Exhausted engineers attached a power cable to the outside of Japan's tsunami-crippled nuclear plant yesterday. The operation raised hopes that it may be possible to restart the pumping of water into the plant's stricken reactors this weekend and cool down its overheated fuel rods before there are more fires and explosions.  "We have connected the external transmission line with the receiving point of the plant and confirmed that electricity can be supplied," said a spokesman for the plant's operator, Tokyo Electric Power Co, in a statement.  However, officials said further cabling would have to be completed before they made an attempt to restart the water pumps at the Fukushima plant, 150 miles north of Tokyo.  It was also reported that health workers had detected radiation levels above safety limits in milk and spinach from farms in Fukushima and in neighbouring Ibaraki, although it was claimed they represented no risk to human health. Officials have asked people living near the plant to follow basic safety advice when going outside: drive, don't walk; wear a mask; wear long sleeves; don't go out in the rain.  Radiation levels in Tokyo were also said to be within safe limits. Nevertheless, the city has seen an exodus of tourists, expatriates and many Japanese, who fear a blast of radioactive material from Fukushima.  at the nuclear plant yesterday, firefighters continued to spray water to cool the dangerously overheated fuel rods in order to keep cores in its reactors from overheating and melting. The UN's atomic agency said yesterday that conditions at the plant remained grave but not deteriorating badly, following Japan's decision on Friday to raise the severity rating of the nuclear crisis from level 4 to level 5 on the seven-level international scale. It put the Fukushima fires on a par with the Three Mile Island accident in Pennsylvania in the US in 1979. The explosion at Chernobyl in 1986 - which sent a plume of radioactive material into the skies 25 years ago - is the only incident to have reached level 7.  Fires and explosions occurred at four of the six reactors at Fukushima last week after the 8.9 Richter earthquake and the ensuing tsunami that hit Japan on 11 March. The earthquake triggered an automatic shutdown of the three reactors that were in operation. The tsunami then damaged diesel generators that were providing back-up power for the pumps driving coolant through these reactors.  as a result, heat could no longer be pumped away and temperatures inside the reactors' cores began to rise, eventually setting off a series of chemical fires. "Hollow rods made of zirconium hold each reactor's uranium fuel pellets in place," said Professor andrew Sherry, director of the Dalton Nuclear Institute in Manchester. "When temperatures rise too much, that zirconium starts to react with the reactor's water. This chemical reaction raises temperatures even further. Hydrogen is also produced. When this hydrogen exploded, it destroyed the buildings that act as each reactor's outer protective shell."  The explosions also damaged two storage tanks in which fuel rods - still hot because of the radioactive material inside them - were being stored in water. Water levels dropped, exposing fuel rods and triggering further chemical reactions between zirconium fuel cladding and the steam that had begun to build up. These set off fires in storage tanks at reactors three and four.  as a result, plant workers, emergency services personnel and scientists have been battling for the past week to restore the pumping of water to the Fukushima nuclear plant and to prevent a meltdown at one of the reactors. a team of about 300 workers - wearing masks, goggles and protective suits sealed with duct tape and known as the Fukushima 50 because they work in shifts of 50-strong groups - have captured the attention of the Japanese who have taken heart from the toil inside the wrecked atom plant. "My eyes well with tears at the thought of the work they are doing," Kazuya aoki, a safety official at Japan's Nuclear and Industrial Safety agency, told Reuters.  Little is known about this band of heroes, except for the few whose relatives have spoken to the Japanese media. One woman said that her father, who had worked for an electricity company for 40 years and who was due to retire in September, had volunteered. "I feel it's my mission to help," he told his daughter.  On Wednesday, the government raised the cumulative legal limit of radiation that the Fukushima workers could be exposed to from 100 to 250 millisieverts. That is more than 12 times the annual legal limit for workers dealing with radiation under British law. Each team works as fast as possible for the briefest of periods. The pilots of the helicopters used to "water-bomb" the plant have been restricted to missions lasting less than 40 minutes.  Nevertheless, the workers have not only managed to link a power cable to one of the plant's reactors, No 2, but they have also connected diesel generators to the No 5 and No 6 reactors, which have so far not suffered serious damage. "If they are successful in getting the cooling infrastructure up and running, that will be a significant step forward in establishing stability," said Eric Moore, a nuclear power expert at US-based FocalPoint Consulting Group. However, the government has conceded that it was too slow in dealing with the crisis at Fukushima. Chief cabinet secretary Yukio Edano said that "in hindsight, we could have moved a little quicker in assessing the situation and co-ordinating all that information, and provided it faster".  The fires at Fukushima have also triggered serious criticism of the plant's design. The decision to place storage tanks close to reactors has been pinpointed as a key design error. When those reactors caught fire, they quickly triggered reactions in the storage tanks which themselves caught fire, and so the fires spread.  In addition, the failure to build defences that could withstand the huge tsunami that struck Japan has also been attacked. "The geological evidence in Japan indicates a history of giant tsunamis over the past several thousand years," said Professor Rolf aalto, an Exeter University expert on tsunamis. "Unfortunately, an engineering and political decision was made to design protection and plan cities around a hypothesized five-metre tsunami - about the size of those experienced in Japan over the last century. However, it was not a surprise to geologists that a tsunami two to three times larger appeared. Both the earthquake and tsunami were exceptional, but were both well within the realm of what can occur within that tectonic setting."  However, Professor Sherry defended the ageing plant - whose six reactors came on line between 1970 and 1979. "These reactors were designed in the 1960s and we have learned a lot since then. Modern plants are much safer. Think of cars in the 1960s: they didn't have crumple zones, airbags or seat belts - features we all take for granted today. It is the same with nuclear reactor design."  The Fukushima reactors, known as boiling water reactors, have active safety features - you have to do something to prevent dangerous heating, such as ensuring that the pumps are activated.  "By contrast, new reactors are designed to include 'passive' safety systems that are