Hello morph-x

I see you are located in the Philippines, and see your Mains Voltage is 110/220V @ 60Hz.

I do not know whether your system is MEN (Multiple Earthed Neutral), but presume that you normally have either one or two phases, plus a neutral - with the Neutral being Earthed at the transformer.

Is the Earth connection also made at each house?

The Earth Connection of a properly isolated UPS should "carry through" to the mains earth connection.

There is a typical "wiring Diagram" of a commercially available UPS here: http://www.ics-timers.com/ups_wiring.html

Note that all the earth connections are connected through to "Mains Earth Wiring".

However, if you use a "Line-Earth-Loop-tester", which gives a half-cycle pulse of many amperes between phase and the earth wire, you risk damaging some of the electronics inside the UPS.

With care, however, the Earth loop impedance may be done by running a cable of known resistance from the measuring meter, to the earthed connection at the switchboard.

Important:

Using a meter with a very low testing voltage, measure the loop resistance from the UPS earth outlet pin, back to the switchboard, thence via that "measured resistance" cable, back to the meter.

You should obtain a low reading, perhaps around 1.0 ohms.

Deduct the resistance of the "measured resistance" cable, (perhaps 0.6 ohms) and the result would thence be 0.4 ohms, for the actual resistance to the switchboard earth connection.

If you have an earthed stake on the premises, which is connected to the switchboard earth point, instead of running the "measured resistance" cable to the switchboard earth, run to the actual earth stake connection, and do the simple sum to obtain the correct result.

Note that the above test is done at DC, and if any inductor is present in the earth wiring, the results at Mains Frequency may be quite different.

I have always preferred the "Line-earth-loop-tester" which fires a ^1/₂ cycle of high current via an SCR, to very quickly obtain a result at Mains Frequency, but this is inadvisable if others are around, because if there is a fault in the earth connection, earthed metal may rise to Mains voltage for the duration of the ^1/₂ cycle pulse, and/or damage equipment of an electronic nature.

Trust that assists you.

If needing further help, please

Kind Regards....