I have a SQ-D panelboard that is overheating and can not get proper help from the factory representative. This is a 400 amp, single-phase 240 volt panel that is feeding ball field lighting. The light fixtures are 1500 watt sport-lighter fixtures. The panel is feeding eight poles with four 1500 watt fixtures on each pole. I measured the voltage and current at all locations and can not find anything unusual, no voltage drop problems or overcurrent problems. All connections at the panel, circuit breakers and fixture junction boxes are tight with no loose connections. The problem is that when the lights are on for more than 15 minutes the circuit breakers are so hot you can not touch them for more than 5 seconds. The Square-D rep. is thinking it could be a harmonics problem. Has anyone else had this problem.
EDIT: I will get more information that you guys are asking for as soon as I get out to the job site with the factory rep.. The weather has been a factor with the job site meeting. I will provide temp. readings, voltage and amprage readings along with wire size and lenght, although I do not believe wire and length have anything to do with this application. Please stand by until I can provide more information.
[This message has been edited by Larry Ural (edited 02-23-2005).]
Your post says you have 32 1500 watt fixtures. Thats about a total of 200 to 250 amps depending on the PF. What are the circuit breaker sizes? What is the measured amp load on each breaker? If you read the load with a true RMS meter you will read any harmonic load that is present.
[This message has been edited by Bob (edited 02-15-2005).]
The meter used to take the readings is a Fluke true RMS meter. The light poles are being fed with a 50 amp, 2-pole QO breaker. The load reading is 26.4 amps and the voltage is 236 volts. The breakers are reading from 129 to 146 deg. F with the ambient temp approx. 78 degrees. I will be on the site next week, if I can provide any other info.
During operation the hotest part of a circuit breaker is the operating handle. According to NEMA AB-1 a non-metallic breaker handle can be 60C above a 25C ambient (this is 140F above 77F for a total temperature of 217F).
If there is no tripping of breakers then the panel is not over heating.
I switched from EE to computers in college. But that still got me some electrical background (and studying for a ham radio license added a bit more). Then working in large data centers (one with as many as 2000 computers) with many switching power supplies, I've learned (and understand why) harmonics can also be an issue with single phase power (and is underrated as an issue with three phase power).
I'll try to explain it with this analogy.
Suppose you have a single phase 100 amp 240 volt circuit with 19200 watts of resistive load (lights, heaters, whatever). That load would pull 80 amps. Now suppose the wiring feeding it has 1/100 ohm on each phase, for a total of 1/50 ohm. That wiring will have a 1.6 volt drop, dissipating 128 watts over its distance. As long as the load is on 100% of the time, that's 128 watts dissipated 100% of the time.
Now let's change the load around. Instead of the load being on 100% of the time, let's make it 50% of the time (for example it might be a large flashing billboard going on an off 1/2 second at a time). Now that 128 watts is dissipated only 50% of the time. The wiring will not get as hot.
But now here's where it gets interesting. You wouldn't do this (right?) but lets suppose the load is doubled to 38400 watts (remember, this is a 50% duty cycle). The current is now doubled to 160 amps, and the voltage drop jumps to 3.2 volts. That 3.2 volts across 1/50 ohm dissipates 512 watts ... when the load is on. With a 50% duty cycle, that averages to 256 watts dissipated.
The total energy used is about the same. But the power being dissipated is four times as much, half the time. So on average it is double the power being dissipated. Remember "I squared R". The wire (or thermal element) gets twice as hot.
If you were going to run TWO 19200 watt billboards in 50% flashing mode on that 100 amp circuit, you'd want to alternate between them instead of having them both on at the same time, right?
Don't forget that the thermal element in a breaker (or fuse) heats up by dissipating a small amount of power the same way. The above system should eventually trip the 100 amp breaker since the load would be the equivalent of 113.137 amps continuous. Fortunately the thermal element does emulate the heating in the wiring, so it should provide the correct protection when the breaker capacity is correct for the wiring. You just have to match the capacity with the load to make it a usable setup.
Switching power supplies do just this kind of thing. They turn on briefly during each AC half cycle, and pull a heavy amount of current for a brief instant, topping off the capacitors, then switch back off. Most do this at about the same point in the AC cycle as all the rest, so the current spikes tend to all happen together when you have many such loads on the same circuit.
Computers can be some of the worst offenders for this. I don't know to what degree high efficiency lighting will do this, since that's not what I have worked with. However, I am starting to get into some of that for my personal research on the effects of non-incandescent lighting on people with Autism. Apparently, electronic ballasts will be new culprits in the harmonics problem. Those that convert to DC first (this would be needed to eliminate the 120 Hz flicker) would likely be a switching power supply. And they are certainly going to be as cheap as the manufacturer can get them UL listed. And those that don't convert to DC and just chop up the AC cycle to limit the total current can still be a problem.
So yes, harmonics can be a problem, even on single phase. It's just different than what people people are aware of on three phase (but this problem can combine with the neutral sequencing problem to make three phase even worse when the switching power supply current duty cycles are shorter than 33%, which is often the case).
pdh, Can you document a problem that has actually occurred in the field in either a single or three phase system that was conclusively caused by harmonics? It seems that almost all of the information on the problems caused by harmonics comes from people (companies) with a vested economic interest in solving the problem. Also how is the heating problem that you described in your post a "harmonic" problem?
The above system should eventually trip the 100 amp breaker since the load would be the equivalent of 113.137 amps continuous.
It is very possible that 113 amps will never trip a 100A breaker. The trip curve for a major brand of breakers show that a 113A load on their 100A breaker will trip between 400 seconds and never. Don Don