At first glance, the piece of metal in Steve Myers's hands could be taken for a harmonica or a pen. Only on closer inspection can you make out its true nature. Myers, director of accelerators at the Cern particle physics laboratory outside Geneva, is clutching a section of copper piping from which a flat electrical cable is protruding.

It looks unremarkable. Yet a piece of cable like this one was responsible last year for the world's most expensive short-circuit. More than £30m-worth of damage was done to the Large Hadron Collider (LHC), the most advanced particle accelerator ever built, a few days after its ceremonial opening. It has taken Myers – and hundreds of other Cern scientists – more than a year to pinpoint the guilty piece of cable and repair the wreckage. "It was a very small piece, but it did immense damage," he said. It remains to be seen whether Myers can fix Cern's tattered technological reputation in the process – when his team restart their great machine in a few weeks. "I am not a nervous person," said the 63-year-old Belfast-born engineer. "And that is probably just as well."

The LHC had been inaugurated at 9.30am on 10 September 2008 to a barrage of global media attention. This was the God Machine that would unravel the secrets of the universe, it was claimed. Beams of protons, one of the key constituents of the atom's nucleus, were successfully fired round the machine's subterranean 18-mile circular tunnel under the Jura mountains outside Geneva.

Over the following weeks, it was predicted, scientists would recreate conditions that existed a trillionth of a second after the universe's birth and start making sensational discoveries as they smashed beams of protons into each other.

Discoveries would include the God Particle, a tiny entity also called the Higgs Boson, which is believed to give objects – including people – their mass. In addition, dark matter, a mysterious, invisible form of matter that permeates the universe, would be uncovered, along with a host of other revolutionary discoveries.

"It was all looking so good," said Myers. Then, at 11.45am on 19 September, things went spectacularly wrong. Faulty soldering in a small section of cable carrying power to the machine's huge magnets caused sparks to arc across its wiring and send temperatures soaring inside a sector of the LHC tunnel.

A hole was punched in the protective pipe that surrounds the cable and released helium, cooled to minus 271C, into a section of the collider tunnel. Pressure valves failed to vent the gas and a shock wave ran though the tunnel.

"The LHC uses as much energy as an aircraft carrier at full speed," said Myers. "When you release that energy suddenly, you do a lot of damage."

Firemen sent into the blackened, stricken collider found that dozens of the massive magnets that control its proton beams had been battered out of position. Soot and metal powder, vaporised by the explosion, coated much of the delicate machinery. "It took us a long time to find out just how serious the accident was," said Myers.

A 400-metre chunk of the £2.5bn device had been wrecked, it was discovered. Worse, when scientists traced the cause to a tiny piece of soldering, they realised that they would have to redesign major parts of the collider's entire safety systems to prevent a repeat event. That has taken more than a year to achieve.

Now Cern scientists have begun firing protons round one small section of the collider as they prepare for its re-opening. Over the next few weeks, more and more bunches of protons will be put into the machine until, by Christmas, beams will be in full flight and can be collided.

The LHC will then start producing results – 13 years after work on its construction began.

"There was so much expectation that we were about to make great discoveries last year and then the accident occurred," said Cern researcher Alison Lister. "Morale was very low when we found out just how bad it was. However, we should now be getting results by Christmas, and you couldn't get a better present than that."

When fully operational, the LHC will soak up 10 times more power than any other particle accelerator on Earth, consuming 120 megawatts of electricity – enough for an entire Swiss canton – to accelerate bunches of protons, kept in two beams, each less than a hair's breadth in diameter, to speeds that will come "within a gnat's whisker of the speed of light", according to Myers.

One beam will circulate clockwise, the other anti-clockwise. Then, at four points along the collider's tunnel, the beams will cross.

Bunches of protons – each containing 100bn particles – will slam into other oncoming bunches, triggering collisions that will fling barrages of sub-atomic detritus in all directions.

These explosive interactions will form the core of the great collider's operations and will generate new types of particle, including the Higgs, that will pop fleetingly into existence before disintegrating into a trail of other sub-atomic entities. New physics will be uncovered with Nobel prizes following in their wake. And that is not all, say sceptics. They argue that miniature black holes will be created and one of these could eventually grow to swallow up the Earth. The LHC would then not only be the world's biggest experiment – but its last. This fear has led protesters to make legal attempts to close down the LHC, one even making it to the European Court of Human Rights. All have failed, though one case – in Germany – has still to be resolved.

Even stranger is the claim by another group of physicists who say the production of Higgs bosons may be so abhorrent to nature that their creation would ripple backwards through time to stop the collider before it could make one, like a time traveller trying to halt his own birth.

"All Higgs machines shall have bad luck," said Dr Holger Bech Nielsen of the Niels Bohr Institute in Copenhagen. Thus the cable meltdown that afflicted the LHC was an inevitable effect of the laws of time, a notion that leaves most Cern scientists scratching their heads in bafflement.

In fact, the real problem facing the LHC is simple. It is a vast device the size of London's Circle Line but is engineered to a billionth of a metre accuracy. Ensuring that no flaws arise at scales and dimensions like these pushes engineering to its absolute limits.

Cern almost succeeded last year. Now it is convinced that it has got it right this time. "All I can say is that the LHC is a much safer, much better understood machine than it was a year ago," said Myers.

Most physicists believe he is right. "If it works, we will have built the most complex machine in history," said one. "If not, we will have assembled the world's most expensive piece of modern art."