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Hazard in latest M. W. article.

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  • Hazard in latest M. W. article.

    I sent the following to Niel Knopf, email. I am also posting it here as I think it bears repeating.

    Mr Knopf:

    By now there has probably been a flood of correspondence concerning the "inexpensive EDM" article in the latest issue of MW. I will add my bit to it, as I just received my copy in today's mail.

    The author correctly points out that the equipment is hazardous. However, the particular version is more than usually hazardous. In fact, I assert the liklihood of at least one electrocution or fire if even a small number of inexperienced persons try to construct the device according to the article.

    1) There is no fuse included. This is a basic safety issue, as otherwise an accidental short depends solely on the branch circuit breaker or fuse for protection. Since these are rated at 15 or 20 amperes, a fire or other hazard is increased dramatically by the lack of a local fuse in the equipment.

    2) Because all parts of the equipment are "conductively connected" to the power line, a shock hazard exists even from touching just ONE wire or part, if a foot or other part of the body touches grounded objects at the same time. The illustrated electrode lowering device lacks the most rudimentary shield against touching the metal directly below the knob. Use of an isolating transformer would protect against this hazard, although the inherent risk of shock from touching two parts of the apparatus at once would remain.

    3) The author used electrolytic capacitors in the device, and failed to note the polarity on the schematic. This type of capacitor conducts well in one direction, but acts as a capacitor if polarity is reversed. Conduction is associated with heating and a very real risk of explosion of the part, particularly with no fuse present. Obviously the polarity is very important.

    4) The suggestion of photoflash capacitors is made, which is reasonable as per ratings, but bad in the use of salvaged ones, which may have used up much of their life prior to the new use and be ready to fail. That type of capacitor has a hard life, and is expected to fail faster than normally used parts. This is despite the fact that they are built to withstand spark discharges such as experienced in this application.

    5) A normal type of electrolytic capacitor is not made to withstand instant discharge as is normal with this type equipment. One would want to use a type intended for it, such as a photo type electrolytic, or certain types of non-electrolytic parts. The author fails to note this.

    All in all, I think this article poses sufficient risks as-is that it may be advisable to go as far as a mailing advising at least subscribed readers not to construct the machine.

    [This message has been edited by Oso (edited 09-24-2002).]

  • #2
    I added more later, after reading even more carefully.

    Additional information:

    6) The author does not list the required voltage rating of the diode, although he does list a barely adequate rating for the bridge rectifier. I would suggest the use of a 400 volt rated device in either case, as the 200 volts is only a few percent higher than the peak line voltage.

    7) Current rating is not discussed other than peripherally, but it should be at least 5 amperes, 10 amperes would be preferable. The 200 watt lamp will draw a little less than 2 amperes maximum, but the inrush current for a 200 watt bulb will be more like 15 or 20 amperes.

    8) The insulation on the toolholder is minimal. It would be better to make the bulk of the toolholder from actual structural electrical insulation material, such as the "glastic" material available from MSC and others. This is a fiberglass material available in thicknesses of 1/4 inch and more, having adequate strength and insulation properties for this usage.

    9) The article shows the clamps for the "tub", which are electrically "live", attached to the wooden base. Use of wood for insulation is not a good idea, and would not be approved by any safety agency. Wood can soak up water and become sufficiently conductive to present a shock hazard.

    10) Covering the electrical wiring is mentioned in passing, but should have been emphasized.

    11) Highlighting the author's lack of electrical knowlege is the reference to a half-wave rectifier giving 30 pulses per second, and a full wave rectifier giving 60. In fact, the numbers are 60 and 120, respectively.

    Overall, as an electrical engineer designing consumer equipment, I am appalled at the hazards created by the lack of knowlege on the part of the author, as well as lack of elementary precautions. The fact that this made it into print unmodified and "un-reviewed" is equally appalling.
    This is particularly perplexing after the more complex EDM article of several years ago in HSM. That prior article was apparently written by someone with actual engineering experience, and it shows.


    • #3
      Very insightful Oso. Two thumbs up.



      • #4
        I have not seen the article yet, but from what you say I am horrified. In Canada Open conductors must have a minimum 1" clearance between each bare conductor and any ground surface for all equipment run on 600V or less. Creepage between conductors should be checked as per CSA requirements with a Dielectric Strength Tester (we tested all 600V equipment to 2500V for one minute in the interest of safety). Proper non-conductive guarding is a must for a device such as this!

        Lake of proper 100,000A interrupting capacity fusing or (thermal, magnetic, AFI, GFI) breakers as required by code is manditory and conducive to safe operation.


        • #5
          Don't I just know it!
          I have done agency compliance work per UL, CSA, and CE mark (europe), and I work with our compliance engineer from time to time still.

          Actually, though, in equipment, through air or over surface clearances and creepage distance is only 3 to 6 mm with approved insulation materials used within ratings.
          That is for voltages up to about 500V.

          You may be referring to facility wiring, which is altogether different.

          The fussiest guys are "TUKES" which is a non-government agency in Finland. They seem to be a sort of "greenpeace" for safety. They will test a product they pull from a store shelf, and then demand payment for their trouble if they claim it does not pass. Kind of sounds like a holdup to me.

          Anyway, most of the special requirements are being "harmonized" which means we all cave in to whatever the europeans say.
          The old CSA "flaming hot oil" test has gone by the wayside. I always thought that was a nice picturesque test description, although we did not have to do it.......

          Compliance folks joke that Americans (and Canadians) live in wood houses and test for fire hazard, while europeans live in stone (or concrete) houses and test for shock hazard.

          Since we use 12AX7, 6L6, EL34, and 6550 vacuum tubes in some equipment, we have in those cases to maintain clearances throughout our equipment, not just in the mains area. Makes things more interesting.

          [This message has been edited by Oso (edited 09-25-2002).]


          • #6
            thanks from me too.

            don't know much about electronics, but this looked 'easy'. i was about to start looking for the parts. by this time i should have learned that anything that looks easy probably isn't.

            all this said, does anyone have any information on a [safe] EDM machine that can be built in a well-equipped shop? i would like to have the capacity to do square holes, and to get all the taps i break out. always the small ones. no place to grab them to get out.

            ........i dremel. therefore i am..........................


            • #7
              That is difficult, as all inherently have high voltages.

              The difference here was that there was no care at all taken to ensure that an unskilled person could build it correctly, and very little to reduce the inevitable hazards.

              With proper components and materials, rated right, decent insulation, enclosure, etc, and an isolating transformer, it will be as safe as it can be made and still work. I will probably make one of that type myself.


              • #8
                Hey, Oso:

                When you make yours, why not submit an article with pictures to HSM so those of us challenged in the ways of the electron can make a SAFE home shop EDM? I realize it's a lot of work, but think of the fame and glory!


                • #9
                  <font face="Verdana, Arial" size="2">Originally posted by billr:
                  thanks from me too.

                  don't know much about electronics, but this looked 'easy'. i was about to start looking for the parts. by this time i should have learned that anything that looks easy probably isn't.

                  all this said, does anyone have any information on a [safe] EDM machine that can be built in a well-equipped shop? i would like to have the capacity to do square holes, and to get all the taps i break out. always the small ones. no place to grab them to get out.

                  A couple of years ago HSM had a series of articles on a pretty sofisticated one. I think it got good reviews from the Elec. Eng. Check the online index. I also think the author was selling circuit boards for this too.

                  Oso. Thanks for the heads up. From now on your the "go to" guy for electrical knowledge.LOL


                  • #10
                    oso is right,

                    A little tuning and it should be ok.
                    But many of our interests are a little risky.

                    Example, that little blow up in Ohio last year.
                    Anyone here ever blown a case in their rifle?
                    or had an unexpected discharge?

                    Fell off your bike, motor bike, vehical, boat, or horse?

                    Heck, I may make one myself.


                    Heck, we could all take up bowling like the former pres. suggested!

                    [This message has been edited by metal mite (edited 09-26-2002).]


                    • #11
                      here here! I still have eight fingers and 30% of my brain cells intact, I'm ready to rock. Really now, a discharge machine would be sweet. But that one in HSM took 2 years worth of issues and a ton of cash to complete. What's the chances there were zero typos? Now it's in book form, so maybe all the bugs are worked out. (Wouldn't bet on it) Maybe Mr. Gregrichs' machine lacks some safety features, but I'd be more willing to tackle this one than the one in the book. So, Oso, we are all ears. Something simple, cheap, and safe, if you get one cooking we would be most appreciative if you share the know-how!


                      • #12
                        Well, I would start by rectifying the items I mentioned.

                        The thing cannot be made non-hazardous, because it uses lethal voltages. So respect is required, just as with your other machines. The difference is the hazards of electrical equipment are somewhat invisible. Also, the other machines will probably just damage you, not kill. Electric can kill.

                        Most important first items to me are the fuse, insulation, and the 1:1 isolating transformer.

                        Oh, and one thing I didn't even mention.....can't believe I missed it.....AN ON/OFF SWITCH AND PILOT LIGHT! I am not by the magazine now, but I don't remember seeing that. I would have the switch break both sides of the line.

                        Correct capacitor polarity is clearly needed, and I think I would consider a non-electrolytic cap for the output cap.

                        400 volt rating on the diode or rectifier, and 10 amp rating minimum.

                        Cover up the electrical parts, and use real insulating material, not wood, for the enclosure. (or, grounded metal, but I would advise insulating material, as the fewer grounded items nearby the tub, the better)

                        Get the fiberglas insulating material and use a chunk for the toolholder support, between the adjuster and the tool. And insulate the top end of the tool with some sort of shroud, like PVC pipe.

                        Cover the stuff that is near the adjusting knob, so your wet hands don't slip off into the hot stuff.

                        That's a good start. basically go through my list and fix it.

                        The transformer is to prevent a shock if you just touch one wire. With everthing direct to the line, a single wire plus a wet shoe can get you.
                        The transformer means you have to touch two wires to get across hazardous voltage.

                        [This message has been edited by Oso (edited 09-26-2002).]

                        [This message has been edited by Oso (edited 09-27-2002).]


                        • #13
                          Whats a "flaming hot oil" test?
                          mark costello-Low speed steel


                          • #14
                            It was a test for certain types of equipment requiring an enclosed chassis for fire hazard reduction.

                            A certain amount of burning oil is poured through any vents in the chassis from the inside towards the outside. It must NOT be burning when it comes out the other side.

                            If the holes are sufficiently small, the oil will be extinguished.

                            That has to be one of my favorite test descriptions in all of the compliance biz.....


                            • #15
                              Yes, I was going by industrial/HD wiring standards that are far stricter than houshold standards and for good reason - safety.

                              The test I thought was neat was the explosion proof enclosure test. Any electrical enclosure rated for hazardous locations had a spark plug installed and were filled with an explosive mixure of gas and pure oxygen and then place in the bottom of a 35' tank and detonated. If the enclosure leaked or distended too much it fails. It is good someone is watching out for our butts! And you wonder why that explosion proof stuff is so expensive...

                              [This message has been edited by Thrud (edited 09-27-2002).]