JOINT DEVELOPMENT AND NOISE FREQUENCY INDUCTION 193 



3. The use of selective devices such as reactors or anti-resonant 

 circuits commonly called "wave traps" in the generator neutral for 

 suppressing the disturbing triple-harmonic components. 



Xon-triple harmonic residual voltages and currents may exist from 

 differences of phase-to-neutral load impedances and from differences 

 in the capacitances to ground of the three phase wires. 



In multi-grounded neutral four-wire systems differences in the single- 

 phase loads connected between the individual phases and the neutral 

 may be important sources of residual current. A considerable measure 

 of control may be exercised by restricting the size of single-phase areas 

 and balancing the load on the different phases. 



Capacitance unbalance to ground may be due to single-phase 

 branches on three-phase distribution systems. Usually, the more 

 important effect is that on the single-phase branch where the residual 

 voltage is practically equal to the phase-to-neutral voltage. The 

 unbalancing effect on the three-phase system may be minimized by 

 equalizing the lengths of the branches connected to the several phases. 

 The residual voltage on the single-phase branch can, where necessary, 

 be eliminated by the use of isolating transformers or by converting to a 

 three-phase branch. 



Capacitance unbalance may also be due to dissymmetry in the ar- 

 rangement of the wires of the circuit to each other and to ground. 

 These unbalances are lowest in triangular configurations of the wires 

 and largest when all the wires are in the same vertical or horizontal 

 plane. With multi-circuit lines, a considerable measure of control 

 may be obtained by suitable phase interconnection of the circuits. 

 Transpositions are also effective in controlling these unbalances. 



CoupUng Factors.— T\iQ coupling between power and communication 

 circuits is, of course, determined by the degree of their proximity, but 

 it may be greatly modified by the balance of the two classes of circuits 

 to each other and with respect to ground. While the most direct and 

 certain method for reducing coupling is to avoid proximity, means 

 are available for minimizing the coupling where necessary. 



The work on coupling of the Joint Subcommittee on Development 

 and Research, in the voice and carrier-frequency range, has been 

 directed toward two objectives: (1) development of improved methods 

 for predetermining the coupling to be expected in new cases of expo- 

 sure, and (2) development of improved methods for reducing coupling 

 for given degrees of proximity. 



Several years ago the California Joint Committee on Inductive 

 Interference ^ completed an extensive series of computations on coeffi- 

 cients of induction which were expressed in the form of curves for 



