THE sudden cessation of the supply of electricity over London and part of the south-east of England during midday on July 29 proves that even with the best machinery a breakdown in the supply is a possibility that has to be guarded against. Luckily it is an extremely rare occurrence. The trouble started apparently when the engineers were re arranging sections of the supply at the Battersea power station. A small section becoming overloaded, the automatic circuit breaker came into action. When the circuit breaker was closed the currents in two of the sections were probably not in synchron ism and so a huge current circulated in the link connecting Battersea with the neighbouring station at Deptford West. The former had an output of 70,000 kilowatts at this moment and the latter of 90,000. The devices at Battersea declined to take the short circuit load, and several of the machines at Deptford shut down. The Barking Power Station had now to take the load, but the circuit breaker at Northfleet opened and the whole system ceased to operate. The effect was that the whole of the south-east area of the grid was suddenly deprived of 280,000 kilowatts of generating plant. The stations at Norwich and Brighton cleared themselves from the grid, the latter for about an hour. This affected the Southern Railway. The trouble was probably caused by the engineers taking advantage of the light load in summer to cut out certain transmission connexions for overhauling and so the grid system was not in full commission. It was not able to face the loss of Battersea, Barking and Deptford and still keep the whole system working. Notes on this shut-down are given in the Electrical Times of August 2.