Waterford — Ballots cast in a union election at the Millstone Power Station this week have been taken to the National Labor Relations Board office in Hartford and will not be counted until the board rules on an appeal to a decision about which workers should be eligible to vote.

About 420 of the 1,080 workers at the nuclear power plant were deemed eligible to vote to decide whether they wanted to be represented by the International Brotherhood of Electrical Workers Local 457 by a May 5 ruling of the labor board, Scott Burson, regional attorney for the labor board’s Boston office, said Friday. The union, however, appealed the ruling, arguing that about 350 plant workers should be eligible to decide whether to join the union.

Dominion, owner of Millstone, also requested a review of the ruling, contending that about 800 workers should have been eligible to vote, said Ken Holt, Millstone spokesman.

When there is an appeal of this type, Burson said, the labor board proceeds with the election and seals the ballot box until it decides on the appeal. If it dismisses the appeal, the votes would be counted and certified, he said. If it finds merit to the appeal, the election would be deemed invalid and a new vote scheduled, he said.

Typically the board takes at least six to eight weeks to decide on an appeal, he added.

John Fernandes, business manager for IBEW 457, said the union believes that union membership should be limited to the “hands-on types” at the plant — workers who maintain plant operations on a daily basis, such as the production operators, maintenance mechanics and electricians. In the NLRB’s May 5 ruling, some of the workers the union sought to include, such as radiation technicians, were excluded, while others were added against the union’s wishes, he said. The range of workers the union wanted included in the vote was similar to those eligible for the union at Dominion’s two nuclear power plants in Virginia, he said.

While it is disappointing to have to wait for the outcome of the vote, Fernandes said, “it was expected.”

Holt said the company believes workers in engineering, procedure writing, chemistry, radiation protection and supply chain areas should have been included in the vote.

NLRB staff was on site Wednesday and Thursday as the workers cast ballots. Fernandes said he did not know how many people voted.

Dominion has been urging workers to vote against the union in a campaign that included DVDs sent to the homes of each of the 420 workers eligible for the vote, Holt said. On the DVD, managers testified about why they believed a union “would not be the best option for the employees, and why we wanted to continue to work individually,” Holt said. The company also hired consultants to meet with workers to persuade them against the union, he said.

“Since there is a potential for the vote to be rerun,” Holt said, “the campaign continues.”

Fernandes said many workers were “put off” by the DVD and some of the company’s anti-union tactics.

“They haven’t been bashful about spending money” to keep the union out, he said.

Main issues for workers supporting the union include pay disparities between hourly workers, securing contract protections in case Dominion sells Millstone to a new owner, outsourcing of jobs, consistency in overtime assignments, and pension and retiree benefits, among others.