HomeEnvironmental Activities and Products

EMC Control of Systems and Power Sources and Correct Selection of Fuses
  1. Preventing Surge Intrusion from Power Supply Line

 A criterion is set in IEC 61000-4-5 Surge Immunity Test for a level to endure a surge entering from a power supply line. Levels and waveforms of surge voltages are varied and "absolutely safe" protection is not feasible at present. Under these circumstances, goodness-of-fit of IEC61000-4-5 is a criterion used to show a level of surge yield strength.		  
 
 
  "Immunity" is the "ability of equipment and systems that enable continued operation without causing deterioration under conditions where electromagnetic interference is caused."
IEC61000-4-5 is a standard used to verify immunity and the surge yield strength is specified in Table 1.
 
    Table1  
 
Level
Surge voltage waveform of 1.2/50 micro sec
(kV)  *1
Surge current waveform of 8/20 micro sec
(kA)  *2

I2t of surge current
(A2s)
1
0.5
0.25
0.7
2
1
0.5
2.9
3
2
1
11.4
4
4
2
45.8
 
    *1 A surge voltage waveform of 1.2/50 micro sec is roughly defined as illustrated in Fig. 1.  
 
 
   
Fig. 1
  
 
*2 A surge voltage waveform of 8/20 micro sec is roughly defined as illustrated in Fig. 1.  
 
 
   
Fig. 2
  
    *1*2 Immunity tests specify a surge voltage waveform and the waveform of a current flowing to a lightning protection circuit of equipment and systems when this surge voltage is impressed. Surge generators capable of generating such outputs are called "hybrid generators" and waveforms of such outputs are called "combination waveforms." The resistance of circuits in which a surge flows is 2 ohm.  
 
*2 The surge yield strength of overvoltage protection devices is generally shown using a waveform of 8/20micro sec. Overvoltage protection devices with sufficiently large surge yield strength (shock discharge yield strength) have to be selected to select overvoltage protection devices commensurate with the compliant immunity level.  
       
 

2.Generation and Prevention of Secondary Faults Through Damage of Overvoltage Protection Devices

If a surge current exceeding a surge yield strength flows, overvoltage protection devices sometimes generate heat and are damaged. When damaged, the zinc oxide varistors used in many circuits are left in a state of low resistance, causing a secondary fault after a short-circuit current flows between circuits. As a measure to prevent secondary faults caused by damage to overvoltage protection devices, a varistor and a fuse are connected serially to blow the fuse by a short-circuit current when the varistor is damaged, thereby isolating the varistor.
However, selection of a fuse of unfitting characteristics sometimes mistakenly blows the fuse due to an unexpectedly small surge current. Fuses that are connected serially to overvoltage protection devices should be selected from those that have a sufficiently large Joule-integral value for blowing (criterion: more than twice) based against the Joule-integral value of a surge current shown in Table 1.

 
     
 

3.Alarm Fuzet

Daito's Alarm Fuzet prevents secondary faults caused by varistor damage.By combining a varistor and an alarm indicator fuse, the varistor is isolated from the circuit in order to prevent secondary faults, even if the varistor deteriorates or a surge exceeding the surge yield strength flows. Varistor damage is indicated and alarm operation activated.
Table 2 shows the compatibility of the Alarm Fuzet with various levels of surge immunity.Type A conforms to Class 3 or lower and Type B, Class 4 or lower.

*Please contact Daito for further information on Type B.

  
   Table2  
Level
Conformity of Alarm Fuzet
FZ**A
FZ**B
1
OK
OK
2
OK
OK
3
OK
OK
4
-
OK
 
 
     
Contact us (Questions and Comments)
Copyright (c) Daito Communication Apparatus Co., Ltd.