Members of the Prospect trade union, who operate the electricity transmission system in NI, have announced a 24-hour strike on 15 April.
Prospect said SONI staff had “voted strongly in favour of the action up to and including strike”.
Prospect said workers were not taking action that would lead to electricity blackouts at this point.
SONI said the action was “extreme, unnecessary and avoidable”.
One of the issues which led to the strike ballot was a dispute over pay increases.
Prospect said it expected an impact on operations and costs for third parties.
‘Trading on goodwill’
“Our members will be there to deal with emergencies and ensure there’s no risk to life,” she said.
Angela Moffat , Prospect negotiations officer, said SONI (System Operator for Northern Ireland) could not “expect to keep trading on that goodwill”.
“If they don’t try to work with us to find a solution, the industrial action will escalate further,” she added.
She also claimed SONI had “refused to budge, despite a proposal from Prospect for an under inflation pay rise to be made”.
“All of our members with the exception of minimum emergency operations cover will now initially strike for one day, but more could follow,” she said.
“We expect an impact on maintenance and operations and costs for third parties like NIE Networks too.”
SONI general manager, Robin McCormick, said it remained “open and willing to engage with the union”, as it had “been doing over the past 12 months”.
He said SONI was “calling on Prospect to withdraw its notice for action and to return to talks”.
“While SONI will do its utmost to maintain core services and security of supply as a priority, this escalating action by Prospect means that we will be operating the NI power system with a greater level of risk,” he added.