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Posted By: Larry Ural Panel Board Overheating - 02/15/05 06:55 PM
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).]
Posted By: Bob Re: Panel Board Overheating - 02/15/05 07:21 PM
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).]
Posted By: mhulbert Re: Panel Board Overheating - 02/15/05 07:23 PM
Is there an actual problem (ie breakers tripping, etc) or is it just "hot"? Fully loaded equipment is allowed to get pretty hot and it will run OK like that.

I think a QO breaker is allowed to get up to 75 deg celsius at its terminals without problems. That's 167 deg farenheit!

What is the measured temp of the panelboard? At the breakers?

Mike
Posted By: cvelectric Re: Panel Board Overheating - 02/15/05 08:40 PM
Might want to check the bus bar for charring (under the main esp). Load balanced?
Posted By: Larry Ural Re: Panel Board Overheating - 02/15/05 09:08 PM
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.
Posted By: iwire Re: Panel Board Overheating - 02/15/05 10:25 PM
I do not know what it is but it is not harmonics.

Quote
This is a 400 amp, single-phase 240 volt panel

Harmonics are not an issue with single phase supplies.

Bob
Posted By: JBD Re: Panel Board Overheating - 02/16/05 03:23 PM
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.
Posted By: pdh Re: Panel Board Overheating - 02/16/05 11:04 PM
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).
Posted By: iwire Re: Panel Board Overheating - 02/16/05 11:15 PM
I say again Harmonics are not an issue with single phase supplies. [Linked Image]

The currents do not add up on the neutral as they can with a three phase supply.

Edit.

OK I will amend this to I have never heard of harmonics bing an issue on single phase system.

I will also point out we are not talking about switching power supplies in this thread.


[This message has been edited by iwire (edited 02-16-2005).]
Posted By: resqcapt19 Re: Panel Board Overheating - 02/17/05 12:13 AM
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?
Quote
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
Posted By: cpal Re: Panel Board Overheating - 02/17/05 12:28 AM
I may have missed this information but I do not see where the poster has mentioned the conductor material. Are we discussing copper or aluminum and what size is feeding the panel??

Has he tried shooting the terminals and breaker body with an IFR sensor???


If this has been discussed let me have it

Charlie
Posted By: dereckbc Re: Panel Board Overheating - 02/17/05 01:15 AM
Larry, I was asked to look at your post and offer some advice. There is some info missing like breaker size, wire size & type, actual current readings etc that could be helpful. But lets start by what I think the problem is not, harmonics. I cannot possible see how harmonics can be a problem on single phase supplying stadium lights. I believe your factory rep is passing the buck.

If the breakers are hot to the touch, then there has to be significant voltage drop and load current in or near the breakers. Since you did not list the size of the breaker, the current, wire size or type. I cannot pin anything down.

Assuming everything is sized properly I would get my hands on some sort of thermal device and determine what is actually heating up. Is it the breaker or the connection to the breaker. A probe would pin it down

Next get you a true reading rms amp probe and make sure your circuits are not overloaded or near max capacity. It could be you are max capacity and everything is normal.

Sorry I cannot be more specific other than to say if something is hot you have voltage drop and current, no other way around it. Doesn't matter if it is normal load current or from harmonics.

Good Luck
Dereck C, PE
Posted By: PCBelarge Re: Panel Board Overheating - 02/17/05 04:15 AM
Mhulbert and JBD seem to have mentioned what I also think this may be.
The breaker is permitted to carry a continuous lighting load of 80%. It may well be operating at it's uppermost limit as related to temperature.

The breaker is rated for 167 degrees F.
167 X .8 = 134 degrees.
I do not know if there is a direct correlation of percentage of temperature to percentage of load, so this may be a real stretch.... but if it is that may be close to the temps you are feeling - try to get a temperature reading when you feel them hot, as I would be very interested to see what it is.

Pierre
Posted By: pdh Re: Panel Board Overheating - 02/17/05 09:06 AM
iwire:

True, this is not a thread about switching power supplies, unless you count some of the new electronic ballast designs intended to eliminate flicker in fluorescent lighting. But I do not know how those units are designed nor have I evaluated them, so I simply do not know. I doubt Larry Ural's issue involves that (but I can't rule it out, either). But I have encountered harmonic issues in single phase power where loads were exclusively switching power supplies. Those issues had nothing to do with overloading the neutral.

resqcapt19:

Other than having actually experienced the problem in one of the places I have worked, there doesn't seem to be a whole lot of documentation on this. But here is one web page that shows some waveforms of loads being considered in the design of AFCI protection devices. Scroll down to number 8 to see a switching power supply. If you're going to put 100 of these in a room, are you going to use the RMS figure, or the peak figure, to calculate load?

http://www.zlan.com/waveforms.htm

Whether or not harmonics is an issue in Larry Ural's case or not, I cannot say (especially since I'm not an electrician ... I just spec work and hire electricians when I need things done). I'd certainly would have suspected loose connections if he had not said everything was tight.
Posted By: capt al Re: Panel Board Overheating - 02/17/05 09:17 AM
Larry, I do Infrared testing as part of my job. I agree with iwire & Dereck that you do not have a harmonics problem. Please post more of the info others have asked for. Also do a millivolt drop reading across each breaker that is getting hot and post those results. Another question for you, do you know the length of the circuits?
Al
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