We were asked to help with the dimming controls on a theater/showroom the other day, and were inadvertently forced to track the history of dimming. It’s funny, not the issues that can arise, but the different answers that can be supplied by different manufactures. I suppose it really boils down to viewpoint and whether you want your product to get specified…but that’s beside the point.
The design of the space utilizes multi-circuit track. Multi-circuit track is awesome in the way it can provide levels of control without forcing the design to split the track into sections; however, the track is the weak point in the design when dimming is introduced in the space. More specifically, the number of neutrals in the track is the weak point.
So the place to start at is what is a shared neutral, and why bother sharing it? Universally, a three phase system is the acceptable method of transmitting and generating energy. Three separate sources generate a sine wave voltage that is identical in magnitude, but 120o degrees out of phase. Think of a grid with an array running along the 0o axis, another array at 120o and a final array at 240o. You now have three phases of power, equal magnitude opposite direction.
Back to that question, when running an electrical circuit, a typical wiring system has 3 wires: a Hot, a Neutral, and a Ground. The hot wire represents the path away from the panel with the neutral being the path of return. When the circuit is completed, the sine wave is generated and travels out from the panel on the hot wire and returns via the neutral wire.
I know, I still haven’t answered the first part of the question, but I’m about to. The previous example looks at one phase, and we need to look at three phases. A standard three phase system, in lieu of 3 wires, has 5 wires: (3) Hots, (1) Neutral, and (1) Ground. It works under the same general concept, with the hot wires representing the path away from the panel, although the three different paths will all be at separate phases, 120o apart from one another. To return to the panel, the (3) sine waves will share the (1) neutral wire, which creates the shared neutral. The waves can share the wire because they are out of phase, 120o apart.
Why is this done? Well that’s a much simpler question, cost. It is cheaper to run conduit with (5) wires as opposed to (3) conduits with (3) wires in it. Copper, after all, is not that cheap.
Great, so we now know that electricity is willing to share, but we’re still not up to speed on why this creates problems with dimming. Remember, we’re talking about non-linear loads when looking at the current supplied to a lighting load. With non-linear loads, Harmonics are introduced to the equation. Harmonics are currents that occur at multiples of the power line voltage frequency. Here in the US, our line frequency is 60 Hz, and we run into harmoics at 60 Hz, 120 Hz, and 180 Hz.
Harmonics cause irregularity in our sine waves, and in a three phase system, the irregularity is the same across all 3 phases. This will cause the sine wave on each phase to be manipulated, and the 3 waves will fall out of sync with each other. On a 1-phase (3) wire circuit, the harmonic is not an issue as it will travel back along the neutral wire. On a 3-phase circuit, there is a much different result. Because the 3 separate phases are no longer 120o out of sync, the waves do not cancel one another out, and a current is induced on the neutral wire. The harmonics, the “noise” on the wire, can cause lamp flickering and noticeable irregularities in the way a dimming system functions.
Harmonic “noise” isn’t the only negative affect that can occur on the shared neutral. The third order harmonic current on the neutral is the sum of the harmonic current on all three of the Hot wires. For three fully loaded circuits, the neutral current can elevate to 3 times what is present on any of the phase conductors. If the wire is not properly rated, neutral conductor overheating or unexplained voltage drop can occur. 3 times is a worst case scenario. Typical distortion runs an average of 1.37 times phase current.
So, how do you avoid running into these situations? The simplest solution was already mentioned in the previous paragraph. Provide a dedicated neutral wire to each circuit to eliminate the cross talk and shared harmonics between the phases.
So now we are back at the beginning, the track, a track with (3) circuits and (1) neutral wire. Even if you run separate conductors, you are still sharing a neutral inside the track. When forced to share a neutral, align the fixtures on the same phase. Please note, that the phasor sine waves will become additive and the neutral wire must be rated to handed the additive value of the current on all (3) Hot wires. Or, the current must be limited so the max additive current will not exceed the 20A rating.
Be careful, and practice safe dimming.