How to ensure fuse selectivity

Selectivity (selectivity) of fuse protection is ensured by choosing fuses in such a way that in the event of a short circuit, for example, on a branch to an electrical receiver, the nearest fuse that protects this electrical receiver is triggered, but the fuse , protecting the network head, does not work.

Selection of fuses according to selectivity conditions

The selection of fuses for the selectivity condition should be carried out using the typical time current characteristics t = f (I) of the fuses, taking into account the possible spread of real characteristics according to the manufacturer's data.

Selection of fuses according to selectivity conditions

When protecting networks with fuses of type PN, NPN and NPR with typical characteristics shown in the figures, the selectivity of the protective action will take place if between the rated current of the fuse protecting the head of the network Ig and the rated current of the fuse of the branch to the consumer Io certain ratios are maintained...

For example, at low fuse overload currents (about 180-250%), the selectivity will be maintained if Ig is greater than Io by at least one step of the standard scale of rated fuse currents.

In the event of a short circuit, the selectivity of NPN fuse protection will be ensured if the following relationships are maintained:

Here Ik is the branch short-circuit current, A; Ig — nominal current of the mains fuse, A; Io — rated current of the branch fuse, A.

The ratios between the rated fuse currents Ig and Io for PN2 type fuses providing reliable selectivity are given in Table 1.

Table 1. Rated currents of series-connected fuses PN2 fuses, providing reliability selectivity

Rated current less fusible link AzO, A

Rated current greater fusible link AzG, A, with the ratio Ik / Io

10

20

50

100 and more

30

40

50

80

120

40

50

60

100

120

50

60

80

120

120

60

80

100

120

120

80

100

120

120

150

100

120

120

150

150

120

150

150

250

250

150

200

200

250

250

200

250

250

300

300

250

300

300

400

more than 600

300

400

400

more than 600

400

500

more than 600

Note. Ik — short-circuit current at the beginning of the protected section of the network.

Protective characteristics (current over time) of fuses PN-2

Protective (current-time) characteristics of fuses type PN-2

Protection (on-time) characteristics of NPR and NPN fuses

Protection (current-time) characteristics of NPR and NPN type fuses

Selection of fuses according to the conditions of selectivity according to the method of matching the protective characteristics of the fusesSelection of fuses according to the condition of selectivity according to the method corresponding to the protective characteristics of the fuses

To select fuses according to the selectivity condition, you can use the method of matching the fuse characteristics, which is based on the principle of comparing the cross sections of the fuses according to the formula:

where F1 is the cross-section of the fuse located closer to the power source; F2 - cross-section of the fuse located further from the power source, i.e. closer to the load.

The obtained value of a is compared with the data in Table 2, which shows the smallest values ​​of a at which selectivity is ensured. The selectivity of the protection will be guaranteed if the calculated value is equal to or greater than the table value.

table 2 The smallest values ​​of a at which selectivity protection is ensured

Metal fuses fuse located closer to the power supply (for each type of fuse)

Behavior a fuse cross-sections of adjacent fuses if the fuse is closest to the load

with filler when melting insert from

without magazine with fuse made of

honey

silver

zinc

I lead

honey

silver

zinc

I lead

Med

1,55

1,33

0,55

0,2

1,15

1,03

0,4

0,15

Silver

1,72

1,55

0,62

0,23

1,33

1,15

0,46

0,17

Zinc

4,5

3,95

1,65

0,6

3,5

3,06

1,2

0,44

I lead

12,4

10,8

4,5

1,65

9,5

8,4

3,3

1,2

We advise you to read:

Why is electric current dangerous?