6.2 Calculate the length of the regeneration section by dispersion

Together with the mentioned above conditions EKD length must satisfy the requirements for dispersion [6].

, (6.4)

where В – transfer rate on the optical interface, bit / s;

– rms linear dispersion of OF, s / km.

Value is chosen as the lowest calculated by formulas (6.2) and (6.4).

Transfer rate on the optical interface is determined according to Table. 4.5 or from the ratio

, (6.5)

6.3 Placement regeneration points along the route of focl

Considerable length of EKD FOCL allows you to place regenerators in localities where there are at least two independent power sources. Placing regenerators made ​​based on budget performance and acceptable dispersion on EKD.

Given the power budget between regenerators FOCK must lie within.

.

Placing regenerators on the route considering the above conditions, it is necessary to determine the length of each EKD. Calculate the margin and dispersion for each EKD on formulas.

. (6.6)

. (6.7)

, (6.8)

. (6.9)

_{This value should be compared with the accepted value , and - with the value [10].}

_{As a result of calculation of lengths EKD a block diagram of FOCL is made up, which indicates non serviced regenerative stations, EKD length, cable type and numbering NRS (Figure 4.1).}

6.4 Constructing a chart levels of energy potential fots at the length of one area of regeneration

Energy potential of FOTS determines energy opportunities of the used equipment for covering the losses in a linear path. The higher the value of EP, the greater the distance on which you can transmit information. Size of EP depends primarily on the radiation power of transmitting optical module (TOM) and sensitivity of optical receiver module (ORM).One can increase the value of EP in the following ways:

1) use TOM laser diodes (LD) instead super fluorescent diodes;

2) apply ORM with better sensitivity (e.g., replace p-i-n photodiode (PD) on the avalanche photodiode );

3) to reduce energy losses when entering it in OF (using microlenses, etc.).

Visual representation of the EP and distribution losses of power radiation in a linear path gives a diagram of the energy levels of the potential FOTS. Chart built for areas of regeneration (Fig. 6.1) in the coordinates: the level of optical power P (dBm), the distance L_p (km). Vertical axis A and B are power level TOM (at point A) and sensitivity ORM (in b), respectively. An A axis contains segments (Р_пер + ), and the axis B - (P_пр+р_з+2α_вп). Along the X axis attenuation of OF is shown as a line with a slope l_буд (dB) for each building length l_буд,, at the junction of building lengths plotted segments, level of attenuation in detachable joints ОК – _н.

You can consider, that FOTS designed correctly, if constructed so diagram of energy levels cross the axis B at a point approximately equal to (or slightly higher) than the value р_з+P_пр.

Chart levels of energy potential (Fig. 6.1) is constructed for FOTS STM-4 with a length = 70 km.

7 Protection of optical cable from the influence of external electromagnetic fields

1. Protection of optical cables from lightning strikes

Methods of calculating the probability of annual damage to FOCL is based on allowable current of lightning in metal surfaces OF, at which there is no damage to the cable and no connection breaks, and the breakdown of external damage to the hose is not considered as the damage of OF. The magnitude of this current depends on the design of OF, as a rule, calculated experimentally[10].

The estimated probable amount of damage per year depends on the number of lightning strikes, which falls on the surface area that is directly subject to lightning or arc that occurs between the point of impact and cable. Chance of apperence of current in underground cables, which can cause it damage is generally found t by the formula [10]:|

, (7.1)

_{where - probabilistic average number of lightning strikes from 1 to 250 kA into FOCL cable, which is projected;}  

_{- rate risk of damage of optical cable with metal elements [11], which is determined from Fig. 7.1;}

– surface coefficient (given in Table 3.2.), which is taken into account when the width of convergence of OC with terrestrial objects is smaller than ;

– maximum radius of the spark zone , m.

Value is calculated by the formula:

, (7.2)

where q – density of lightning strikes per year per km² earth's surface, year/km²;

– conventional radius of the spark zone, m;

L –length of the line, km.

Let us consider L = 100 км.

The value of q is calculated by the expression:

, (7.3)

where С = 0,067 – average number of lightning strikes per 1 км² of ground during one hour of thunderstorm;

Т – average duration of storms in hours.

Conventional radius of the spark zone is defined by the formula:

, (7.4)

where – resistivity of the soil on the route length L of FOCL, Ohm • m;

– breakdown voltage of electric field in the soil, kV / m.

Value varies from 250 kV/m and = 1000 Оm·m till до 500 kV/m and 1000 Оm·m.

To compare the results with the permissible limits of the expected probability of damage of OF with metallic elements it is necessary to calculate for L 100 km from the formula:

, (7.5)

where –norm for the possible number of damage per 100 km cable route for transport FOCL = 0,1.

If , then the protection to the cable is not needed, if this inequality is not satisfied, then apply methods of protection, such as:

• using one or two storm protected cables of type PS-70, or other types;

• replacement of cable by dielectric one

• |.

_{Calculating protection of OF cable is as follows. To evaluate the effectiveness of a rope, factor screening is used} [11].

, (7.6)

where – allowable test pulse current, kA;

–current in the circle "protective rope wire-ground.".

From the value the amount of possible damage to the cable after cable laying of protective rope is estimated, while instead of in calculation of К_р the value is taken.If you need to use two ropes. The coefficient of screening for different types according to [11] cables are listed in Table. 7.1 ... 7.4.

Table 7.1 - Coefficient of screening one copper wire or bimetallic cable with a diameter of 4 mm

The distance between the protective wire and cable , mm	Coefficient of screening one copper wire or bimetallic cable with a diameter of 4 mm
	Diameter of cable jacket, mm
	15	20	25	30
300	0,576	0,596	0,612	0,626
400	0,571	0,590	0,605	0,617
600	0,566	0,582	0,596	0,607

Table 7.2 - Coefficient of screening two copper wire or bimetallic cable with a diameter of 4 mm

Factor of screening or two copper wire or bimetallic cable of diameter of 4 mm and a depth of laying cable 1.2 m and depth of cables laying 0.4 m
The distance between the protective wires b, mm	Diameter of cable jacket, mm
	15	20	25	30
300	0,428	0,443	0,456	0,467
400	0,418	0,433	0,446	0,457
600	0,404	0,419	0,431	0,442
800	0,394	0,409	0,421	0,432
1000	0,387	0,401	0,414	0,424
1500	0,675	0,389	0,401	0,411
2000	0,369	0,383	0,394	0,403
3000	0,363	0,376	0,386	0,395
4000	0,360	0,372	0,382	0,390

Table 7.3 - Coefficient of screening a galvanized steel wire ПС-70

The distance between the protective wire and cable , mm	Coefficient of screening a galvanized steel wire ПС-70
	Diameter of cable jacket, mm
	15	20	25	30
300	0,662	0,685	0,704	0,720
400	0,657	0,678	0,695	0,710

Table 7.4 - Coefficient of shielding of two galvanized steel wires ПС-70

Factor of screening of two galvanized steel wires or bimetallic cable of diameter of 4 mm and a depth of laying cable 1.2 m and depth of cables laying 0.4 m
The distance between the protective wires, mm	Diameter of cable jacket, mm
	15	20	25	30
300	0,518	0,536	0,552	0,565
400	0,506	0,524	0,540	0,553
600	0,488	0,507	0,522	0,535
800	0,477	0,495	0,510	0,523
1000	0,468	0,486	0,500	0,513
1500	0,454	0,471	0,485	0,497
2000	0,447	0,463	0,477	0,488
3000	0,439	0,454	0,467	0,478
4000	0,435	0,450	0,462	0,472