Re: GFIs, Hairdryers, and Bathtubs ...
I have been reading some of the responses to this note. It appears many of us product safety professionals are missing some key points. I would like to thank the author for bringing this up for discussion. This is what makes the bulletin board so valuable. A GFI is a very simple devices that works by monitoring the amount of current that flows in through the hot and out through the neutral. If there is a difference of more than about 3 mA (and no more than 5), the device trips. Basically speaking, out must equal in. If it doesn't, current has found another path to travel to ground. A persons resistance to fatal shock varies with a multitude of factors, but it is generally safe to say that anything above an amp flowing through the body is fatal. Our nerves are designed to handle milli and micro amps, and anything significantly above that will burn them out rapidly. If the nerves don't work, key muscles like the heart become ineffective. Unless you are filling your bath tub with distilled water, there are enough conductive impurities to allow small amounts of current flow. In many municipalities that are large amounts of iron in the supplies. In addition, almost every city adds chlorine, which is somewhat corrosive, and will pull the copper and iron ions off of the distribution system piping. Again, remember that we are talking about very small amounts of current. Being a parent myself, I know what it is like to let two young children in the bathtub at the same time. There was likely water all over the floor, the sides of the tub, various toys around the area, etc. Since there was no GFI breaker installed, I am guessing that this was a fairly old dwelling (GFI's have been required by code since the 70's). This means the appliances were probably scratched, dinged, dented, etc, and the pipes were copper and cast iron. It is also code required that metal supply piping in houses be tied to ground somewhere in the dwelling, so there ire ample routes to ground. Most lighting circuits in houses are either 15 or 20 amp circuits. The breakers are designed to allow for "turn on" surges, meaning they will not immediately trip if current flow crosses the threshold. Some breakers will operate at 20% overage for u to 5 minutes. Also, breakers are generally only guaranteed to be accurate for a limited number of trips. There may in the case have been a full 15 or 20 amps going to ground. Even though these girls were at the opposite end of the tub, they were able to get enough of that current flow to kill them, even if it wasn't the entire amount Any way you look at this, it was a tragedy. I reject the idea that this was just a bad set of circumstances. The NFPA claims that bathroom electrocutions have drooped by over 90% since the requirement for GFI breakers was added to the Code. This is a compelling statistic. The bottom line is that the newspapers were right when they stated that this event would not have occurred for want of a $20.00 item available at any hardware store. If you are evaluating system designs, and there are single phase low power systems where people could come in contact with electricity and water at the same time, remember this. The Code requires GFI's essentially anywhere there is water, and/or portable hand tools (drills, mixers, and yes even hair dryers). The way implement this requirement determines if we consider the code a proactive approach, or an obstacle. I have often seen convenience outlets in machine tools that are not GFI protected, and heard the argument that a GFI is not required by the code. This misses the point. Lets make sure that we all learn the important lessons from this event. Rick Koski Director, ES&H SEMI/SEMATECH __ Reply Separator _ Subject: GFIs, Hairdryers, and Bathtubs ... Author: dmck...@paragon-networks.com List-Post: emc-pstc@listserv.ieee.org Date:8/13/97 11:51 AM Recently somewhere back in the news (couple of weeks ago), two children died when one of them used a hairdryer in the tub. A discussion this accident with some lead me to a counter-intuitive result from my experience in product safety. ** Given Situation #1: 1. Person in a tub of water sitting at the opposite end of the tub than the drain. 2. The tub is ungrounded with water in it. Ungrounded meaning that the drain is plastic piping. 3. A 2-wire hair dryer (either On or OFF) is dropped into the water at the drain end of the tub. The person in the tub is neither in contact with the hairdryer, not anything that would be grounded. The hairdryer has a plastic case. Result: 1. Since t
Re: New test finger
Juan, Thanks for bringing this up. I thought I would check into for use at our company also, so I checked with ED&D. I've been happy with the other probes that I previously bought from them. Regards, Ray Russell ray_russ...@leco.com Here is their response: Thank you for your message! Yes, we do offer that probe. It is the Fingernail Probe (FNP-01) and it is $895.00. We are currently out of stock for these, and the lead time is approximately 4-6 weeks. Please let me know if I can be of any more help, or if you would like to go ahead and place the order. Thank you and best regards, Rita Fischer Educated Design & Development email: prodsaf...@aol.com __ Reply Separator _ Subject: New test finger Author: =?iso-8859-1?Q?Juan_Pedro_Pe=F1a?= at Internet List-Post: emc-pstc@listserv.ieee.org Date:9/2/97 10:33 AM As many of you probably know, the last version of IEC 335-1 introduces a new test finger including a nail, rather different to the usual one. I have asked my usual suppliers for it, but I have only received negative or no answer. Does anybody know where can I buy that new test finger? Thank you very much. Juan P. Pena / Electrical Safety Area Company: CETECOM, S.A. (http://www.cetecom.es ) e-mail: jpp...@cetecom.es
Re: GFIs, Hairdryers, and Bathtubs ... -Reply
Don't forget, the low impedance path from the black (high side of switch) to water to human to water to neutral (white wire.) It doesn't go to ground and may not even trip the GFI! >>> 09/02/97 06:33pm >>> You forgot two major assumptions: 1) The impedance of the water and 2)the impedance of the human in water. In water, the human has orders of magnitude of lower impedance, and therefore, the majority of the current will flow through the human assuming there is a path to ground. It is highly unlikely that there is no path to ground. In most home building codes, plastic piping is not allowed due to fire codes (they melt) perhaps with the exception of waste pipe (most of them are cast iron) Recently in the bay area, a girl was electrocuted in a swimming pool due to a faulty lighting wire. Yes, the breaker triped but too late. Also, don't forget that there are many unsuspecting grounds. The faucet, dings and scrapes on the porcelain or enamel, wet wood, concrete, stucco, morter etc. Those are all better conductors than tap water. Hans __ Reply Separator _ Subject: GFIs, Hairdryers, and Bathtubs ... Author: Non-HP-dmckean (dmck...@paragon-networks.com) at HP-ColSprings,mimegw5 List-Post: emc-pstc@listserv.ieee.org Date:8/13/97 9:51 AM Recently somewhere back in the news (couple of weeks ago), two children died when one of them used a hairdryer in the tub. A discussion this accident with some lead me to a counter-intuitive result from my experience in product safety. ** Given Situation #1: 1. Person in a tub of water sitting at the opposite end of the tub than the drain. 2. The tub is ungrounded with water in it. Ungrounded meaning that the drain is plastic piping. 3. A 2-wire hair dryer (either On or OFF) is dropped into the water at the drain end of the tub. The person in the tub is neither in contact with the hairdryer, not anything that would be grounded. The hairdryer has a plastic case. Result: 1. Since there is no path to ground from the hairdryer through the person in the tub to ground, no current should flow *thru* the person in the tub. The person is in no harm. 2. Since no current is flowing through ground, the GFI won't trip. If the water allows sufficient current to flow by shorting between the HOT and NEUTRAL in the hairdryer, the breaker will trip. ** Given Situation #2: Same as situation #1 except that the drain is now a grounded metal pipe. Is there a sufficient parallel path to be lethal to the person in the tub? Seems as though the parallel path for the hairdryer is straight to the drain. Result: 1. Person is still unharmed. 2. GFI trips. ** Are my conclusions correct? Is the only time one can be electrocuted in a tub when they are in contact with a ground and holding the hairdryer? Is a person really killed in a tub with hairdryer by drowning rather than electrocution? I'm beginning to doubt that I know exactly how a person is killed in a tub with a hairdryer. Or, am I making this way more complicated than it is? Comments?
Immunity of Optocouplers to Electromagnetic Disturbances
The Society for Reliability and Environmental Testing, SPM has issued a report with a comparative analysis of optocouplers* immunity performance when exposed to typical EMC tests. The general results of this report are: * Some - and not all - optocouplers can successfully be used in apparatus for galvanic insulation of inputs/outputs in industrial electromagnetic environ*ments and without additional common mode filters. * The present data sheets on optocouplers do not provide sufficient information to design an apparatus and predict all the normal electromagnetic immunity parameters. This report can help the design and prediction, but tests are still required. * All optocouplers will produce a short wrong output signal, when exposed to normal fast common mode transients/burst. 24 different optocouplers from six different manufacturers were subjected to electro*magnetic immunity tests, which are commonly used for test of commercial apparatus aimed at industrial environments. The disturbances were surge transients, fast tran*sients/burst, conducted radio frequency signals, and radio frequency electromagnetic fields. The report is: "Immunity of Optocouplers to Electromagnetic Disturbances", SPM-139 by Viggo Brøndegaard Nielsen, DELTA Electronics Testing. Members of SPM automatically receive the report, and others can buy the report from SPM - price DKK 1200.-. Contact Ms. Lisbeth Valentiner, DELTA Danish Electronics, Light & Acoustics, Telephone: +45 42 86 77 22, e-mail: s...@delta.dk. SPM is an independent organisation consisting of more than 100 company members in Scandinavia. SPM initiates and finances unprejudiced investigations of common interest for its members - mainly in the field of reliability problems and testing of electronic components and materials. List of some other SPM reports: http://www.delta.dk/spm/rapplist/welcome.htm
GFI Function
In all of the discussions of hair-dryer in a tub scenario, no one has discussed the GFI and how it works. It senses the current in both power leads, and if they are not equal, it trips OFF. Thus it assumes that any current flowing between phase/neutral is good, any current in the phase lead not returning in the neutral is not-good. Ignoring the physical fact that pure water is an excellent insulator--- dropping the hair dryer in an older home with metal piping will probably result in instant trip-out, since some current will find a path back via the pipes to electrical earth. dropping the hair dryer in a newer home with all plastic piping is probably a more hazardous situation. The GFI will not trip out, and a person may provide a lower impedance path between phase/neutral connections in the dryer than the surrounding water. Our bodies are flooded with a saline solution (blood) and are very conductive once the skin is bypassed. I have done some work in the crawl-space of my daughters' home in Durham NC, and know for certain that the waste/water lines are all plastic. There could be a grounding circuit but I am not aware of it. Naturally the tub & surround are all plastic also. The warnings provided with hair dryers and similar appliances warning against their use in wet areas must be observed. bo...@vnet.ibm.com
Re: New test finger
-- Original message from Juan Pedro Peña -- > As many of you probably know, the last version of IEC 335-1 introduces a new > test finger including a nail, rather different to the usual one. I have asked > my usual suppliers for it, but I have only received negative or no answer. > > Does anybody know where can I buy that new test finger? > > Thank you very much. > > > Juan P. Pena / Electrical Safety Area > Company: CETECOM, S.A. (http://www.cetecom.es ) > e-mail: jpp...@cetecom.es Juan We bought one of these test fingers just over two years ago from the following manufacturer: Startrite Designs Ltd Courteney Road Hoath Way Gillingham Kent ME8 0RZ UK Phone: +44 1634 233216 Fax: +44 1634 373516 Contact: John Masters It cost about GBP 800 to buy and GBP 30 a year for calibration. I hope this helps. Regards -- Ben Wrigley Senior Approvals Specialist KTL Saxon Way Priory Park West Hull HU13 9PB UK Phone: +44 (0)1482 801801 Fax:+44 (0)1482 801806 Web:http://www.ktl.co.uk Email: bwrig...@ktl.co.uk
50,000 and Counting -- Thank you!
Hello pstc-ers, The Safety Link (http://www.safetylink.com) noted the entry of its 50,000th visitor today, 2 September 1997. Our counter was started in December of 1995 and the website has been active since the Summer of that year. We would like to thank all of our visitors as well as our Corporate Supporters. Your continued visits and support affirm our intent that the Safety Link serve as a useful resource for electrical product safety professionals. And, for those not familiar with the sites offerings, we invite you to surf in and check out the web's most comprehensive collection of electrical product safety resources. ThanX again, Art Michael, Webmaster of the Safety Link.
Re: IEEE Conference
Charles: Kudos to you!!! Your offer is one that should be a "standard operating procedure" to our community. Sharing knowledge, interpretations, and opinions in discussions seems to be a lost art that was practiced in the 18th and 19th centuries, but (contrary to expectations) died out in the 20th century that features "improved communications" with the advent of electronic-based communications. The sharing of insights in our EMC community will enhance the abilities of participants and help earn the respect of those outside the community. That said, I was unable (because of contractual commitments) to attend the EMC conference in Austin and would greatly appreciate seeing commentaries from any attendees on the most informative papers, and what they learned. I will soon have a copy of the conference proceedings; however, our activities move in real time and delays mean less-informed decisions in the here and now. (Not to resurrect a controversial subject, but the use of attachments to the forum E-mail would probably simplify consolidation of member's commentaries by those who wish to download the comments into a separate directory for perusal as time allows.) Respectfully, Mike Conn, Owner Mikon Consulting
Re: GFIs, Hairdryers, and Bathtubs ...
You forgot two major assumptions: 1) The impedance of the water and 2)the impedance of the human in water. In water, the human has orders of magnitude of lower impedance, and therefore, the majority of the current will flow through the human assuming there is a path to ground. It is highly unlikely that there is no path to ground. In most home building codes, plastic piping is not allowed due to fire codes (they melt) perhaps with the exception of waste pipe (most of them are cast iron) Recently in the bay area, a girl was electrocuted in a swimming pool due to a faulty lighting wire. Yes, the breaker triped but too late. Also, don't forget that there are many unsuspecting grounds. The faucet, dings and scrapes on the porcelain or enamel, wet wood, concrete, stucco, morter etc. Those are all better conductors than tap water. Hans __ Reply Separator _ Subject: GFIs, Hairdryers, and Bathtubs ... Author: Non-HP-dmckean (dmck...@paragon-networks.com) at HP-ColSprings,mimegw5 List-Post: emc-pstc@listserv.ieee.org Date:8/13/97 9:51 AM Recently somewhere back in the news (couple of weeks ago), two children died when one of them used a hairdryer in the tub. A discussion this accident with some lead me to a counter-intuitive result from my experience in product safety. ** Given Situation #1: 1. Person in a tub of water sitting at the opposite end of the tub than the drain. 2. The tub is ungrounded with water in it. Ungrounded meaning that the drain is plastic piping. 3. A 2-wire hair dryer (either On or OFF) is dropped into the water at the drain end of the tub. The person in the tub is neither in contact with the hairdryer, not anything that would be grounded. The hairdryer has a plastic case. Result: 1. Since there is no path to ground from the hairdryer through the person in the tub to ground, no current should flow *thru* the person in the tub. The person is in no harm. 2. Since no current is flowing through ground, the GFI won't trip. If the water allows sufficient current to flow by shorting between the HOT and NEUTRAL in the hairdryer, the breaker will trip. ** Given Situation #2: Same as situation #1 except that the drain is now a grounded metal pipe. Is there a sufficient parallel path to be lethal to the person in the tub? Seems as though the parallel path for the hairdryer is straight to the drain. Result: 1. Person is still unharmed. 2. GFI trips. ** Are my conclusions correct? Is the only time one can be electrocuted in a tub when they are in contact with a ground and holding the hairdryer? Is a person really killed in a tub with hairdryer by drowning rather than electrocution? I'm beginning to doubt that I know exactly how a person is killed in a tub with a hairdryer. Or, am I making this way more complicated than it is? Comments?
