In the event of a power loss during a science run, the APC Smart-UPS (Model: 1500, Max Configurable Power: 980 Watts/1440 VA, Mfg. Part: SUA1500R2X122) will continue powering the Pre-Stabilized Laser (PSL) to prevent laser damage. The UPS (Uninterruptible Power Supply) draws for its transformer a large amount of current, so we measured the device's magnetic field at various distances to determine the 'minimum distance' --  how close the UPS can be to other equipment, especially the interferometer.

We defined the maximum allowable magnetic field at 60 Hz from any electronic device to be 0.4 nT, which is one tenth of the root mean square 60 Hz magnetic field during iLIGO science runs.

For magnetic field measurements, we used two 500 Watt lights (to simulate the PSL load), a Bartington magnetometer (mounted on a tripod), and the UPS (placed horizontally and face-up on a plastic bin).

During preliminary tests, we found that the system's on/plugged-in and off/plugged-in configurations produced similar magnetic field magnitudes. Therefore, the UPS must be placed at its minimum distance whenever the device is plugged-in (and either on or off).

Next, at 1m, we measured the magnetic field at three different angles relative to the physical center of the device. Of these measurements, The lowest value was 55% of the highest value. The magnetic field at 60 Hz was strongest when the magnetometer was aligned with the device's front (face with buttons) left edge, and the remaining measurements were taken at this angle. 

The final step in determining the minimum distance was measuring the attenuation of the device's magnetic field with distance. To minimize noise caused by surrounding electronic devices, I collected data in the VEA of End X instead of in the LVEA. I took data from 3 to 39 feet, with a constant interval of 3 feet. At each distance, I used Diagnostic Test Tools (DTT) to record the power spectra of the magnetic field when the UPS was unplugged and off and when the UPS was plugged-in and on. I took two data sets--both were numerically similar.

The data was slightly noisy, so I used Grace to perform a linear regression on, at 60 Hz, the natural log of the magnetic field  vs. the natural log of the distance. I found the fitted curve to be described by:
 
y=(170.579)x^-1.739

In conclusion, when y is 0.4nT (see above), x is about 33 ft -- the minimum distance.

The attached plot shows the UPS magnetic field attenuation.

Here is a link to the UPS at the LIGO Hanford Observatory: http://www.apc.com/resource/include/techspec_index.cfm?base_sku=SUA1500R2X122

Christina Daniel, Robert Schofield