The dispute between the HSE and the TEEU has been simmering for some time and has been before the Labour Relations Commission on a number of occasions.
The HSE says the union's members won't agree to a protocol which would allow staff other than electricians to change light bulbs.
The TEEU however says this is not the issue. It claims it is about pay and conditions for call-out and standby arrangements for its members, and the fact that the HSE is employing contractors to carry out work for less than the registered employment rates.
TEEU members are also angry they have been denied pay increases due since 2003 under the parallel benchmarking process, as well as increases due under the Towards 2016 agreement.
The HSE says TEEU members withdrew from providing on-call cover except in life-or-death situations to acute hospitals in the south and southeast from July.
In an escalation of the dispute on Tuesday, TEEU members, according to the HSE, refused to carry bleeps, answer pagers/mobile phones and it suspended them from the payroll.