Germany's environment minister Sunday announced a check of electrical systems at nuclear power plants after an incident forced one of them to be taken off the electricity grid on Saturday, according to dpa. Sigmar Gabriel said the fault at the Kruemmel reactor near the north German port of Hamburg required "an examination of the electrical systems in all nuclear plants." A short-circuit in a transformer triggered the automatic shutdown of the power plant, which had only reopened two weeks earlier after a two-year closure caused by fire. Swedish energy giant Vattenfall, which operates the plant, was forced to apologize Sunday for failing to inform authorities quickly enough about the latest disruption. The shutdown knocked out around 1,500 of the 1,800 sets of traffic lights in Germany's second-biggest city. Thousands of homes were left without water because there was no electricity to drive pumps. Vattenfall said it was investigating what caused the short-circuit, which came as the reactor was slowly being powered up again, following the June 2007 blaze. Opponents of nuclear energy demonstrated outside the Kruemmel plant on Sunday, calling for it to be shut down completely. There are 17 nuclear power plants in Germany, producing around a quarter of the nation's electricity. Germany's Social Democrat-Greens government decided in 2000 to mothball the reactors by 2020, but Chancellor Angela Merkel's conservative party has been calling for a review of the decision.