BS EN 61663-2-2001 pdf download.Procedure for evaluating the protection needs against lightning of telecommunication or signal lines using metallic conductors.
where
Ub is the impulse breakdown voltage between the conductors and the shield of the cable (Ub is 1,5 kV or 5 kV for paper- or plastic-insulated conductors respectively):
USPD is the impulse breakdown voltage of SPO which can be estimated at 500 V;
T1 is the front time which is assumed to be 10 ts for conductor overvoltage and 1 .LS for voltage drop due to current injected in the shield by the SPD firing;
v is the propagation speed of the surge voltage which is estimated in 200 m/Its.
— the second is the connection to earth of the shield in the transition points Tk. in order to reduce the voltage drop in the cable shield to values lower than the breakdown voltage. The earth resistance value of this earth connection is evaluated in annex B.
Bonding conductors shall withstand the overcurrent induced in the telecommunication conductors I (see annex B) or the part of the lightning current l flowing through them evaluated according to annex C. Therefore the minimum cross-section area Smin of the conductor connecting one or several n SPDs to the EBB is given by the following equations:
where
S,, is in square millimetres (mm2); I, or I in kiloamperes (kA).
In any case, the cross-section area shall not be lower than the total cross-section area of one telecommunication conductor pair and, if one conductor bonding cable is used, it shall not be lower than 1,2 mm2.
7.3 Protective measures against direct lightning to the structure that the telecommunication line enters
At the entrance of the structure, SPDs can be used at the cable termination between conductors and metallic sheath (if it exists) or EBB of the structure. The current which flows into the sheath I and/or the conductors I shall be evaluated using annex C. The probability p(i) related to this current I or I is evaluated using figure 5.
If equation (34) is not fulfilled, then at the entrance of the structure, the additional protective measures indicated in table 3 can be used.
The length of the protective measure I in metres, (i.e. lightning protective cable or metal conduit or reinforced cable ducts or shield wires) is given by the following practical equation:
The conductors coming from the exposed structure should be protected with SPDs at the point where they enter another cable. If the cable is unshielded, the SPOs shall be installed between all the conductors of the cable (see figure 7a); whereas if the cable is shielded, the SPDs can be installed only between the exposed conductors and the shield (see figure 7b). SPDs between the shields, shown in figure 7b. are recommended to reduce the risk of corrosion.
7.4 Protective measures against direct lightning to the telecommunication lines Feasible protection methods include the following.
a) Buried lines
1) Shield wire(s), generally a galvanized steel wire with a diameter of 8 mm. Other materials or diameters are also possible, for example, for corrosion protection problems. The shielding factor q is evaluated by annex C of IEC 61663-1.
2) Lightning protective cable (see 3.43).
3) Steel tube(s), generally a galvanized steel tube. It should be assured that interruptions at cable collars are kept as short as possible: the interruptions should be bridged by a close metal jacket or at least three shield wires by a cage arrangement, each offset by 1200,
4) Lightning protection cable duct (see 3.44).
The protection factors K of these protective measures for buried cables are indicated in table 3.
b) Aerial lines
1) Use of a supporting wire, as air-termination of a lightning protection system dedicated to the aerial cable. As an example, a protection factor K value of 0,3 can be achieved when the supporting wire is connected
• to ground at a distance of about 200 m with the following minimum length of each electrode
— = 5 m for radial horizontal electrode,
or
— = 2,4 m for vertical (inclined) electrode:
• to the metallic shield of the cable, if any, or to the cable conductors through SPDs, If the cable is shielded, SPDs should be installed between conductors and shield at both ends of the shielded section.
2) Substitute all or part of the aerial cable with a buried one and use the protective means Indicated In a) above,
3) Substitute the aerial cable with a non-metallic transmission system, for example, metal-free optical cable or radio link (K = 0).
The reduced frequency of damage F’,b is evaluated using the following equation:BS EN 61663-2-2001 pdf download.