Hi,
Thanks for everyone's reply, sorry if my question is not clear.
The step down transformer (240 to 110VAC) is supplying power to the 110VAC UPS. The UPS will then use to backup power the PC & some field devices.
The transformer 110VAC output only has 2 pin (0V & 110VAC), the transformer incoming 240VAC has 3 pins (Life, Earth & Neutral). Life & Neutral will be going into the coil, the Earth will be grounded to the transformer casing (body).
Our UPS (110VAC) has 3 pins, so obviously it cannot directly plug into the transformer 110VAC output. We drill a hole on the transformer 110VAC output socket, on top of the 0V & 110VAC pins, so that the UPS 3 pins can go into the 110VAC socket & the UPS Earth pin is shorted with the transformer incoming 240VAC Earth.
1) I've tried connecting the step down transformer (500VA) into the UPS without any load, once I press the ON button on the UPS, my office CB also tripped. Same thing happen in my customer office.
2) We also tried connecting an extension (no load) into the transformer, it also tripped our CB.
3) We also tried to wrap the Earth pin of the UPS with insulator tape and plug in into the transformer, it's fine, no tripping, but all the devices & PC powered by the UPS are not grounded, so we got electricity short when we touch them!!
4) We tried to power up the UPS with another transformer at my office. This transformer 110VAC comes with 0V, 110VAV & Earth pin. It fitted the UPS socket nicely! When we power up the UPS, everythin's fine! No tripping the building CB. It's a 750VA step down transformer.
So, is it possible the low VA (500VA) of the transformer that cause the tripping problem?