KOCHI: As the monsoon picks up momentum, electric posts and power lines pose a threat to the public and electricians. Statistics shows that 74 people died of electrical mishaps in Thiruvananthapuram during 2009, 2010 and 2011 while 60 were injured, according to figures tabled in the Assembly.
The victims included staff and contractors of the Kerala State Electricity Board (KSEB), and the general public.
Why do overhead lines and other electrical installations tend to create electric shock hazards even when the mains supplies are switched off for taking up repair work?
Standby power supply equipment, generator sets, UPS and inverters and offline
UPS are vulnerable to backfeeding their power supplies to the main supply that leads to electric shock hazards and secondary effects to the people working on overhead lines and doing connected work.
*When a work is being carried out on overhead lines of a pole after removing section fuses or switching off the AB switch of the respective distribution transformer and if the the neutral conductor is opened, there is a chance of electric shock from an offline UPS or inverter installed under the distribution transformer area in such a way that the standby power supply unit starts functioning because of the interruption of the mains supply. One end of the unit comes through the neutral conductor and the other end through the earthed pole( where the magnitude of the shock varies with respect to the magnitude of the earth leakage/earth fault of the equipment connected to the standby supply or the type of standby unit itself). As a secondary effect, the worker falls down.
Likewise, there are several chances of back-feeding power supply from the standby power supply equipment such as generator, inverter and UPS.
When a work is being carried out on switch board or distribution board of a building where generator, UPS or inverter is installed, the standby power supply unit starts functioning because of the interruption of mains supply (isolating the mains supply both phase and neutral by switching off the MCB or main switch) then there is a chance of electric shock.
■ Some Precautions
There are always chances of backfeeding from standby power supply units. Any breakage of neutral conductor, wire of termination will provide a path for the standby power supply and cause electric shock to the worker.
Before working on overhead lines, be sure that the neutral conductor is properly earthed. Adhere to the standard method of earthing of neutral conductor at one of every six poles. It will be safer if the neutral conductor is earthed at each and every pole where service connection is provided.
Make sure that offline UPS installed is pulled out the mains input pin from the mains supply sockets. In case of generator, inverter or UPS installed in the building; check out the changeover switch and ensure that both phases and neutral are isolated from mains properly.
■ Protection against backfeeding of standby power supply
■ UPS should be given a protective unit that is capable of providing isolation between the mains supply and the standby power supply. Likewise automatic or manual changeover should be installed wherever generator, UPS or inverters are installed and the changeover switch and relays should be checked periodically to ensure that there is sufficient isolation between the standby and the mains supply. For this, I already have designed two protective units (one for providing isolation between mains supply and offline UPS and the other for mains supply and generator, UPS, or inverter. The latter unit has been installed at the CUSAT administrative building in 2006.
S P Prakasan (The writer is retired assistant engineer Electrical (Cusat) and available at 9746688677