The Early Bird is an Electrician, and He Wants Single Neutrals
Why automated lights should have separate neutrals
by Richard Cadena
“The fool wonders, the wise man asks.” – Sir Benjamin Disraeli (1804-1881), former Prime Minister of England
When the telephone in my office rang just after seven o’clock this morning, I guessed that it was probably an electrician. No one else that I know calls me before 8:00 a.m. unless something is very wrong. Of my friends and associates, only electricians start work that early. There must be something about being around all that high voltage and electromagnetic fields that synchronizes your biological clock so your have to start work at 7:00 a.m. There must be a bio-magnetic connection. When I was in college I worked for several summers as a plant electrician and we started work at 7:00 a.m. every day. I won’t lie to you – it was not easy. When I first went to work as a lighting technician and they told me to report to work at 9:00 a.m., I knew I had found a career.
I picked up the phone, and sure enough, it was an electrical contractor. I had been working on a lighting design and he was the electrical contractor on the job. He was working on an estimate for the electrical system based on a one-line diagram that I had drawn up and e-mailed to him the night before. The electrical requirements for this job were very simple. After a budget meeting we found that we were over budget (hard to believe, I know) and we had cut the conventional lighting and dimming and went solely with automated lighting. So the electrical design called for a series of non-dim circuits.
The EC wanted to know if he should pull a separate neutral for each circuit. I had drawn it up with individual neutrals, but he said the National Electrical Code didn’t require separate neutrals since it was a balanced three-phase load. In theory, a balanced three-phase load with three hot legs and a neutral leg conducts no current in the neutral conductor. That’s because the current flowing from the three hot conductors into the neutral conductor cancel each other out if the amount of current in every conductor is the same and the waveform is a pure sine wave. The three waveforms are each 120° out of phase with each other, so when one leg is at its peak value, the other two are at their negative 50% value, canceling the first.
So as long as those two conditions exist – equal amplitude and a sinusoidal waveform, then there are no problems. But rarely, in real life, does it happen the way it was drawn up on paper. If there is a potential for problems it is our job as responsible lighting designers and consultants to anticipate and minimize them.
In this case, the fixtures going in this installation use electronic switching power supplies. Electronic switching power supplies are wonderful things. They are very efficient, lightweight, and their operating frequency is faster than video of film frame rates, so they produce no flicker on film or video. Many automated lighting fixture manufacturers are turning to electronic switch-mode power supplies to reduce the weight of the instrument, to increase the brightness of the fixture, and to offer auto-voltage sensing so that it can operate at any voltage. But this type of power supply converts the incoming voltage sine wave into a square wave. The input voltage is rectified, and then a control circuit opens and closes a switch several hundred times per second to pass the current. The opening and closing of the switch creates a square wave and that produces harmonic distortion. These harmonics can cause large currents to flow in the neutral.
How does it do that? I’m glad you asked. A square wave is made up of a combination of sine waves with odd-order multiples of the fundamental frequency. For example, in North America, our power grid operates at a frequency of 60 Hertz, or 60 cycles per second. The first odd-order harmonic frequency is 180Hz (3 x 60Hz). The next is 300Hz (5 x 60Hz), and so on. If you were to add all the odd-order harmonic frequencies with decreasing amplitudes you would get a square wave.
Conversely, squaring the voltage waveform, like a switching power supply does, produces odd-order harmonics that feed back into the neutral conductor. Each of the three phases feeds these harmonics to the neutral conductor but each one is 120° out of phase with each other. When the three components are summed in the neutral, the result is a voltage sine wave with a peak of 1.5 times the peak input. The overvoltage results in increased current and that can cause the conductor to heat up and possibly melt down.
The job in question was on a very tight budget and asking for individual neutrals would increase the cost of the electrical installation. It’s possible that we could have gotten away with a common neutral and saved the customer a little bit of money. But I would rather err on the safe side. Safety should always be the overriding concern. It was easy to make this decision. I asked the electrician for individual neutrals and he was happy to oblige.
Don’t be neutral. E-mail the author at rcadena<at>swamicandela.com.