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A diagnostic catalog of the causes & cures of of dimming light fixtures or flickering lights & power losses.
How to diagnose the causes of flickering or dimming lights at or in buildings.
This article gives simple diagnostic steps that a homeowner can do to determine the type of electrical system problem that is causing flickering or dimming lights or intermittent loss of electrical power. We list the common causes of these problems and suggest what to do about them.
Watch out: flickering or dimming lights often indicates a dangerous condition. Switch off the bad-acting appliance or circuit and ask for help from a licensed electrician.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Dimming or Flickering Lights Indicate Electrical Hazards in Buildings: What to Do
Watch out: flickering or dimming lights at a building may be more than an inconvenience. While some causes of flickering lights may be normal and harmless, such as flickering in certain types of light bulbs, many others indicate a dangerous condition, risking fire, shock, injury or worse. The safest approach is to turn off electrical circuits or components that are causing flickering or dimming or acting strangely in any way.
If you cannot safely access your electrical panel to turn off dangerous circuits or components, leave the building and call your local emergency services.
Photo: a fluorescent light fixture may be the only electrical device that is flickering, for any of several reasons we will explain below. Other causes of flickering lights are very dangerous.
Six Tips for Diagnosing Dimming or Flickering Lights at Buildings
In diagnosing the cause of flickering or dimming lights at a building the following diagnostic questions can help narrow down the cause of the trouble.
Is the dimming light or power loss problem happening in just your building or do neighbors see the same problem at exactly the same time?
If your neighbors are seeing flickering or dimming lights too, chances are the problem is in the electrical supply network or possibly at local electrical wiring in your neighborhood or its power transformer. If your neighborhood frequently loses electrical power
Is the dimming problem or power loss showing up in the whole building or just parts of it? If all lights in the building dim, flicker, or if all power is lost, then the problem is probably in the electrical panel or at the service entry cabling to your building.
The electrician will look for a problem in the electrical panel, at the main breaker, at the service entry wiring connections or at the SEC wiring itself, including the two hot wires and the service neutral wire. But see the exception in step 4 below.
If the dimming, flickering, or lost power occurs in multiple circuits in the building the problem could still be in the electrical panel itself, or in the service entry wiring to the building.
Is the dimming or flickering light problem or power loss associated with circuits in just half of the electrical panel? Depending on electrical panel layout, damage to one panel bus can cause failures or odd behaviour on electrical circuits connected to that panel bus.
Is the dimming light problem or power loss associated with the operation of a single, specific appliance or motor? This is an exception to the case described in step 2 in that all lights may dim when there is a developing failure in a single major appliance such as an air conditioner compressor motor.
Such motors can draw very high amps (current) for just a brief moment when the motor is starting.
If the current surge is very brief, no circuit breaker may trip but the load may be sufficient to dim all of the incandescent lights that are on or all of fixtures in just part of the building, depending on how the appliance circuit is wired in the electrical panel. (Fluorescent lights probably won't dim).
If the problem never occurs when the circuit to that appliance has been turned off you've probably found the trouble.
If the problem is traced to a specific appliance but is intermittent, other variations in conditions such as temperature, humidity, vibration, or moving around of a loose wire may explain that inconsistency.
Is the dimming or flickering light or power loss showing up on just a single circuit? In this case we suspect a faulty circuit breaker or bad connection in the circuit.
Some circuit breaker brands are particularly prone to failure such as FPE Stab-Lok and Zinsco-Sylvania. In this case the problem may ultimately show up on additional circuits in the same building, depending on building age and circuit usage levels and other conditions.
Is the dimming or flickering light or power loss showing up just at some fixtures or appliances? A likely cause of this problem is a failing appliance or motor that is drawing abnormally high current as we cited in step 3.
But sometimes the problem may occur only at lights or appliances plugged-in electrically "downstream" from a specific electrical circuit, or at lights operated by a specific switch. In this case we suspect the trouble is not the appliance itself but in connectors within the receptacle or switch or immediately "upstream" (electrically) from it.
Some receptacle and switch types such as older push-in back-wired devices are more likely to have loose or failing electrical connectors with age and usage.
Try plugging in the light fixture or appliance at a different location on a different electrical circuit in the home. If the bad behavior continues then the trouble is in the light or appliance itself.
Is the flickering light showing up at just one light fixture? Look for loose wiring or an overheating fixture.
Is the flickering light found in a fluorescent fixture?
If so, while checking for a loose wire or poor fluorescent bulb connection is important, the most common causes of flickering fluorescent light fixtures are
A bad fluorescent light bulb - take a bulb from a fixture working properly and test it in the flickering fixture; also look for black or other discoloration in the flickering bulb. Replace it.
A bad starter in the fixture
A bad ballast in the fixture
In the photo above I am showing the original data tag for this Fox Co. fluorescent fixture installed in the bathroom of a Minnesota home built in 1963.
To stop the flickering light in this luminaire fixture I needed to replace the ballast.
Watch out: for flickering light safety hazards and take the immediate safety measures listed here.
18 Causes of Dimming or Flickering Lights or Electrical Power Loss at Buildings
Start by turning off any electrical appliance or circuit that is misbehaving. If you are not trained and familiar with safe electrical practices and repairs keep your fingers off of the wiring - you could be shocked or killed or could cause a fire. Call for help from a licensed electrician.
Aluminum electrical wiring: if your building's branch circuit wiring (such as lights or electrical receptacles) was installed in the 1970's solid-conductor aluminum wiring may have been used. With age, use, and mechanical disturbance the connectors in an aluminum-wired circuit become unsafe, ultimately overheating.
These connections can become hot enough to start a building fire without ever tripping a circuit breaker or blowing a fuse. We have had reports from homeowners of both conventional buildings and mobile homes or doublewides who described flickering lights that indicated trouble traced to aluminum electrical wiring.
Watch out: for this fire hazard. If your home's branch circuits (receptacles and lighting) are wired with aluminum wire, flickering, dimming lights or even sparking may show up on just one circuit but the hazard is building-wide and significant. Turn off the misbehaving circuit and have the electrical system inspected and repaired by an electrician familiar with the hazards of and proper repairs for aluminum wiring.
Appliance or motor drawing high current: Any appliance that draws high current (amperage). Sometimes this is normal such as a brief high current draw when some large electric motors start, such as an air conditioning or heat pump compressor.
An air conditioner or heat pump compressor motor may draw considerably higher amps at start-up than it does once the motor has begun to run.
That's why installers should use a slow-blow fuse or slow-trip circuit breaker and it's why air conditioning circuit over-current protection ("breaker size" or "fuse size") is typically permitted to be one size larger (more amps) than the rating of the wire supplying the circuit.
For example, an air conditioner may run on a #10 copper wire 30 Amp circuit but may be fused with a 35A or 40A circuit breaker to avoid nuisance tripping when the A/C unit is starting normally.
But dimming lights can also mean that a motor is drawing high current because it is having difficulty starting.
That in turn can be due to a motor that is seizing or due to a failing start-capacitor. Some of these conditions are unsafe. If the motor or appliance trips a circuit breaker, leave that appliance turned off until it can be repaired or replaced.
There you'll see that the data tag on an air conditioner or heat pump specifies two different amps or current ratings:
the maximum circuit ampacity (for example 15A) that must be supported by the actual wiring - this is the current drawn when the motor is running
the maximum overcurrent protection (for example 20A) that is permitted on the circuit. Typically the data tag will also specify that the installer should use a time delay fuse or HVACR type circuit breaker.
Watch out: many air conditioning and heat pump units are wired with multi-strand aluminum electrical wire - a perfectly legal choice.
But because there is a higher risk of corrosion, resistance, and overheating at the connections of aluminum wiring, if your A/C or heat pump unit is causing severe light dimming or light flickering it makes sense to ask your electrician to check the condition of the wiring and its connections - that will reduce the risk of a total failure or worse, a fire.
Watch out: When you can trace flickering or dimming lights to a single appliance, un-plug it and stop using it until it can be inspected and repaired. If the appliance is a unit such as central air or a heat pump, switch it off at the electrical panel.
Corrosion from unanticipated sources: homes where Chinese drywall was installed were reported to have strange electrical problems including flickering lights. Off gassing from Chinese drywall was found to cause corrosion of copper components throughout some of these buildings, including HVAC components, cooling coils, and copper electrical wiring or connectors.
Electrical circuit breaker defects: some brands or models of circuit breaker are known to have significantly-higher failure rates than that equipment in general. Examples include Federal Pacific Electric Stab-Lok circuit breakers and ZInsco circuit breakers.
Watch out: fire or shock hazard: When you trace flickering or dimming lights to a specific electrical circuit, turn that circuit off at the electrical panel until your electrician can inspect and repair the problem.
See CIRCUIT BREAKER FAILURE RATES - a bad circuit breaker or electrical panel connection can cause flickering lights or loss of power. This article lists common or well-known problem circuit breaker or electrical panel brands or models.
Since a failing circuit breaker or device sometimes (not always) suffers internal arcing that produces a buzzing sound, that clue may also be diagnostic. Switch such circuits off.
Electrical circuit wiring defects: specific electrical circuits may be dimming or intermittently losing power if the wiring has become damaged or its electrical connectors are loose. Typically this problem shows up first on just one electrical circuit, or on all of the lights or receptacles ("wall plugs or outlets") downstream from a specific point (where a problem has occurred). A double-wide homeowner reported loss of lights on just one side of a room.
After confirming that no GFCI device had tripped causing a power loss and no circuit breaker had tripped, the owner needed to ask for help from an electrician to find the open wire. For examples
Electric fan or LED bulb induced "light flicker": when certain fluorescent or LED lights or in particular when an overhead fan is in use, depending on factors such as the location light sources and fan in the room, fan speed, fan blade size and who knows what else can cause an apparent flickering of light where the fan is located.
This is not an electrical problem but one of light and shadow. However depending on the flickering light speed, this condition can be uncomfortable for some people and may be hazardous for some who have particular medical concerns such as light-sensitive epilepsy. Video games and TV's can also produce flickering light that can be a problem for the same occupants.
Dahlquist, Neil R., James F. Mellinger, and Donald W. Klass. "Hazard of video games in patients with light-sensitive epilepsy." JAMA 249, no. 6 (1983): 776-777.
Romero‐Gómez, Manuel, Juan Córdoba, Rodrigo Jover, Juan A. del Olmo, Marta Ramírez, Ramón Rey, Enrique de Madaria et al. "Value of the critical flicker frequency in patients with minimal hepatic encephalopathy." Hepatology 45, no. 4 (2007): 879-885.
Wilkins, Arnold, Jennifer Veitch, and Brad Lehman. "LED lighting flicker and potential health concerns: IEEE standard PAR1789 update." In Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, pp. 171-178. IEEE, 2010.
Electric motor defects: motors drawing high or excessive current can cause dimming or flickering lights
Electrical panel defects: as we cite for circuit breakers, some electrical panel brands experience failures far more often than is generally the case among their peers, often because of innate design or manufacturing problems. Some of these electrical panel defects show up as flickering lights, dimming lights, or loss of power on one or more electrical circuits.
Electrical service supply defects & voltage fluctuations: depending on where you live weather, loads on the community electrical system, age and condition of the electrical grid and power generation can cause dimming or flickering lights. If this is the source of trouble at your building, your neighbors will see the same conditions as you.
Sometimes we monitor voltage being delivered to a building to check the range of voltage variation but of course if your electric company's supply is varying significantly you and your neighbors will all see dimming lights or total loss of power. Problems at a local power transformer can cause the lights of your home and those of your neighbors to flicker or dim or go out entirely.
Some light flickering, outages that last 60 seconds or less, are referred to in the power industry as "momentary outages" while voltage drops - not a total loss of power but a reduction in the voltage being delivered to a building are referred to as "voltage drops".
According to Florida Power and Light (FPL), these may be caused by lightning strikes, damaged electrical equipment, animals interfering with electrical equipment (including a mouse in your electrical panel), and in coastal areas, salt spray that affects power company equipment or wiring.
A "voltage sag" - momentary drop in the voltage level below its nominal 120VAC - may also occur, which explains why some lights may dim. These conditions can re-set or stop operation of computers and other electrical devices unless you have a battery-backup uninterruptible power supply (UPS) installed. - FPL "Power Flickers - Reducing Power Flickers", Florida Power and Light, retrieved 9 Nov 2015, original source: https://www.fpl.com/reliability/power-disturbances/flickers.html
According to Canada's CCOHC, "Usually voltage fluctuations are small and do not have adverse effects on electrical equipment. However, in offices, for example, voltage fluctuations of just a few tenths of one percent can produce very annoying flickers in the lighting, especially if they are regular and repetitive in the 5-15 Hz range." - CCOHS (2015 cited below)
Electrical service connection failure: a loose connection at the building's electrical service entry cable or damage to the service entry wiring itself can cause flickering lights or loss of power. I've seen this problem occur in one side of a 240V electrical panel, causing ultimate loss of power to half of the electrical circuits in the home.
Electrical neutral connection failure: if the electrical system's neutral connection or neutral wiring is faulty, such as a loose connection in the electrical panel, lights in the building may sporadically flicker or dim or power may even be lost intermittently.
I've seen this problem occur when a metal fence post was driven through an underground feeder. A case history that was finally resolved by finding a problem with the electrical utility company's neutral is given
Fluorescent light ballast & bulb problems: fluorescent lights that use a ballast to produce the voltage needed to drive the light fixture may suffer from a ballast failure. The ballast, a "black box" found inside of fluorescent light fixtures may cause humming or buzzing sounds heard near the light fixture as well as flickering lights.
This problem occurs where older magnetic type ballasts are installed. Newer electronic ballasts don't hum or buzz.
Lightning strikes can damage both area electrical wiring or the supply grid as well as damaging components at an individual building. A frequent sufferer of lighting strikes are well pumps and well pump wiring at properties where a submersible well pump is in a steel-casing well in an area subject to frequent lightning storms.
In this case the problem is usually confined to the well circuit, but I have participated in investigation of more extreme damage to all of a building's wiring and even plumbing systems due to lightning strikes.
Light fixture or light bulb defects: flickering lights that occur only at a single light fixture may be caused by a failing or defective light bulb, particularly if the bulb is a traditional incandescent type.
As the filament in an incandescent light bulb begins to fail it may become loose and intermittently "open" electrically causing the light to flicker. Tapping gently on such a bulb may cause it to go out completely. Changing the bulb should fix the trouble.
If the individual light fixture continues to flicker after a new bulb has been installed, check that the bulb itself is ok by trying it in a different light fixture.
If the light bulb is not defective then I suspect a bad bulb socket in the light fixture or a loose wire connection in the light fixture's connection to its power circuit. Turn the fixture off until it can be repaired or replaced.
Loose electrical connections in an electrical circuit: loose splices or connections anywhere in an electrical circuit can cause flickering lights. Usually this defect shows up just on the circuit that contains the faulty connection. I've seen this problem at an electrical receptacle that received very frequent use: devices were constantly plugged-in then removed.
The wiggling and jiggling of the device loosened contacts intended to be made between the receptacle and the wall plug. Other receptacles or switches may be wiggled around in their electrical box (if poorly-secured) causing loose, failing electrical connections.
In some cases, such as back-wired push-in type electrical receptacles, the wiring connector itself (a small spring) is inherently weaker than screw-type connectors. On older homes these connections can fail, occasionally leading to a fire.
Outdoor electrical wiring defects: aside from the service entry cable problems I've cited, other outdoor wiring defects can cause flickering lights on those circuits. Examples include lighting fixtures or receptacles that have become wet.
Something else is wrong: the list above is certainly not exhaustive and there may be other causes of flickering or dimming lights that we should add here. Please use the page bottom CONTACT link to let us know what we've missed.
Watch out: flickering or dimming lights may, depending on the cause, indicate a dangerous condition. Arcing or overheating at electrical circuits can ultimately lead not only to power loss but to a building fire. That's why we recommend turning off misbehaving electrical equipment while you wait for the electrician.
What Level of Flickering Light Can People See?
Canada's CCOHS has written some of the most easily-understood description of who sees flickering lights, what people can see or sense, and what health effects may occur when exposed to flickering light. Excerpts are below:
People can see lights flashing on and off up to about 50 flashes per second (50 Hz) - they are most sensitive to time-varying illumination in the 10-25 Hz range. The actual critical flicker frequency increases as the light intensity increases up to a maximum value, after which it starts to decrease.
When a light is flickering at a frequency greater than 50 or so Hertz, most people can no longer distinguish between the individual flickers. At this frequency - the critical flicker frequency or flicker fusion threshold - the flashes appear to fuse into a steady, continuous source of light. This happens because the response to the light stimulus lasts longer than the flash itself.
Most people cannot notice the flicker in fluorescent lights that have a flicker rate of 120 cycles per second (or 120 Hz).
The light flicker may be detected by its stroboscopic effect. When objects move or rotate rapidly, they may be lit at or about the same position during each cycle or rotation.
This makes objects look as if they are moving more slowly than their actual speeds - they may even appear stationary if the object is moving at the same rate as the flicker frequency (or a multiple of it).
This fact is the principle behind a strobe light but it is not the desired effect in general lighting. In fact, it could be a safety hazard if someone mistakenly thought that some equipment was stationary or was moving slowly. - CCOHS, "Lighting Ergonomics - Light Flicker", Canadian Centre for Occupational Health and Safety, CCOHS, retrieved 2015/11/09, original source: http://www.ccohs.ca/oshanswers/ergonomics/lighting_flicker.html
Also, from a different source we see that flickering light can be a source of eye strain and headaches even if it is not related to electrical malfunctions that we listed earlier in this article:
... the use of high frequency electronic ballasts (20,000 Hz or higher) in fluorescent lights resulted in more than a 50% drop in complaints of eye strain and headaches.
There tended to be fewer complaints of headaches among workers on higher floors compared to those closer to ground level; that is, workers exposed to more natural light experienced fewer health effects. - Wilkins, A. J., I. Nimmo-Smith, A. I. Slater, and L. Bedocs. "Fluorescent lighting, headaches and eyestrain." Lighting Research and Technology 21, no. 1 (1989): 11-18.
Reader Question: flickering lights traced to bad utility company neutral wire
(Nov 10, 2015) Phil said:
I've been searching and can't find the answer to my problem. All of a sudden (last couple of months) my lights go dim whenever an appliance cycles on. All of them. Even when the coffee maker cycles on and off you can see it happen.
The lights even flicker to the cadence of the washing machine during the wash cycle. The dryer is the worst as the lights go waaay dim, but only for a second or two.
I have no electric heat, a 100 amp service with nothing special in the house requiring anything more than 100 amps. I have a well pump, septic pump and a freezer and all the normal things a house would have requiring intermittent electrical draw.
Nothing special, but why are my lights flickering upon appliance demand all of a sudden? I do have some electrical experience and would like to know some simple troubleshooting I could do before calling in the experts, and the power company etc. Thanks.
Reply: Look for what changed in the electrical system, look for aluminum wiring
As you are describing what is a new problem, not one that has been present since original construction, I infer that something has changed. Most likely there is a loose connection or a failing appliance.
The fact that it's "any appliance" is puzzling as I don't expect all appliances to be on the same individual electrical circuit.
Check this for me: take a look at the circuit layout in your electrical panel.
If the flickering only occurs when appliances run, let me know how those appliance circuit breakers are located in your panel. For example if they are all on just one side of the panel that would be diagnostic (and would point to a problem in the panel itself).
While you're there see if you can see any aluminum wiring, and let me know the brand and model of the electrical panel.
Reader follow-up: flickering appliances or lights on more than one electrical circuit
(Nov 11, 2015) Phil said:
None of the appliances are on the same circuit. The washing machine and septic alarm are on one circuit/one breaker, the refer. is on one circuit/one breaker, as is the dishwasher, septic pump, water pump, dryer, and range. I did observe that the range has alum.
wiring which has been treated with a type of grease at the breaker connection. The coffee pot is on my "kitchen receptacle" circuit, but shouldn't be an issue in my opinion. I've checked and re-checked every connection in the panel and all are tight and in good condition.
Washing machine, septic pump and water pump are on the left side of the panel, while the refer, dishwasher, dryer and range are on the right. The panel is Westinghouse and all the breakers are too. The freezer is on the basement receptacle circuit. Hopefully, this doesn't confuse you.
Reply: check the incoming service entry wires
As you're seeing flickering across multiple circuits I suspect the problem is
1. in the panel, a bad connection at the incoming service wire, neutral or hot, or bad connections of breakers to bus - since I don't know of defective breaker/bus issues specific to Westinghouse (and haven't seen many of those) I'd look with care (watch out you can be killed touching anything in the panel) at the connection of the mains and neutral to the main lugs in the panel - a job for an electrician.
An example of that problem is at
The SEC wires coming into the panel are secured in (in some brands) an aluminum lug with a steel screw or set-screw. The screw can feel tight because it's binding in the lug but it might not actually be tight on the wire.
If all of the bad-actors happen to be fed from breakers that pick up one of the two 120V sides of the panel that'd point to the culprit.
If the bad actors are on both sides of the panel the problem can also be outside: e.g. a buried (UF) service entry cable that's damaged or other damage to the utility company's SEC wires or overhead wires or even a pole transformer
Reader follow-up: hiring an electrician to check the panel for cause of flickering lights
Nov 12, 2015) Phil said:
Ok, I'm gonna hire an electrician to troubleshoot the panel, although it appears to be ok it my estimation. I have to start digging somewhere. I will let you know the outcome.
(Nov 16, 2015) Phil said:
My estimation was right. The panel is sound, very clean in the electrician's opinion. Took off every breaker and inspected them, all ok, very tight connections to the bus bars. Measured the power coming in and it was 120v on both legs. I'm miffed.
Reply: check the service neutral
If the panel seems OK I'd expect the electrician to start working outwards from the main lugs that connect the service entry wiring to the panel and onwards;
I've found, for example, that the service neutral was disconnected and the panel was "working" through its local ground path - a dangerous condition.
(Nov 19, 2015) Phil said:
Everything in the panel is satisfactory. There are no problems with the service neutral. Panel is 100% good.
As I sit here around 5:00 AM the only things running are my pellet stove (about a 200 watt draw), and 2 lights, and they continue to flicker with each cycling of the coffee pot going on and off. Oh, the refer. is going on and off too causing much flickering.
Reply: check the panel again or look outside the panel
As flickering lights are occurring on two different circuits I'd want to trace back those circuits to their origin, either in a shared junction box or in the panel.
I would look for a loose connection, aluminum wiring, or a problem in the panel that was not at first so obvious such as a poor connection of the panel neutral to the bus.
It would help to know the panel and breaker brands, and also to know if any aluminum wiring is present.
OR the problem is outside the electrical panel as suggested earlier.
(Nov 20, 2015) Phil said:
I'll attempt to trace back the circuits. Could take a while.....No aluminum, other than the SEC. The panel is Westinghouse and all the breakers are too.
Reply: check the SEC (service entry conductors) wiring
(Nov 21, 2015) (mod) said:
Phil the SEC could be the problem: particularly if the binding-head screws in the panel lugs are not really making sound contact with the SEC wires.
In some electrical panel, tightening the screw binds in the lug and you think it's tight on the wire when in fact the wire is loose. An electrician, who knows how to do this without being killed, might pull the meter, then remove and re-make the SEC connections.
Reader follow-up: flickering light problem found: bad neutral wire from the utility company
Had the local power company here yesterday. They installed a device in the meter socket that simulates a 1500 watt draw, and then measured the voltage on both legs. One side was 135v and the other 108v. That prompted him to check the transformer at the pole.
Transformer was fine, but found that the aluminum ground wire on the cable assembly from the pole to the house was 99.9% severed, with only one strand of aluminum left holding on.
They replaced the cable and VOILA, no more issues. My question now is
: What would have happened if it was 100% severed?
Reply: effects of a lost neutral at the SEC
Thanks for this important feedback, Phil. I've come across this issue before and have described it at InspectApedia.com in an article LOST NEUTRAL Shocks Homeowner - found at inspectapedia.com/electric/Electrical_Neutral_Lost.php
What happens is this: if you lose the electric utility's neutral AND if your panel is properly installed, you still have a local connection to earth - to ground - at your building. IN the panel where neutral and local ground are tied together, all of the neutral wires in all of your circuits continue to work but all of their return current is flowing out on the local ground.
Watch out: a lost neutral or bad electrical grounding are both very dangerous. Once the neutral has been lost the ground system will be doing work it's not designed to perform.
If there is anything marginal about the local ground - which is common - you can have flickering lights or even overheating and burn-ups on individual ground wires or circuits in the home. Ultimately the result is very dangerous as loss of safe grounding can result in someone being electrocuted.
Continue reading at DIRECTORY OF ELECTRICIANS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Boyce, Peter R. "Review: the impact of light in buildings on human health." Indoor and Built environment 19, no. 1 (2010): 8-20.
Girgis, Adly, John W. Stephens, and Elham B. Makram. "Measurement and prediction of voltage flicker magnitude and frequency." Power Delivery, IEEE Transactions on 10, no. 3 (1995): 1600-1605.
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Veitch, Jennifer A., and Shelley L. McColl. "Modulation of fluorescent light: Flicker rate and light source effects on visual performance and visual comfort." Lighting Research and technology 27, no. 4 (1995): 243-256.
Wilkins, A. J., I. Nimmo-Smith, A. I. Slater, and L. Bedocs. "Fluorescent lighting, headaches and eyestrain." Lighting Research and Technology 21, no. 1 (1989): 11-18.
Abstract: The weekly incidence of headaches among office workers was compared when the offices were lit by fluorescent lighting where the fluorescent tubes were operated by (a) a conventional switch-start circuit with choke ballast providing illumination that pulsated with a modulation depth of 43-49% and a principal frequency component at 100 Hz; (b) an electronic start circuit with choke ballast giving illumination with similar characteristics; (c) an electronic ballast driving the lamps at about 32 kHz and reducing the 100 Hz modulation to less than 7%. In a double-blind cross-over design, the average incidence of headaches and eyestrain was more than halved under high-frequency lighting. The incidence was unaffected by the speed with which the tubes ignited. Headaches tended to decrease with the height of the office above the ground and thus with increasing natural light. Office occupants chose to switch on the high-frequency lighting for 30% longer on average.
Winterbottom, Mark, and Arnold Wilkins. "Lighting and discomfort in the classroom." Journal of Environmental Psychology 29, no. 1 (2009): 63-75.
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Simpson 260® Series 6XLM
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 Kenneth Kruger: Original author of the sidebar on testing VOM DMM condition: Kenneth Kruger, R.A., P.E. AIA ASCE, is an ASHI
Member and ASHI Director in Cambridge, MA. He provided basis for this article penned by DJ Friedman.
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 "Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
 "Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
original source: http://www.nationalpumpcompany.com/pdf/Betta_Flo_IOM_Jet_Pump.pdf
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones