How safe is "low voltage"?

[ryan_618]

Re: How safe is "low voltage"?

Here are a couple of pictures from the fire that I was describing:

 

[tom baker]

Re: How safe is "low voltage"?

Ryan thanks!
I'm sure Mike Holt would like to see these.
I'd like to see an article about this in the IAEI Journal.
I'll be using this in my next class.

 

[LawnGuyLandSparky]

Re: How safe is "low voltage"?

Originally posted by lquadros:
I agree with you Ron. Any thoughts about electrocution on low voltages?
Anybody have any experience?

Were you really expecting a reply from someone electrocuted by low voltage? :roll:

 

[LawnGuyLandSparky]

Re: How safe is "low voltage"?

Originally posted by ryan_618:
----------------------------------------------

>snip<

How Safe Are ?Low Voltage? Systems?

As you would agree, the world of electricity is full of misconceptions and misunderstandings. Many of the things we were told to be true simply aren?t, and many of the concepts that many people have held dear are not true either. One of these misconceptions that I?m sure we have all been told is that low voltage systems are safe from an electrical shock or fire perspective. Nothing could be farther from the truth.

>snip<
Ryan Jackson
2nd Vice President Utah Chapter of IAEI

Here's another real-life senerio:

A few years ago, while running conduit homeruns, an apprentice asked why the lighting conduits are seperate from the power conduits. My answer touched on the topic of high-voltage 277v lighting being more efficient in an office building, and how we frown upon mixing conductors of different voltages, or from different electrical closets, etc...

Later on in the project, that apprentice asked why low(12)volt mr-16 hi hats and 120 volt track w 12v heads were used in some rooms? I answered how 12v lighting was supposed to be more energy efficient, safer, run cooler.

Sharp as a tack the apprentice wanted to know why higher-voltage lighting and lower voltage lighting was better and more efficient but not 120v, the most common voltage?

This lead us both on a quest for the truth. His path was to ask more people the same questions, but I did some bench testing and took amprobe readings.

It's obvious 277v lighting is more efficient as it cuts down on the voltage drop problems in hi-rise or sprawlingly large buildings. But the claims from low-voltage fixture manufactures turns out to be patently false. Not only would a consumer have to suffer the 25-50.00 added cost for each fixture to house an additional transformer to step line voltage down to 12v, but also have to pay to remove that added heat that is generated in the process.

A 12v 50w MR-16 generates no less heat than it's 120v PAR-16 counterpart.

I suspect the real reason LV lighting is being touted as "safe & efficient" is to draw attention away from the fact that the only component of these systems that has a UL listing is the step-down transformer. Many of the "artsy" track heads or wild-looking 12v distribution cables, trolleys, and bendable track are not listed at all.

 

[tom baker]

Re: How safe is "low voltage"?

"A 12v 50w MR-16 generates no less heat than it's 120v PAR-16 counterpart."

Part of the reason for the 12 V lamps is the fixture is smaller, looks better, and with the smaller lamp the photometrics are better (get better light control with a point source)

[ February 05, 2006, 12:18 AM: Message edited by: tom baker ]

 

[GeorgeB]

Re: How safe is "low voltage"?

Originally posted by bphgravity:
Remember that 24 volts DC is over 30 volts AC RMS.

Bryan, you're one of the sharper tacks in this box; where you get this idea is puzzling. RMS is the value equivalent to DC from a heating perspective; using resistive loads, 30VAC RMS will deliver 1.5625 times (30*30 / 24*24) the power to the same load.

If you are talking about peak voltage, the "peak-to-peak" of 30VAC RMS will be 2.83 x 30 or 85 volts.

 

[sparrott4]

Aside from the danger of fire, let's be clear about the risk of electrocution with low voltage lighting.

I did an extensive search reviewing the past 20 years of OSHA, medical and engineering records on the subject. Here's a quick summary of the findings:

1. In the past 20 years there are no records of any serious injury or death resulting from contact with conductors carrying less than 30V.

2. A widely respected organization, the International Electrotechnical Commision (IEC), has recently concluded studies on the effects of low voltage shock on humans. They concluded that there is no risk of death with any current under 30 mA.

3. To calculate the mA exposure of low voltage lighting, you need to consider two things ? voltage and resistance. The highest voltage we encounter in landscape lighting is 22 volts. The applicable resistance is the path from the conductor to the hand through the body to either another conductor or through ground. The lower the resistance the higher the amperage going through the body. A reasonable (but unlikely) worst case scenario would be grasping a terminal block (say, a common) in one wet hand and the 22 volt terminal in the other wet hand. The resistance going from hand to hand would be approximately 1000 ohms. Since Amps=Voltage/Resistance, this worst case scenario would result in a current of 22 mA (well below the danger threshold of 30 mA).

4. A more likely scenario would be touching bare wires (at 22 volts) with wet bare fingers - this would result in in a current of around 2 mA (just above the 1 mA threshold of a tingling sensation.)

5. Touching two wires with dry hands (at 22 volts) results in a current of 0.02 mA.

 

[winnie]

A large portion of human body resistance is the resistance of the skin.

The resistance of dry skin is high enough that you can _probably_ touch both terminals of a 24V source (even a multi-thousand-amp-hour battery) and not have any sensation of current flow, let alone harm from the shock.

With wet skin, the resistance drops considerably, and you will probably have a sensation of current flow.

I'd bet that there are some circumstances that could lower the resistance enough that 24V could deliver a fatal shock. Perhaps contact with other body parts (the palms of the hands have very thick skin) combined with salt water, or actually getting _cut_ with energized metal.

-Jon

 

[zog]

1. In the past 20 years there are no records of any serious injury or death resulting from contact with conductors carrying less than 30V

.

Navy electrician was killed from output of a Fluke (4-7V) in 1990. Not found in OSHA records, you have to check public DOD records, this event is rare but possible unbder perfect comditions (Skin was punctured).

2. A widely respected organization, the International Electrotechnical Commision (IEC), has recently concluded studies on the effects of low voltage shock on humans. They concluded that there is no risk of death with any current under 30 mA.

For what duration? 10-20 mA can cause loss of muscle control (For AC) and cause one to be stuck on the circuit, increasing the duration will increase the damage to tissue. For a short duration I would agree (< 1/2 second), longer durations will cause heating of tissue per Ralph Lees tables.

3. To calculate the mA exposure of low voltage lighting, you need to consider two things ? voltage and resistance. The highest voltage we encounter in landscape lighting is 22 volts. The applicable resistance is the path from the conductor to the hand through the body to either another conductor or through ground. The lower the resistance the higher the amperage going through the body. A reasonable (but unlikely) worst case scenario would be grasping a terminal block (say, a common) in one wet hand and the 22 volt terminal in the other wet hand. The resistance going from hand to hand would be approximately 1000 ohms. Since Amps=Voltage/Resistance, this worst case scenario would result in a current of 22 mA (well below the danger threshold of 30 mA).

You forgot about contact area and pressure, if you were grasping the conductors with a thumb and forefinger with wet hands the skin resistance would be 2-5 k Ohms for ewach hand plus internal resistance of about 250 Ohms per arm, so it would be between 4500 Ohms and 10500 Ohms(Depending on pressure). There are several other factors (Weight, sex, age, etc) that will skew these values but they are close. All this does is backup your arguement that the current would be below any dangerous levels.

4. A more likely scenario would be touching bare wires (at 22 volts) with wet bare fingers - this would result in in a current of around 2 mA (just above the 1 mA threshold of a tingling sensation.)

Wouldnt touching bare wires with dry hands be more likely?

5. Touching two wires with dry hands (at 22 volts) results in a current of 0.02 mA.

It sounds like your research has been based on Charles Dalziel reasearch. If you are interested in continuing your research, try contacting Dr Michael Morse PhD, he is one of the leading guys in human shock reasearch today. Or PM me, I always welcome discussions with people interested in resaerch and understanding of electric shocks.

 

[zog]

I forgot, for LV shocks you may want to reasearch Diffuse electrical injuries. Electrical injuries can produce physical, neurological and neuropsychological sequelae that exist even in the total absence of a theoretical current path that includes the brain. Diffuse Electrical Injury (DEI) is a rarely occurring class of electrical injury that can occur even after low voltage contact.

 

[e57]

Ryan, I'm noticing no primary or secondary circuit protection at that transformer..... Most of those units are shipped with at least secondary, and it is still a transformer... Had they been the fire may not have happened.

As for the people who think they can not get a good shock from low voltage, I have two things NOT to try.... Niether are pleasurable

Touch a ringing phone pair.... I have done this, it hurts.

Stand wet and naked in a tub, and handle a low voltage fixture like the one shown with exposed conductors. I have not done this, but I have gotten a small shock and tingle from those types of fixtures, and would not want to try it wet, or naked.... Would nieither be fatal, may be not. (Unless I had some type of arithmia, may be, or a pace maker) A lot would depend on the circuit path, and a number other variables. But it could happen. Remember, not all Darwin canidates get the award, just the winner.

 

[iwire]

Touch a ringing phone pair.... I have done this, it hurts.

Yeah, been there....an unpleasant lesson.

I have since learned the ring voltage is around 80 to 90 volts DC. Not so 'low'.

 

[69boss302]

Touch a ringing phone pair.... I have done this, it hurts.

You ain't a kidden it hurt's. I've been bit by this. I was down in the basement tapping into add a line into another room, when of all things the phone rang. I was yelling up to people in the living room. Somebody answer the phone. They said it aint ringing. I told them, OH YES IT IS!. They went ahead and answered it and I just sat down and waited as I explained to myself: "self" I said, "that hurt".

 

[The Timinator]

I agree Ryan Although some LV systems are different from others.

You have a satellite system which sends 13, and 18 Volts. I can give you a buzz, and it can arc if connections are lose. I've got some good buzzes of phone lines to. At ringing voltage it's just enough to get locked up. I agree that there should be a standardized test for LV, but I don't want the States to create a huge Bureaucracy out of this. Like Washington State. Comm. techs don't need to be licensed electricians just licensed Comm. techs. Most Comm. techs would not be wiring lighting anyway. That should be for electricians. How true that statement is about misconceptions. They right the code like it's some ancient prophecy