But is there a "Rule of thumb" distance to keep in mind?
This is not a "
rule of thumb" but is a ROW (
right of way) in most cases and in most areas:
high voltage transmission lines regardless of size have a 30m (100ft) ROW - meaning to be safe you should be flying and calibrating beyond that ROW.
All electricity transmission lines produce a magnetic filed - of course, dependent of amount of voltage being carried by them and the conditions (
damp or dry day) the interference and range of same may vary. Let's face it, when you are in an area with very high EMI it is no different than standing on a large magnet while trying to calibrate your Phantom - not something any of us would consider doing.
The very high voltage transmissions lines (
atop high metal towers) can be in the range of 300-500kv+ and the magnetic field surrounding them can be very significant and has been measured out and into the 50m range (165ft.) - also dependent on shielding provided for the lines (
most high voltage lines offer little or no shielding). The EMI from those does drop quite a bit around the 20m or so (65ft) mark (
not to be interpreted as being a safe area at all with regard to flying or calibrating).
More common high voltage transmission lines on towers where I am are in the 230kv range and obviously don't have as large a magnetic field surrounding them, hence less EMI - but still do pose an issue as to same. In fact in my business where I do vibration testing and IR studies, I have noticed on my vibration analyzer FFT plot - a significant peak at 60hz and 120hz frequencies just by holding the attached vibration accelerometer in mid air! Walking away from the area and observing "
live time" on the plot the peak is reduced in value and disappears the further I walk away (
shielding the accelerometer also reduces the value at those frequencies).
I have been testing the resistance and/or the effect of EMI around high voltage towers/lines on the Phantom as I plan on adding an IR camera to an Inspire 1 Pro or the Matric 100 as soon as the Flir XT camera is available (
or I should say, I hope to). Having the ability to shoot for "hot spots" on any power lines (
transmission or distribution lines) and/or transformers would be a great addition to the business I do now.
IMO I would never fly
under any hydro lines high voltage or not, for two reasons: one, high voltage EMI has been measured to be higher under these lines due to distance to the ground and less dissipation of EMI as compared to above or around them where the effect is less due to higher dissipation of same (
NOTE: I am not saying here it is safe to fly around or above in near proximity here at all) . Secondly, if I lost communication or whatever with the Phantom and it went into the RTH stage - it very well could fly up and into the wires on its own (
high voltage transmission or even just neighborhood distribution lines)!
Interesting to note here as to EMI and the effect from same: Walking under high voltage lines (
especially on a damp morning or day) you will see and feel the hair on your arm stand up. If one takes a close look at water droplets on blades of grass you can often see them dancing around from this same effect. Depending on the voltage lines, time of day and conditions, people have held a 4ft fluorescent light at ground level under the power lines and sometimes see it flicker - pretty cool, but also very telling!
EDIT:
Disclaimer, I am not in any way shape or form, advocating, encouraging or accepting any responsibility here for anyone with regard to high voltage towers to go out and do some "trial and error" flying and/or calibrating at or near high voltage towers or lines, or even to walk under them or test out the effect of same on oneself or the fluorescent bulb effect from same. We all have to identify and obey whatever the ROW is for high voltage or any power lines be they transmission or distribution regardless of voltage when it comes to our UAV's. 
