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Can a standard 3 phase Dry Type transformer rated 480v.pri.-208v.sec. be run in reverse?....i.e., 208v.pri.-480v.sec.?
If not, why not? Wyatt October 22 2014, 11:29 pm EDT Reply to this comment |
Mike,
I received the email last week about EMFs folks have in their homes and need help resolving. I really enjoy reading and researching issues on your web site, so keep up the good work. However, I totally disagree with where the EMF newsletter is leading folks. I was disappointed with how this article mislead people into thinking that they could find an EMF expert to solve their minor problems within their home and this may lead to them not having an EMF health issue. Yes, there may be some miss-wiring within the structure and if found it should be corrected. As a contractor I've seen my share of this, but I think 100% of these would not cause any direct health issue. I'll explain this statement further below.
As a licensed electrical engineer (NM) and electrical contractor with 14+ years experience at electric utilities and 22+ years in the semi-conductor industry, I have worked on numerous EMI & EMF issues. A majority of these were with the utilities' residential customers.
I did some research about 25 years ago on effects of EMF and how it may impact folks health. It basically boiled down to that the numerous scientific studies completed resulted in no known correlation between EMFs and negative impacts on people's health. Look at the following two links for a bit more on this:
* emfandhealth.com
* www.who.int/peh-emf/about/WhatisEMF/en/index1.html
While these are just two examples, there are numerous studies completed on animals and humans and in all true data-based cases the medical community can find no major link to issues with human health. I know not everyone is the same, so there may be some folks out there that are more sensitive to EMF and EMI; however, in the cases where I ran into someone that stated that they had an issues with a power line or a field in their home, I was able to show them that they had larger concerns than the electrical wiring within or outside of their homes. In most cases, I hate to admit, I think these people could use a good psychiatrist, but since I'm just an EE, I couldn't tell them that. Maybe having an expert come and say their wiring is fine, would also cure them.
I remember one person stated that they were sick because of the magnetic field at her residence. As an engineering supervisor for the utility I was told to go investigate this issue, since they kept complaining about power problems. I met the customer at their home and took an ac Gauss meter along with me. I began questioning them about where it was they were getting the worst feelings about this. They took me to their property line and said it occurred along the north side of her lot. It just so happens that there is a single-phase 7200V line paralleled with this lot line. The power line was properly installed and only about 10 years old at that time. The highest EMF reading along the stated line, that I observed with the Gauss meter, was 7 mG. Yes, this would vary with the load, but the hot summer day I measured the field, would most likely have been within a few percent of when the line saw its peak load.
I then asked the person if they used the electrical power within their home. They gave me a strange look, but answered the following questions:
1. Do you have an electric range?
2. Do you use a hair dryer?
3. Do you have an electric clothes dryer?
4. Do you have any fluorescent lighting in the home?
The individual answered yes to all of these questions. I asked them how they felt when near these locations while operating the appliances or lights. They said they felt fine near them but only had problems near the power line outdoors at the property line.
I asked them when they started having the health problems they associated with the EMF. I asked them if they had done any reading on the health effects of power and the magnetic flux density associated with them. They stated they had gone to the library and read about it. It turned out their problems started after the library trip. I then asked if I could measure the field strength at the appliances within the home. They said that it would be okay and here are my findings:
1. Range = 511 mG at 1 foot above surface, with oven on and largest burner on high. It varied depending on which element was energized and at what level it was set.
2. Hair dryer = 235 mG at 1 foot from the dryer on the low setting. I don't remember the high number, but it was worse.
3. The clothes dryer had a 400 mG reading at 1 foot in front of the dryer.
4. The worse reading (highest) was at the surface of the fluorescent fixture mounted on the ceiling in the kitchen. It was over 1 Gauss (>1000 mG). At 5' off the floor is was just under 900 mG. That is 128 times the reading taken under the medium-voltage single-phase line.
I told them to stay away from the property line if that is where they felt ill, but I told them I didn't know how they were going to live in the home with the field strength in the home being many times larger than was outdoors.
Yes, this is just one example, but I have lots of them. Some were with 345 kV transmission lines that were within 100' of the back of residences. Yes, at times it may induce a low-level current on their rain gutters, but as far as the flux density goes, there are many more devices within a home that have a significantly larger field.
This leads me to the ethical statement that I wrote above. I really feel it is unethical to tell people to go find an expert on EMF and have their homes looked at. Yes, the wiring may have the non-grounded conductor and the neutral separated for some distance, especially at the panelboard; but the field nearby is so low, compared to the other electrical appliances, it is insignificant.
In most of my dealings with EMF/EMI, especially with residential customers, I think these people would be better off spending their money getting some psychosomatic medicine.
Regards,
Dwight D. Dupy, P.E. Dwight Dupy September 15 2014, 8:09 pm EDT Reply to this comment |
Ernie, you are describing the assumptions engineers made up until the late 1960s, when health researchers discovered statistical links between non-thermal levels of power frequency magnetic fields and health. You are basing your opinions on short-term thermal risk.
There is a huge amount of published info on the results of the health research, funded by the Department of Energy (DOE), the Electric Power research Institute (EPRI) the Bioelectromagnetics Society, and many European governments, all relating to non-thermal effects.
However, this is a forum for the electrical industry, and all you have to know is that if you walk into a building and turn on a gaussmeter, and get a reading of over a mG or two in areas (not spots) of the building, you have either a Code violation or a current traveling on grounding conductors.
Karl Riley Karl Riley September 13 2014, 2:36 pm EDT Reply to this comment |
If you don´t think EMF can do harm you have never used a microwave
or know that the term microwave means high frequency EMF. Bob September 10 2014, 5:02 pm EDT Reply to this comment |
Just to add. Both the magnetic field are blocked by metal conduit
or cabinet. The magnetic field is present only when current flows
but the electric field is present when there is an electrical
potential difference. The higher the potential the stronger the electric
field and accordingly the more current the stronger the magnetic field.
If the load is balanced they are canceled out. This is why the NEC does not allow
a single conductor in a metal conduit. It would induce a current in the conduit
and heat it up. In this case the conduit has a magnetic field. Bob September 10 2014, 4:52 pm EDT Reply to this comment |
Re; The EMF QUERY-I was unaware that EMF experts exist . Engineers can calculate field magnitudes and show you "Yup ,you have a problem situation ". Unlikely that all can trace and be licensed to correct the problem. The info provided is sketchy. Why would the h/owner moving to california consult an expert in N.H. . All things aside,this sounds like new Homeowners who are so 'system involved ' that moving into any new home will still result in what a system sees as "Noise" on their lines . This can be from proximity our different lighting sources,motors on the same line [or even same phase with sensitivity of many heavy draw server systems which are more common than we think]. You're in a home . Easiest way is to of course run some new dedicated,isolated feeds to supply the main equipment areas . Most residential work involves dedicated server rooms and feeds these days-even renos. Running the clean feeds is inevitably cheaper than hiring an expert who in the end says "but you need a lic'd electrician to do the work . You'd be surprised at what a lic'd elec can do . I was an insp and a lic'd elec==I can even do calculations-imagine that-but I'm not an EMF expert . That is a new one on me . i'm a former M.Holt forum member but Sandy washed away all my pswrds and computers . I am mthead from LBNY Jim Owens September 10 2014, 5:27 am EDT Reply to this comment |
BS. It has been long proven, or no proof at all that magnetic fields cause illness.
I did research in 1992 for a large west coast utility that showed household applicances have some of the highest magnetic fields and they are all properly wired. The fields generated by a microwave, when you stand within a foot or so is very high. Electric blankets are also very high. Power lines - very low due to the distance from the lines. Magentic fields drop off by the inverse square of the distance.
If they are so concerned, then throw out all your household appliances, hair dryers, curling irons, flat irons, heating blankets, and so forth. Use oil lamps, wood burning stove and get a gas water heater. Next, take your incoming service and raise it up about 20ft; burying it puts it closer and soil or conduit does not block magentic fields. kingpb September 9 2014, 8:41 am EDT Reply to this comment |
I'm going past just low frequency issues which are harmless
to humans and so will expand on the low to RF spectrum.
Disregard this if not interested in more than the magnetic
field issue. Bullets below.
Background: A background in electrical/electronic
engineering/product safety engineering and a career
overlapping Navy (electronic crew chief) deep into this
field from EMF/MF, radar to communications to microwaves.
I will list some points covering ELF-MFG-RF. I can address
quickly from 40+ years hands on work and research and
actually measuring RF and related fields. Bottom line,
there is some over-reacting to an unknown (to them) anxiety
working here.
Some quick points: (And conclusion in short, following)
•All of us are exposed continually to MF/RF energy
every minute of every day and night. (20 year Cancer rates
and other tell-tales remain flat or are falling, etc.,
yet exposure continues to increase over 50 years.)
•RF covers the spectrum from 3 Hz to about 3,000 GHZ
and is referred to as Non-Ionizing.
•Non-ionizing (RF) covers everything from power-lines
to microwaves, infrared through the visible spectrum and
approaches Ionizing radiation at wavelengths shorter than
ultraviolet.
•Ionizing Radiation gets into the very high
ultraviolet (very small wavelengths) to X-RAY and Gamma
rays.
•RF: Very Low Frequency (VLF or ELF extra low
frequency) like power lines can induce low currents in
humans SAR units or Specific Absorption Rate limits,
have been recommended) but harm is non-thermal and rare
even with high-focused close distances. I would absolutely
not be concerned for virtually any normal work environment.
•RF: Covering Radio to microwaves: Harm can only
occur thermally and it is the only mechanism for injury
yet shown by any scientist for RF exposure to date.
Very high energy density or time exposures required!
Some Exceptions for UV/IR. Some positive effects shown
like RF to accelerate bone growth.
•RF: Infrared to ultraviolet wavelengths approaching
energy levels beginning to threaten from an optics
standpoint (eyes) and of course photochemical effects
(e.g. Skin).
•Ionizing: Once wavelengths get shorter than 100 nm,
energy (electron-volts) begins to exceed 12eV which can
disrupt/corrupt cell structures.
•US NAVY: Exists surrounded with radar,
communications, etc., bristling with high energy yet
career sailors have lower cancer etc., rates that the
average citizen the last time I looked. (Full disclosure…
they have excellent training in safety from energy sources
up to Nuclear which they’ve used for about 55 years.)
Short conclusion(s):
By and large considering Low Frequency electrical, RF, Non-ionizing radiation has
some risk optically, to skin (UV/IR) and RF Burns (circuit
measurement exposures) which can literally punch a
self-healing hole in a finger. Other than sitting in front
of, or on, an operating radar dish for too long the average
worker will be fine. Microwaves (along with other sources)
can interfere with asynchronous pacemakers but Doctors
brief all patients on that risk. Microwave emissions have
a TLV (10mw/sq. cm) or 40 minutes at 100mw/sq. cm. And so
on…
There is very little to fear from EMF MF.
Bottom line: You rarely need to take any drastic action
unless it is to suggest they investigate significant energy
sources where they work and request assurances.
Ernie Harper, CSP, DABFE, DABFET, CFC September 9 2014, 12:22 am EDT Reply to this comment |
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