* Copper rises on planned strike in mining powerhouse * Unions criticize worker deaths, use of contractors * Electoral year seen galvanizing union labor action By Felipe Iturrieta SANTIAGO, April 8 (Reuters) - Chilean state miner Codelco's unionized workers said on Monday they would conduct a 24-hour work stoppage at all units of the world's top copper miner on Tuesday for safety improvements and greater job security. The union representing Chile's private miners, which include BHP Billiton, Anglo American and Antofagasta Minerals, initially said it would also strike to halt output from world No.1 copper producer Chile. But union leader Gustavo Tapia said later individual mine unions would decide on their own whether to join. Sources from private mines in the Andean country told Reuters they were likely to see milder labor action, such as one-hour shift delays, as unions sought to avoid legal problems. The effects from the planned strike will be "marginal," Codelco's chief executive Thomas Keller told reporters earlier on Monday. Codelco is seen producing around 1.7 million tonnes of copper this year, or an average of just under 5,000 tonnes per day. A 24-hour strike at all of Codelco's units would cost $35 million, Gustavo Lagos, mining professor at the Universidad Catolica told Reuters. At stake more widely in Chile is a third of the world's copper production. The country is seen producing 5.58 million tonnes of copper this year, or an average of roughly 15,000 tonnes per day. Copper prices rose on Monday, rebounding from falls the previous week, in part as the planned strike raised concerns over temporary supply constraints. "I expect the principal short-term impact to be primarily monetary. There will likely be delays in shipments and settlement payments," said Colin Becker, mining partner with PwC in Santiago. "However, these events are like exposure to the sun. It is not only another sunburn, but rather there are also long-term impacts to the mining sector's health. All this does is make Chile a little bit less competitive." While Chile boasts enviable copper reserves, miners are reeling from steep costs, tumbling ore grades and an uncertain power supply. Extending the strike or adding extra measures "will depend on the response by Codelco and authorities," said Raimundo Espinoza, head of the powerful Federation of Copper Workers umbrella union group. "We're hoping for concrete answers from Codelco with regards to the demands we've made," he said during a press conference. Espinoza, who sits on Codelco's board, also lamented that "more than 70 comrades have died" in Chilean mining accidents since October 2010, when 33 miners were successfully rescued from underground in a globally-broadcast event. Chile's November presidential elections are seen galvanizing labor action as unions seek to make their issues heard. For analysis, see The planned strike will hit just as the copper industry is gathered in Santiago this week for the CESCO/CRU copper conference, the world's biggest gathering dedicated to the red metal. Memories of a shock two-week strike at Escondida in 2011 are fresh, and union activity could prove a growing pain for Chilean mining companies, which are already battling soaring prices, slipping ore grades and energy woes. While stoppages in 2011 often sought a wider share of record copper prices, workers now say they fear being displaced by subcontract workers, losing their jobs, or seeing their benefits cut as companies seek to rein in spiraling costs. Keller said earlier Monday that he does not expect potential labor strife during an electoral year to have any significant impact on copper production. Codelco has restarted its copper shipments following the end of an extended port strike, though it will take roughly a month for a normal rhythm to resume, he added. Most ports in Chile returned to normal operations on Saturday after the extended strike, which hurt the world's top copper producer's exports. Key ports in the mineral-rich North are operational.