The residual currents generated by some lights can be of sufficient magnitude to trip a GFCI without the aid of transient leakage currents. For instance, the harmonics drawn by non-pfc HMI ballasts can be a large source of residual current. If a HMI ballast is not power factor corrected it draws a harmonically distorted current waveform. In addition to snubbing the switching noise of transistors to ground, an RF filter will also snub the high frequency harmonics drawn by the ballast - significantly increasing the residual current generated by the light. For instance, some non-power factor corrected 1200W HMI ballasts will draw 1.15mA as soon as you throw its breaker to power it up, and an additional 15.32 mA after you strike the light, for a total residual current of 16.47 mA (which is why these lights are guaranteed to trip inexpensive hardware store style GFCIs and wall receptacle GFCIs.)
The reason these lights generate such high residual current is that non-power factor corrected ballasts are a type of Switch Mode Power Supply (see simplified schematic below) that first converts the AC waveform of the supply to DC current by means of smoothing capacitors. It then employs IGBTs to switch the DC to a square waveform used to ignite and maintain an arc in the lamp (the purpose of the square waveform is to eliminate flicker appearing on the exposed film.)
A simplified schematic of a non-pfc electronic HMI ballast (Courtesy of Harry Box)
Since its smoothing capacitors only draw current when they charge at the peak of the rectified voltage, the ballast draws a pulsed current consisting of abrupt peaks that are rich in harmonics (see waveform and corresponding FFT reading below.)
The waveform and FFT of current drawn by a 1200W non-pfc electronic HMI ballast
The RF filter snubs the higher frequency harmonics drawn by the ballast to earth, contributing to the additional 15.32mA of residual current measured. In the waveform and FFT of the ballast's residual current (below) one can see the small amount of mains current inadvertently passed by the filter as well as the harmonic currents it is designed to pass to ground.
The waveform and FFT of the residual current generated by a 1200W non-pfc electronic HMI ballast
These high frequency leakage currents will cause a hardware store type GFCI to nuisance trip because they do not return via the neutral conductor and so cause the GFCI to see a difference between the current leaving on the hot line and the current returning on the neutral line. It is a nuisance because they in fact pose no hazard.
These high frequency leakage currents do not pose a hazard because, not only are they safely confined to the EGC, but they are also the result of the system capacitance (the harmonic currents are drawn by the charging of the ballast's smoothing capacitors.) The human body is a resistive load and according to Ohm's Law (I=V/R) shock current (I) is a function of voltage (V) and resistance (R) only. In this case it is not the residual current that causes a shock, but the current generated by the resistive value of the individual making contact with the potential to ground. Since the residual currents do not increase the voltage in the fault circuit there is no increase in shock hazard even though their accumulation on the EGC can cause nuisance tripping of hardware store type GFCIs.
To avoid GFCIs that have been sensitized by residual currents from nuisance tripping as a result of transient conditions that are not of a sufficient duration to pose a hazard, in 2003 UL published a new standard (UL 943) for GFCIs designed to prevent nuisance tripping. The new standard allowed GFCIs to incorporate high frequency filters and trip on an "Inverse Time Curve." Attenuated by a filter, residual currents don't sensitize GFCIs and so pose less of a problem. An inverse time trip curve permits transient conditions that are sufficiently short in duration so as not to pose a hazard while keeping current through the body to safe levels. To assure the latter, UL943 requires that as fault current increases the maximum allowable time to open a circuit and interrupt power decreases, with an almost instantaneous response time required (no more than 20ms) if the fault current is greater than 300 mA compared to 5.59 seconds at 6 mA. In other words, the higher the current, the faster the GFCI must trip. The advantage of UL943's inverse trip curve is that it minimizes nuisance tripping from transient conditions while providing protection from shock currents generated by an individual coming into contact with ground potential.
