stems can enable the largest fault SO called valve sealed recombination lead acid battery. It shows current to be drawn without an unacceptably high voltage disturb- promise of being far lighter and smaller than present cells, albeit that ance. Fault currents can of course be limited by the use of fast acting much development work will need to be carried out for it to meet its fuses and the smallest practical conductor sizes. considerable potential. The maximum noise on telecommunications power supplies used Work generally in competing cells is a process of continuing to be simply specified as 2mV weighted to 800Hz when related to development, and progress with the production of international a psophometric curve as defined by the CCITT 1960 (New Delhi) standards is receiving increasing attention. Such standards will have Standard (Howard, 1979). The weighting factors are set out in the obvious advantages of uniformity of electrical performance and CCITT Recommendation P53. The more recent recommendations physical compatibility where they are adopted. IEC International for 48 and 60 volts d.c. supplies for instance, are far more precisely Committee TC21, WG3, is for instance responsible for the interna- defined to take account of narrow and wide band noise. These tional standards of lead acid batteries (Harrison, 1985). recommendations also cover electromagnetic radiations emitted into the immediate surroundings. Electromagnetic compatibility is a factor which must be taken into account in all equipment and Power supply distribution distribution design, and IEC International Special Committee on Radio Interference (CISPR), and the Conference of European Posts The d.c. system distribution is not simply a function of providing and Telecommunications, (CEPT), as well as CCITT, are involved conductors adequate to handle the current involved. For the most in the promulgation of recommendations concerning this topic part the voltages involved are about 50 volts or less, and clearly the (Wilson, 1984; Wilson, 1985; Scuk