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: [email protected] List-Post: [email protected] 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 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?
