Shocks due to Back feed from UPS into utility line .

[Satish Chaparala]

I am investigating and analysing the fatal shocks which occured in India due to back feed from battery based standby power home inverters - off line UPS. Some cases of line men working getting shocks due to this are reported. Generally in homes these are single phase inverters off line UPS used as a back up to utility power in homes as the weak utility power fails often in India. In these off load UPS used in homes, only phase wire comes out of the UPS and taken to load through a transfer switch
(change over of supply from utility to UPS. The existing neutral of home wiring is used and the utility side input neutral is connected to the output side neutral inside the UPS. I request you to analyse and tell reasons for back feed and remedies

C. Satish, Life SMIEEE
India

 

[tom baker]

This is a NEC forum, your wiring and codes would be different.
Others may answer. but asking us to anayize and tell reasons, that should be done locally by persons familar with your codes and installations.

 

[GoldDigger]

If proper transfer switches are installed, there is no way that energy can be fed back into the power grid through the ungrounded (switched) conductors. If the neutral is not switched, the only way that energy can be fed back into the grid is if there is a problem with the ground reference of that neutral and the ground reference of the grid secondary wires.
An improperly installed backup power source could apply voltage to the neutral relative to the earth/protective ground wire or the local ground electrodes.
If the ungrounded grid secondary conductors have conductivity to ground via connected loads at other sites, then applying a voltage to the neutral alone could cause a voltage across the secondary of the grid transformer which could in turn generate unsafe voltages on the primary of the same transformer.
If the workers did not apply protective shorting jumpers to the primary wires before doing their work they could be exposed to hazardous voltages.

This is just one hypothetical mechanism which requires several simultaneous errors and faults to cause a hazardous condition, but could still happen even with a correctly operating transfer switch.

Any real analysis would, as noted by another member, require a lot more knowledge of the details of the distribution system as well as the workers' standard practices and the likely faults and weaknesses in the system.

I am sorry that we are not really able to help you in your research.

 

[Sahib]

Satish, the utility supply side neutral is usually grounded on the utility side and then there would not be any safety issue as for a 3 pole transfer switch. But when there is no ground or poor grounding for the neutral, people could get shocked by touching it.