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  　　This kind of Resistance-reduce-agent is composed of many ingredients, including electrolyte, curing agent, lubricant, electro-conductive cement and stuffing materials. As one excellent electric conductor, it is used between grounding part and earth. On the one hand, it can be closely connected to metal grounding to form enough current surface; on the other hand, it can infiltrate into soil near it to lower resistance and form one low resistance district around grounding parts.
 
 
     
 
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5 . Choose Method of Surge Restrain Protection Device

The choice of surge restrain protection device should be:
US.max (power network) ＜ U; p(surge restrain protection device) ＜ Uchoc(overload)
The choice of surge restrain protection device should consider from the following three aspects:

Ⅰ. Characteristic of protection over-load

According to GB50343-2004/IEC60364-4, the voltage of three phase power network should be 230V/440; the shock pressure resistance of protected equipment (8/20 μ s) is divided into four sorts:
Shock Pressure Resistance
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Sort
Low
Common
High
Very High
　
Over-load Type

Electron Needing Special Protection

Information Equipment:

Communication equipment such as television, acoustics, video recorder and computer etc

Electric Equipment:

Washing machine, refrigerator, electric tools and heater etc

　
Line of Power Distribution and Last Embranchment

Line Equipment:

Motor, power distribution cabinet, power, socket and transformer etc

Power Equipment:

Electric measuring apparatus, over current protection equipment et

Uchoc (Shock Pressure Resistance)
1.5KV
2.5KV
4KV
6KV

Ⅱ . Choosing nominal discharge current of surge restrain protection device

Protection Grade Junction of LPZ0 area and LPZ1 area  Junction of LPZ1 area and LPZ2 are; LPZ3 are and LPZ3 area
First Nominal Discharge
Current (KA)
Second Nominal Discharge
Current (KA)
Third Nominal Discharge
Current (KA)
Fourth Nominal Discharge
Current (KA)
Nominal Discharge of Direct Current
Current (KA)
A Grade
≥80
≥40
≥20
≥10
≥10
B Grade
≥60
≥40
≥20
In the system of direct current power distribution, it should choose the suitable SPD of the nominal discharge current ≥ 10KA according to the length of line and the voltage of working.
C Grade
≥50
≥20
D Grade
≥50
≥10

Note: 1. All the above is the 8/20 μ s undee nominal discharge current;
2. LPZ0-direct lightning area, LPZ1-first protection area, LPZ2, LPZ3-second, third protection area

Appendix: the affirmation of thunder protection grade for building

　 Thunder and Lighting Protection Grade Electronic Information System
A Grade

1. Large computer center, large communication hinge, national finance center, bank, airport, large port and railway hinge station etc.
2. A grade security system and guard system such as closed circuit television monitoring and warning system of national cultural relic and archives.
3.Large electronic medical treatment equipment and five-star hotel
B Grade

1. Moderate computer, moderate communication hinge, communication base stations, large palestra (gymnasium) and negotiable securities center.
2. B grade security system and monitoring system such as closed circuit television monitoring and warning system of provincial cultural relic and archives.
3. Radar station, microwave station, monitoring and charge system of speedway.
4. Moderate electronic medical treatment equipment.
5. Four-star hotel. 　

C Grade

1. Small communication hinge, telephone office.
2. Small cable TV system
3. Hotel under three-star.

D Grade
 Electronic information system equipment for common use except A, B, C grade.

Ⅲ . Choosing Uc of max continuous working voltage

The max continuous working voltage Uc of surge restrain protection device has relation with the key parameter of working stability of surge protection device. When choosing the max continuous working voltage of surge protection device, it should consider the normal wave and max continuous malfunction voltage in the course of installing power network except the relative criterion.
According to the explanation of IEC61643-2, the max continuous malfunction voltage of phase line and earth line can reach 1.5 times of Uo(220V) in the system of TT cross current power supply, that is to say, it can reach 330V; therefore, it recommends to choose the max continuous working voltage Uc is 385V or 440V module of surge restrain protection device in the area with unstable current.
In the system of direct current, it has no uniform ratio between max continuous working voltage (Uc) and normal voltage Uo; but the ratio should be in the range of 1.5 times and 2 times according to experience.

Ⅳ . Air circuit breaker choosing of surge protection device

Nominal discharge current of surge protection device In(8/20 μ s) Rating current of air circuit breaker
　100KA 　100A
　50KA 　63A
　10-40KA 　25-32A