# Loss factor shield (screen/sheath)

The power loss in the sheath or screen consists of losses caused by circulating currents and eddy currents, thus $\lambda_1=\lambda_{11}+\lambda_{12}$.

Special cases are

• For cables with non-magnetic armour or reinforcement, the general procedure is to combine the calculation or the loss in the reinforcment with that of the sheath. When a non-magnetic armour is present, $\lambda_1$ is then reduced by the armour losses after the evaluation of $\lambda_2$.
• For single-core cables with magnetic armour, bonded to sheath at both ends, the loss in sheath and armour may be assumed to be appoximately equal, so that $\lambda_{11}=\lambda_2=W_{sA}/2W_c$
• A concentric return cable has a conductor around the core for a return path. These are typically used for single phase systems or for single-core HV AC subsea cable circuits where specially bonded cable systems are not possible. Also DC cables may be of concentric type as is the case in an integrated return conductor cable, where the return conductor is installed around the core conductor.
• Two phase concentric cables are not directly covered by IEC 60287.The suggested method is to assume that the current in the outer conductor is equal to the current in the central conductor. The heat loss in the outer conductor can then be calculated and hence a revised sheath loss factor can be evaluated: $\lambda_1=R_{outer}/R$, where $R_{outer}$ = a.c. resistance of outer conductor at operating temperature. Normally for the outer conductor the AC resistance can be assumed to be the same as the DC resistance as the skin and proximity effect will be negligible. For the inner conductor the proximity effect can be assumed to be negligible although the skin effect will still need to be calculated. Any subsequent metal sheaths or armour layers can then be assumed to have no heat losses as the magnetic field outside the outer conductor generated by the cable will be negligible.

Symbol
$\lambda_1$
Formulas
 $\lambda_{11}+\lambda_{12}$ general case $\frac{R_e}{R_c}$ single-core concentric return cable $\frac{R_{encl}}{R_c}$ single-core concentric return PAC/GIL
Used in
$I_c$
$T_{eq}$
$T_{int}$
$\theta_{ar}$
$\theta_{c,z}$
$\theta_e$
$\theta_f$
$\theta_{sp}$
$W_{encl}$
$W_h$
$W_s$
$Z_d$