Dim lights, correct voltage

Status
Not open for further replies.
H

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
One of my techs called today, saying he was at a store were all of the lights were dim, 277 volt overhead floresents, 120 volt display lights,ect. The voltage he was reading was correct, 480 between phases, 277 to neutral. Had him go to another tennant in the shopping center, but on a different transformer, same thing. Had him call the local poco, they came out, and found the same thing, voltage good, but lights appeared dim to the linemen too. Had tech go pick up a light meter, when he returned about an hour later, lights were bright again. The linemen said that the poco was building two new power plants, so I would assume that maybe their exsisting capacity may have been at peak load, but since the voltage was good, it really couldn't be called a brown out. Since all of the fixtures are electronic ballast, could it be possible the frequency was low? I don't know if this is even possible with the poco being tied to multiple generating plants.
 
As the O/P states that the voltage was correct when first checked, but that the lights were dim, but were brighter later, perhaps the voltage was above nominal later.
I would not however expect much variation in brightness between 277 volts and 277 +5%.

What was the ambient temperature like ? some newer flourescent lamps are sensitive to air temperature and might perform well at 25 degrees, but not at 10 degrees. If the lit space was cold initialy, but warmed up later, that could explain it.

I presume the ballasts are intended for 277 volt operation ? If some one had mistakenly supplied 377 volt ballasts (Canadian) these might give acceptable light on say 290 volts, but not on 277.

If the utility supply is heavily loaded, then the voltage waveform may start to resemble a square wave, rather than the expected sine wave.
Electronic ballasts contain a bridge rectifier that charges a capacitor up to a voltage approaching the peak of the AC input.
If the voltage supplied is a sqaure wave, then the peak voltage will be much reduced, despite the RMS value being correct. That might impair correct operation.

If the supply is from stand alone generating plant, then frequency might be an issue, but not if connected to a grid network.
 
LarryFine said:
You should have measured the voltages again.
Click to expand...

He did, but I went out of cell range before he could call back. The lights in the racks were so dim, that he said they looked blue. When he came back, they were the proper color. Ambient temp inside and outside the store was around 68 to 70 degrees, so temp was not a factor.
 
Where was the voltage measured ?
Might it be correct at the service entrance, but low at the lights due to an overloaded feeder or sub-panel, perhaps intermitanly low due to other loads cycling on/off.
 
broadgage said:
Where was the voltage measured ?
Might it be correct at the service entrance, but low at the lights due to an overloaded feeder or sub-panel, perhaps intermitanly low due to other loads cycling on/off.
Click to expand...

Lights storewide were dim, Big blue box, neighboring stores were dim also, definite utility problem, but what? I talked to him this morning, and he said the voltage was the same as before when the lights were dim. He does not have the frequency function on his meter, so I could not have him check that when they were dim. Really strange. PWM is used to dim floresents, so I figured that maybe low frequency input may cause the same thing.
 
A slight amount of low frequency will cause what looks like a brownout effect--lights dimming and flourescents blinking. Remember the formula for impeadence --- frequency is a direct multiplier ! Once again, it tells contractors to set aside money for a data logger -- it takes the guesswork out of many trade related problems. Power companies will be the last one to admit they have a problem. Hand them a data read out from a certified data logger and watch them wiggle !
 
charlietuna said:
A slight amount of low frequency will cause what looks like a brownout effect--lights dimming and flourescents blinking. Remember the formula for impeadence --- frequency is a direct multiplier ! Once again, it tells contractors to set aside money for a data logger -- it takes the guesswork out of many trade related problems. Power companies will be the last one to admit they have a problem. Hand them a data read out from a certified data logger and watch them wiggle !
Click to expand...

And then... what?

Customer: "Our lights were dim for 1.5 hours while your frequency dipped 12%...here are 2 independant certified data log reports..."

Utility: "So...? GFY.
Bye!"
 
12% would relate operating frequency of 52.7 !! That would be tripping everything "offline" . Just a cycle or two has a major effect on alternating current circuits . Utility companies do not admit to anything until caught ! To catch them your need to document the problem with certified equipment. The poster stated they were building two new plants -- today's economy is not the time to expand your business -- unless your in serious trouble. Generators begin to loose RPM when operating above their design capacity. The fact that the voltage remained within normal specs and there was dimming suggests a frequency drop. Remember the IR formula and you can easily see the effect of even two cycles !

From my experience, you provide certified proof of a problem to a utility company, and they don't react, your next step is calling your Public Utility Commissioner. And believe me, they don't want that to happen, especially knowing you have the proof in your hand. They get very co-operative with proper proof of providing out of spec power. Just consider the damage this causes in computerized equipment?
 
Last edited:
charlietuna said:
12% would relate operating frequency of 52.7 !! That would be tripping everything "offline" . Just a cycle or two has a major effect on alternating current circuits . Utility companies do not admit to anything until caught ! To catch them your need to document the problem with certified equipment. The poster stated they were building two new plants -- today's economy is not the time to expand your business -- unless your in serious trouble. Generators begin to loose RPM when operating above their design capacity. The fact that the voltage remained within normal specs and there was dimming suggests a frequency drop. Remember the IR formula and you can easily see the effect of even two cycles !

From my experience, you provide certified proof of a problem to a utility company, and they don't react, your next step is calling your Public Utility Commissioner. And believe me, they don't want that to happen, especially knowing you have the proof in your hand. They get very co-operative with proper proof of providing out of spec power. Just consider the damage this causes in computerized equipment?
Click to expand...

I had a feeling that frequency had to be the culprit, I carry three different meters that have frequency capability because I do a lot of generator work, but most of my guys do not. Probably suggest monitoring to the customer if it gets to be a frequent issue.
 
Install one of them on the incoming service and let it run --charge customer for seting it up and rental per day -- then you'll have it documented....
 
charlietuna said:
Install one of them on the incoming service and let it run --charge customer for seting it up and rental per day -- then you'll have it documented....
Click to expand...

We do it all the time for this customer and others, the company I usually rent the recorder from went bankrupt (maybe thats why I always got a good price from them) but another company bought their assets.
 
I rather doubt that low frequency is the problem, with a copper/iron ballast low frequency should result in a brighter light, since the ballast will have a lower impedance and therefore pass more current to the lamp.

In the case of electronic ballasts, low frequency might in theory reduce the light slightly, but many are dual rated for 50 or 60 cycles.

Also most utilities are connected to each other via the grid, and therefore cant have a different frequency to the rest of the country.

This might be a stand alone utility, but such are rare these days.

Waveform distortion might be the cause, though it would have to be very severe.
 
lakee911 said:
The utility does not care.

Jason
Click to expand...
It's been MANY years since I worked for our POCO, but I seriously doubt there to be a frequency problem. The entire USA is tied together. The generators are locked to the line ... there are angles of lag (not power factor concept) between rotating field and stationary coil which relate to excitation. If New York is low, San Francisco is low at the same time.

I would be very surprised to see an error as much as 0.1 Hz for a full second, or 0.01 Hz for a full minute. We've some real utility engineers here; perhaps they will hop in here.
 
broadgage said:
I rather doubt that low frequency is the problem, with a copper/iron ballast low frequency should result in a brighter light, since the ballast will have a lower impedance and therefore pass more current to the lamp.

In the case of electronic ballasts, low frequency might in theory reduce the light slightly, but many are dual rated for 50 or 60 cycles.

Also most utilities are connected to each other via the grid, and therefore cant have a different frequency to the rest of the country.

This might be a stand alone utility, but such are rare these days.

Waveform distortion might be the cause, though it would have to be very severe.
Click to expand...

All fixtures are T-8 with electronic ballast, overhead 277 volt, racking 120, Found out it was SC Power and Light, Don't know how big they are, as far as grid connection, plants can be disconnected at certain junctions just to prevent them from dragging down the grid, otherwise the grid would be brought down. (like the northeast was a couple years back when the plants were overloaded, and the failsafes failed)
 
GeorgeB said:
It's been MANY years since I worked for our POCO, but I seriously doubt there to be a frequency problem. The entire USA is tied together. The generators are locked to the line ... there are angles of lag (not power factor concept) between rotating field and stationary coil which relate to excitation. If New York is low, San Francisco is low at the same time.

I would be very surprised to see an error as much as 0.1 Hz for a full second, or 0.01 Hz for a full minute. We've some real utility engineers here; perhaps they will hop in here.
Click to expand...

Agree,though the frequency control is probably not quite that close, the legal limit here in the UK is from 49.5 cycles to 50.5, and the normal range is from 49.8 to 50.2 cycles.

I would expect that rules in the USA would differ, but still be somwhat similar.

UK grid frequency may be viewed here
http://www.dynamicdemand.co.uk/grid.htm
 
Last edited:
GeorgeB said:
It's been MANY years since I worked for our POCO, but I seriously doubt there to be a frequency problem. The entire USA is tied together. The generators are locked to the line ... there are angles of lag (not power factor concept) between rotating field and stationary coil which relate to excitation. If New York is low, San Francisco is low at the same time.

I would be very surprised to see an error as much as 0.1 Hz for a full second, or 0.01 Hz for a full minute. We've some real utility engineers here; perhaps they will hop in here.
Click to expand...
Actually the entire US is not tied together. There are 3 grids...east of the Rockies, west of the Rockies and Texas. The only ties between the grids use equipment to match the frequency.
 
Status
Not open for further replies.
Top