a fuse wire, called the fuse element, secured between two screw terminals in a fuse carrier. The fuse element is connected in series with the load and the thickness of the element is sufficient to carry the normal rated circuit current. When a fault occurs an overcurrent flows and the fuse element becomes hot and melts or blows . 

Their operation is more rapid than semi-enclosed fuses. Operating time is inversely proportional to the fault current, so the bigger the fault current the quicker the fuse operates. 

A 230V ring main circuit of socket outlets is wired in 2.5 mm single PVC copper cables in a plastic conduit with a separate 1.5 mm CPC. An earth fault loop impedance test identifies Zg as 1.15Verify that the 1.5 mm CPC meets the requirements of lET Regulation 543.1.3 when the protective device is a 30A semi-enclosed fuse. 

A TN supply feeds a domestic immersion heater wired in 2.5 mm PVC insulated copper cable and incorporates a 1.5 mm CPC. The circuit is correctly protected with a 15 A semi-enclosed fuse to BS 3036. Establish by calculation that the CPC is of an adequate size to meet the requirements of lET Regulation 543.1.3. The characteristics of the protective device are given in lET Regulation Fig 3A2(a) of Appendix 3. 

So, in this case the table states that 90 A will trip a 15 A semi-enclosed fuse in 0.4s... 

Cf the 0.725 correction factor to be applied when semi-enclosed fuses protect the circuit as described in item 5.1.1 of the preface to Appendix 4 of the lET Regulations. 

The cartridge fuse breaks a faulty circuit in the same way as a semi-enclosed fuse, but its construction eliminates some of the disadvantages experienced with an open-fuse element. The fuse element is encased in a glass or ceramic tube and secured to end-caps which are firmly attached... 

In the event of a fault occurring on an electrical installation only the protective device nearest to the fault should operate, leaving other healthy circuits unaffected. A circuit designed in this way would he considered to have effective discrimination. Effective discrimination can he achieved hy graded protection since the speed of operation of the protective device increases as the rating decreases. This can he seen in Fig. 12.5(h). A fault current of 200A will cause a 15 A semi-enclosed fuse to operate in about 0.1 s, a 30A semi-enclosed fuse in about 0.4 s and a 60 A semi-enclosed fuse in about 5.0 s. If a circuit is arranged as shown in Fig. 12.6 and a fault occurs on the appliance, effective discrimination will be achieved because the 15 A fuse will operate more quickly than the other protective devices if they were all semi-enclosed types fuses with the characteristics shown in Fig. 12.5(b). 

The semi-enclosed rewirable fuse consists of a tinned copper wire placed in the circuit conductor path. The fuse will blow when it is approximately 160% overloaded. In the case of a short circuit fault, which can allow thousands of amps to flow, the fuse will blow within 10 milliseconds (half a cycle). The tinned copper is ionized and can be thrown from the fuse into the surrounding area and, in some cases, can cause secondary fires and ejqtlosions. The advantages of the semi-enclosed fuse are few except for their low cost the fact that a blown fuse is easify seen and the ease with which a user can replace them. The disadvantages are that they may be open to abuse with the wrong size wire being used on rewiring to risk of fire to deterioration of fuse element with age and use and to slow operation when compared with HRCs (see below). 

Figure 6.39 Time/current characteristics of (a) a Type B MCB to BS EN 60898 and (b) semi-enclosed fuse to BS 3036.

Semi-enclosed fuse The semi-enclosed fuse consists of a fuse wire, called the fuse element, secured between two screw terminals in a fuse carrier. 

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