Author Topic: Help! Total beginner- Just recieved my new UNI-T multimeter and I've killed it  (Read 22459 times)

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Online Fraser

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That last comment made me shiver... I know you may have been kidding but...........

Anyone who is not familiar with the workings of the Microwave oven..... PLEASE do not use it or it's parts in experiments unless/until you learn about the correct safety procedures and take precautions.

I still break into a sweat when working on RADAR magnetrons and Microwave Ovens PSU's. There are absolutely no second chances if you get across the transformer HT winding or smoothing capacitor. The HT will happily punch straight through a set of old (degraded)  or inadequate test leads and I am personally aware of two deaths in the UK as a result. If you get it wrong on the HT supply, you die, no maybe about it. The reason is simple.... microwave ovens power high energy magnetrons using HT voltages at deadly currents. The HT capacitor will store enormous amounts of energy (in terms of Joules) and, if allowed to, release it into a victims body tissues at HT voltage and unrestricted current...... your heart will beat no more. Remember, it is the current that actually kills .... you just need a high enough voltage to set up conduction.

For those reading this with knowledge of the voltages and currents inside a microwave oven you will already know the dangers, but I beg those without the knowledge to not experiment or tinker with these things internals.  

And finally.... Anyone who thinks 110V is 'safe' just check out the domestic accident electrocution figures for the USA.... 110V ac can still kill if across your chest (arm to arm). We don't mess around with the light weight stuff here in the UK, we go the whole hog and use 240V ac to ensure quality conduction through peoples bodies ! It's a form of natural selection....  Also known as the Darwin effect for which a victim may win one of the famous Darwin Awards.

Sorry, a bit of a heavy posting but with good reason.
« Last Edit: October 21, 2010, 08:17:14 pm by Aurora »
 

Offline sonicj

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Offline Simon

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I am a firm beleiver in natural selection  :D
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Offline djsb

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Yep, 33,000 V/cm is the breakdown field strength of air at standard temperature and pressure.  So if you put 33,000 volts across a 1 cm gap it will breakdown.  Inside of a multimeter the field strengths between traces needed to breakdown are even less because the arcs can track along the surfaces, known as surface flashover.  Even further complicating the situation and raising the probability of breakdown are the points inside the meter where air, metal, and dielectric meet.  These points are called triple points and provide dense amounts of free electrons under HV excitation which can initiate a surface flashover event, making it even easier to break the device.

Gas discharge processes like surface flashover and volume discharge are somewhat chaotic and slow processes, so under pulsed excitation it takes larger field strengths for them to occur compared to a DC excitation.  This is why he probably doesn't see any visible carbon tracking (burns) from the failure because the pulses from the fence probably just blew up the first solid state device they ran into, internally.

Are there any references or other sources of information on high voltages? It could be useful so people can do their own research and become more informed on what to expect.

David
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Offline saturation

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http://en.wikipedia.org/wiki/High_voltage

It has all the basic elements for anyone to read more after.

Yep, 33,000 V/cm is the breakdown field strength of air at standard temperature and pressure.  So if you put 33,000 volts across a 1 cm gap it will breakdown.  Inside of a multimeter the field strengths between traces needed to breakdown are even less because the arcs can track along the surfaces, known as surface flashover.  Even further complicating the situation and raising the probability of breakdown are the points inside the meter where air, metal, and dielectric meet.  These points are called triple points and provide dense amounts of free electrons under HV excitation which can initiate a surface flashover event, making it even easier to break the device.

Gas discharge processes like surface flashover and volume discharge are somewhat chaotic and slow processes, so under pulsed excitation it takes larger field strengths for them to occur compared to a DC excitation.  This is why he probably doesn't see any visible carbon tracking (burns) from the failure because the pulses from the fence probably just blew up the first solid state device they ran into, internally.

Are there any references or other sources of information on high voltages? It could be useful so people can do their own research and become more informed on what to expect.


Best Wishes,

 Saturation
 

Offline Time

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Honestly, I can't think of any readily available online sources that address high voltage safety other than what one could find by just googling it.  Most of what I know comes from my time with my practical work and schooling (expensive text books :( ).  I spent a few years studying plasma discharge physics and just kind of picked up the know how through tinkering and exposure to high voltage, high energy systems (the kind used to generate plasma discharges).  I have been shocked many times over by something as low as the mains (I don't even know how many times, getting mains shocked is barely noticeable sometimes - its just a gradual tingle) to as much as 70 kV in my hand (definitely notice this as voltage of this magnitude will make a large pop sound and stings worse than any other sting I have ever felt).

The best advice I have for anyone handling anything live or charged is use one hand to touch the system.  A gloved hand, preferably.  i.e. If you are dealing with a large system don't lean over the chasis and support your weight on it with one hand and reach into the device with another.  Since the chasis is normally the reference potential for the dangerous voltages, your wandering hand suddenly becomes a lightning rod with your heart as the load.  If you use 2 hands than you can risk the current going across your chest and through your heart.  Using one hand is not even fail safe though. It takes very little current across the heart to kill you.  You can take much more current through less significant appendages and only receive painful burns or shock.  Though if its a large enough system it could very well dismember whatever its discharged through (yikes).  Another good rule is never lose your fear with what you are dealing with and relax when handling dangerous or live systems.  A lot of people in this kind of work become desensitized to the danger from their day to day dealings with these kinds of systems and relax causing them to become unattentive and careless.

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Offline Kiriakos-GR

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Honestly, I can't think of any readily available online sources that address high voltage safety other than what one could find by just googling it. 


I have not find either on-line training for any true profession , soooo  they it must teach all those at those large white square buildings called as technical schools and universities .   ;)
 

Offline PetrosA

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About 20-30 mA is all the heart can take. Class A GFCI (personnel) protection in the US is designed to shut off at ~5 mA. RCD protection in EU countries doesn't kick in till about 30 mA and is only designed to protect equipment.

Even I've made dumb mistakes before. I used my Agilent once to check the output of a HID socket not thinking that without a bulb in it and running, the ignitor was outputting 4 kV pulses. The Agilent survived, my pride was humbled ;) Interestingly, the Agilent wouldn't give a voltage reading on the pulse and the display flashed all kinds of numbers while a Fluke settled down at an incorrect 200V reading.
« Last Edit: October 22, 2010, 03:43:39 am by PetrosA »
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Offline Simon

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well in the EU a 30mA RCD is supposed to be the life saver, I think it takes 30mA to turn off fastest but will still kick off with lower currents taking more time to do so. 100mA is for equipment protection only and that's what some dick head put in my house before i bought it.
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Offline Jon Chandler

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My first meter was a $7 analog meter from RadioShack.  Amazing that the arcing from connecting it across a 750 VAC high voltage supply for a tube circuit in ohms mode can be seen through the case.  The burned electronics smell was overpowering too!

At least that was a cheaper lesson.
 

Offline Zero999

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For those reading this with knowledge of the voltages and currents inside a microwave oven you will already know the dangers, but I beg those without the knowledge to not experiment or tinker with these things internals.  
Yes, a microwave oven is certainly the most dangerous household appliance. People often go one about supposedly deadly 25kV old CRT TV tubes which are really quite low risk, the low voltage (300V to 600V) stored in other capacitors around the TV set is the real hazard.

Quote
And finally.... Anyone who thinks 110V is 'safe' just check out the domestic accident electrocution figures for the USA.... 110V ac can still kill if across your chest (arm to arm).
I think this is directed at my remark about voltages below 50VAC or 120VDC being classed as extra low voltage which is fairly low risk. I think you missed the fact I said 120VDC not 120VAC 50Hz or 60Hz which is much more dangerous. Of course 50VAC or 120VDC can kill as well, it's just unlikely so appliances designed to work from such voltages are designated as class 3 so don't need to conform to the same standards as those connected to low voltage mains power.

Don't forget that AC is really 1.414 times the RMS voltage and the human body is more sensitive to 50/60Hz AC than it is to steady DC, so from the US mains you're really receiving a 170V shock not a 120VAC shock and the changing current is interfering with your heartbeat more than steady DC.
 

Offline Kiriakos-GR

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Don't forget that AC is really 1.414 times the RMS voltage and the human body is more sensitive to 50/60Hz AC than it is to steady DC, so from the US mains you're really receiving a 170V shock not a 120VAC shock and the changing current is interfering with your heartbeat more than steady DC.

I need to see the scaned page of the book that says such things ..
 

alm

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Edit: Never mind, poor reading on my part.
« Last Edit: October 22, 2010, 02:20:20 pm by alm »
 

Offline gonnafail

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Well the 1.41 times the RMS voltage is definitely correct. I cannot comment on the human body being more sensitive to AC than DC but its plausible in my opinion.
A quick google search gave me multiple sources that stated the same saying anywhere from 5 to 10 times more sensitive to AC. Sometimes a google search can be faster than a post calling someone out.
 

Offline allanw

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alm

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I cannot comment on the human body being more sensitive to AC than DC but its plausible in my opinion.
Agreed. More current will flow (reactive impedance is likely to be lower at 50/60Hz than resistive impedance), and it's in the same order of magnitude as the heartbeat. Can't imagine that it's hard to find references in medical journals about electrocution with DC and 50/60Hz if anyone is so inclined.
 

Online Fraser

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 Hero999,

Thanks for the comment... please be assured my comment was not aimed specifically at your statement... it was really highlighting what I have heard many times from people who believe the 110/120VAC mains system is 'safe' or 'safer' whereas they believe the UK's 240VAC system is dangerous... an interesting concept that I personally would not wish to test on myself  ;) I was trained to respect all voltages, but for differing reasons.

An example of which was a colleague who decided to investigate an oil leak on his car. Whilst delving around the engine he managed to get his stainless steel watch strap across the starter motor solenoid terminals. 12V at hundreds of AMPS flowed through it instantly turning it into a heating element. He reported that the pain was excruciating and the smell ghastly as the watch strap melted itself into his wrist. Imagineā€¦ the strap links and release catch had welded together so it was not removable and was red hot until he could get to water :(  He still bears the scar to remind him of his mistake.

I think it has already been said here.... before people start 'playing' with test kit on live circuits they should know the possible risks and consequences in order to stay safe. I am certainly not perfect  ;D I managed to get some pretty nasty 240VAC belts on my hands whilst live working on mains powered CRO's, amplifiers and the like. The 240VAC shocks made my arm ache for some time after, but the 100 to 300 VDC valve anode shocks I also experienced just stung a bit. I was a teenager at the time so learnt my lesson well before being released on the professional world.
« Last Edit: October 22, 2010, 06:23:40 pm by Aurora »
 

Offline Zero999

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I need to see the scaned page of the book that says such things ..
Better still, I can provide a link to a good book on Google:

http://books.google.co.uk/books?id=B0aPhJn0YcAC&pg=PA963&lpg=PA963&dq=let-go+current+shock&source=bl&ots=eELpQzCycv&sig=xMzkz9AeJWLghhYrWU7B4Sa4VS4&hl=en&ei=SejpSajnJcONjAf64LibCg&sa=X&oi=book_result&ct=result&resnum=5#v=onepage&q=let-go%20current%20shock&f=false

Here are some more links:
http://www.pat-testing.info/electric-shock.htm
http://www.esdjournal.com/techpapr/elechazd/ehaz.htm
http://www.health24.com/medical/Condition_centres/777-792-2557-2577,15144.asp

Agreed. More current will flow (reactive impedance is likely to be lower at 50/60Hz than resistive impedance), and it's in the same order of magnitude as the heartbeat. Can't imagine that it's hard to find references in medical journals about electrocution with DC and 50/60Hz if anyone is so inclined.
Actually it's got nothing to do with the reactive impedance, the human body is a poor conductor so the resistance will be the dominant component of the impedance.

One myth I encounter often is that high frequency AC is less dangerous because of the skin effect which is not true because the human body is a poor conductor so the skin depth is very deep. If the skin effect offered considerable protection then a microwave oven would only heat meat on the surface, like a grill but in reality it penetrates quite deeply.

Thanks for the comment... please be assured my comment was not aimed specifically at your statement... it was really highlighting what I have heard many times from people who believe the 110/120VAC mains system is 'safe' or 'safer' whereas they believe the UK's 240VAC system is dangerous... an interesting concept that I personally would not wish to test on myself  ;) I was trained to respect all voltages, but for differing reasons.

I agree. I think part of the problem is that in the UK 110VAC is used on building sites for safety reasons and is classed as reduced voltage. The reason why the 110VAC used in the UK is safer than that used in the US is because in the UK it's derived from a centre tapped transformer or two phases of a three phase transformer so the maximum voltage with respect to earth is 55V (single phase) or 63.5V (threes phase), in the US 110V is 110F with respect to earth so is much more dangerous.
 

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Agreed. More current will flow (reactive impedance is likely to be lower at 50/60Hz than resistive impedance), and it's in the same order of magnitude as the heartbeat. Can't imagine that it's hard to find references in medical journals about electrocution with DC and 50/60Hz if anyone is so inclined.
Actually it's got nothing to do with the reactive impedance, the human body is a poor conductor so the resistance will be the dominant component of the impedance.
Your point that impedance at DC and 60Hz is the same may be valid, I couldn't find a proper reference in the few minutes I had available (the Google Books link doesn't work anymore, and the other links didn't mention impedance), but I don't see how your argument supports this. The ideal capacitor has an infinite DC resistance, but depending on capacitance, may have a low reactive impedance. PTFE is a pretty poor conductor, but that doesn't mean that it will dominate the impedance in a PTFE capacitor with an AC signal applied.

Also, the human (dry) skin is a pretty poor conductor, the rest of the body is not as far as I know, and is often considered a good conductor compared to the skin in models of the human body.

One myth I encounter often is that high frequency AC is less dangerous because of the skin effect which is not true because the human body is a poor conductor so the skin depth is very deep. If the skin effect offered considerable protection then a microwave oven would only heat meat on the surface, like a grill but in reality it penetrates quite deeply.
I don't follow this argument either. A microwave oven is about absorption of an electrical field, it's not the induced currents that are dangerous, but the energy (heat). With RF burns, the opposite is the case: the heat is generated by the current, since the body doesn't absorb most of the RF spectrum very well.
 

Offline Time

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One myth I encounter often is that high frequency AC is less dangerous because of the skin effect which is not true because the human body is a poor conductor so the skin depth is very deep. If the skin effect offered considerable protection then a microwave oven would only heat meat on the surface, like a grill but in reality it penetrates quite deeply.


Ehhhhh, I can't disagree with this statement but the physicist in me is not sure the microwave oven analogy is a good one.  Yes, the skin depth is deeper on a more resistive element but in some cases when the magnetic permeability of something is high enough than the skin depth is shallow.  Consider iron vs. copper in this case.  Iron is more resistive than copper but is a terrible high frequency conductor because of its magnetic properties.  The higher the permeability of something the longer it takes the magnetic field from the current to penetrate the material.  This limits the current flow to the 'skin' of the conductor.

A convential microwave oven heats materials by producing microwaves at frequencies that cause molecular bond structures to harmonically resonate with the oscillating electric field component of the microwaves.  This frequency is usually optimized around the OH in H20 since water is present in all foods.  A good conducting material will heat or cause sparks because the electrons in the material are easily influenced by the field and allowed to move inside the conductor to oppose the influence of the e-field (more so, no electric field can exist in a conductor which gives rise to some complicated boundary conditions in the case of EM theory which I dont know how to describe without differential equations).

Yes, microwave energy does not penetrate a good conductor and yes it penetrates a poor conductor more thoroughly but the governing phenomenon is different than what gives rise to the skin effect.  I don't know if any of that even makes sense.  I have already rambled a lot on this thread.

edit:

In regards to alms post below, tissue is a dielectric just as PTFE.  Dielectrics and insulating materials behave differently than materials with any considerable amount of bulk conductivity.
« Last Edit: October 22, 2010, 07:54:56 pm by Time »
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Offline Zero999

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Your point that impedance at DC and 60Hz is the same may be valid, I couldn't find a proper reference in the few minutes I had available (the Google Books link doesn't work anymore, and the other links didn't mention impedance), but I don't see how your argument supports this. The ideal capacitor has an infinite DC resistance, but depending on capacitance, may have a low reactive impedance. PTFE is a pretty poor conductor, but that doesn't mean that it will dominate the impedance in a PTFE capacitor with an AC signal applied.

Also, the human (dry) skin is a pretty poor conductor, the rest of the body is not as far as I know, and is often considered a good conductor compared to the skin in models of the human body.
I wasn't intending to back up my argument regarding impedance of the human body, just reinforcing what I was saying about the effects of AC vs DC current on the human body being the reason why power frequency AC is more dangerous than DC.

You're right about the impedance of dry skin increasing the reactive component, but if the skin is dry not much current will flow at 50/60Hz because the capacitance will be really low, probably under 100pF. I don't have any information on the permittivity of human skin but I doubt it's high enough to make the capacitance high enough to allow a significant current to flow, if it was it would be used to make capacitors :D. At higher frequencies, it's true a significant current can flow due to capacitive coupling but at those frequencies the nerves won't be sensitive to shock so it's low risk. Besides, at low frequencies the insulating properties of the skin have to be compromised anyway to allow a harmful current to flow and damp or carbonised skin will have very poor dielectric properties.

Quote
I don't follow this argument either. A microwave oven is about absorption of an electrical field, it's not the induced currents that are dangerous, but the energy (heat). With RF burns, the opposite is the case: the heat is generated by the current, since the body doesn't absorb most of the RF spectrum very well.
You're right there to but if the skin depth of flesh was very shallow the radiation wouldn't penetrate the surface so it would work like a grill.

Ehhhhh, I can't disagree with this statement but the physicist in me is not sure the microwave oven analogy is a good one.  Yes, the skin depth is deeper on a more resistive element but in some cases when the magnetic permeability of something is high enough than the skin depth is shallow.  Consider iron vs. copper in this case.  Iron is more resistive than copper but is a terrible high frequency conductor because of its magnetic properties.  The higher the permeability of something the longer it takes the magnetic field from the current to penetrate the material.  This limits the current flow to the 'skin' of the conductor.
Human flesh has a low permeability so I don't see what your point is there.

Quote
A convential microwave oven heats materials by producing microwaves at frequencies that cause molecular bond structures to harmonically resonate with the oscillating electric field component of the microwaves.  This frequency is usually optimized around the OH in H20 since water is present in all foods.  A good conducting material will heat or cause sparks because the electrons in the material are easily influenced by the field and allowed to move inside the conductor to oppose the influence of the e-field (more so, no electric field can exist in a conductor which gives rise to some complicated boundary conditions in the case of EM theory which I dont know how to describe without differential equations).

Yes, microwave energy does not penetrate a good conductor and yes it penetrates a poor conductor more thoroughly but the governing phenomenon is different than what gives rise to the skin effect.  I don't know if any of that even makes sense.  I have already rambled a lot on this thread.
That's another myth, the frequency of microwave radiation has nothing to do with the resonance of water and everything to do with the skin depth and dimensions of a microwave oven. Liquid water has no strong resonant peaks because any resonance is damped by the hydrogen bonds fixing each molecule to its neighbour. Only gaseous water has strong resonant modes because the molecules are free enough to move and the 2.45GHz used in a microwave oven is an order of a magnitude too low to excite any resonance.

http://amasci.com/weird/microwave/voltage3.html
http://www.zyra.org.uk/microw.htm

« Last Edit: October 22, 2010, 08:31:26 pm by Hero999 »
 

Offline Time

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Ehhhhh, I can't disagree with this statement but the physicist in me is not sure the microwave oven analogy is a good one.  Yes, the skin depth is deeper on a more resistive element but in some cases when the magnetic permeability of something is high enough than the skin depth is shallow.  Consider iron vs. copper in this case.  Iron is more resistive than copper but is a terrible high frequency conductor because of its magnetic properties.  The higher the permeability of something the longer it takes the magnetic field from the current to penetrate the material.  This limits the current flow to the 'skin' of the conductor.
Human flesh has a low permeability so I don't see what your point is there.

Quote
A convential microwave oven heats materials by producing microwaves at frequencies that cause molecular bond structures to harmonically resonate with the oscillating electric field component of the microwaves.  This frequency is usually optimized around the OH in H20 since water is present in all foods.  A good conducting material will heat or cause sparks because the electrons in the material are easily influenced by the field and allowed to move inside the conductor to oppose the influence of the e-field (more so, no electric field can exist in a conductor which gives rise to some complicated boundary conditions in the case of EM theory which I dont know how to describe without differential equations).

Yes, microwave energy does not penetrate a good conductor and yes it penetrates a poor conductor more thoroughly but the governing phenomenon is different than what gives rise to the skin effect.  I don't know if any of that even makes sense.  I have already rambled a lot on this thread.
That's another myth, the frequency of microwave radiation has nothing to do with the resonance of water and everything to do with the skin depth and dimensions of a microwave oven. Liquid water has no strong resonant peaks because any resonance is damped by the hydrogen bonds fixing each molecule to its neighbour. Only gaseous water has strong resonant modes because the molecules are free enough to move and the 2.45GHz used in a microwave oven is an order of a magnitude too low to excite any resonance.

http://amasci.com/weird/microwave/voltage3.html
http://www.zyra.org.uk/microw.htm



The point was not refuting your argument against its occurence in tissue.  I was just explaining the phenomenon and than going on to explain why its not the same as what is occuring in microwave heating.  

Oxygen is inherently very electronegative so its presence in a molecule usually causes a net charge distribution across the molecule.  Any bond with a charge distribution will move in some manner under the influence of a field , of course.  Anything with movement will have an inertia or momentum and momentum transfer with surrounding molecules (thermal transfer) and because of this a frequency (or range of frequencies) can be optimized for the most ideal energy transfer from these movements.  By resonating and harmonic I was not refering to spectroscopic data and peaks like the literature you presented refers to (nice mickey mouse color schemes, by the way :P ).  Anyhow, all your sources even go to say "microwaves work because they shake water up" and shaking water up has nothing to do with the skin effect.  Skin effect refers to electron flow.  Microwave heating in a non-conducting material has nothing to do with electron flow.
« Last Edit: October 22, 2010, 09:21:24 pm by Time »
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Offline Time

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http://www.martin.chaplin.btinternet.co.uk/microwave.html

Excellent link.  It was a citation in your first source, Hero.

The electromagnetic penetration portion might be more what you are refering to.  Dielectric loss tangents are not the same as skin effect.  The loss factor which is dependent upon relative dielectric constant which is largely governed by water, especially in food.
« Last Edit: October 22, 2010, 09:16:18 pm by Time »
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Offline Simon

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I used to have a 24V AC/DC power supply i made from salvaged parts from a TV, at 24VDC I could feel nothing, at 24VAC I could feel a tingle in my fingers
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