40 Power Technicians for The London Underground (LUL) are expected to strike until Tuesday 22 July.
The employees, based at the power control room in Blackfriars Road, have been on strike from 1 July over unfair pay and worsening conditions.
Unite the union is urging LUL management to return to discussions at Acas.
The union has written to the Office of Rail Regulation (ORR) challenging LUL management.
Unite regional officer Hugh Roberts said: "Our members have voted to continue strike action to 22 July and we would urge LUL management to get around the table at Acas to achieve an equable settlement.
"Our members are monitoring the safety issues very carefully. For example, last Saturday (5 July) on the Metropolitan line, inexperienced and poorly trained staff switched off the wrong item of plant that could have caused the loss of electrical supplies to stations and signals at the top end of the Metropolitan line.
"These safety concerns will mount as the strike goes into a third week and this, in itself, should be enough incentive for the management to start negotiating in a constructive fashion."
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