On 01/01/2016 08:47 AM, Dave Cole wrote: > On 1/1/2016 11:26 AM, Bertho Stultiens wrote: >> On 01/01/2016 05:13 PM, Dave Cole wrote: >>> Anytime you have a 120 volt source for computer, misc power etc, you >>> need to declare one side of the 120 vac winding the neutral (white wire) >>> and tie that terminal to the machine frame. >>> It similar to what is required at the service entrance of your house. >>> The neutral is always tied to the ground at the entrance box. >>> Oftentimes they drive a screw through the neutral buss bar in the >>> service entrance box into the box sheetmetal and then tie a green or >>> bare copper ground wire to the same neutral buss bar and run that to a >>> metal water pipe or ground rod. >> You should never tie a secondary winding of a transformer to ground. The >> transformer makes the secondary float wrt. the primary and that is just >> fine. >> >> In this scenario, you would create a (capacitive) feed-through from the >> transformer to ground because the primary is not referenced at ground. >> The primary has two phases attached, which creates a virtual circuit at >> the primary side. >> >> The difficulty here is that you must take account for the floating >> references, which make most of the things you measure local phenomena. >> Once you go through the transformer, your reference to the primary line >> input is lost and you must not try to reestablish it. It would only make >> things bad and worse. >> > Hi Bertho, > > That's done all of the time. In fact it is part of the NEC (National > Electric Code) that is followed (for the most part) in the US.
(for the most part) unfortunately. Books on this subject matter say the same thing. When cities have their own codes I wonder about it sometimes. It's about physics, not about politically correct or union prescribed codes that matters. I've seen silly codes in EU as well as in the US. Code is not going to kill, but large amount of moving electrons will. > Pretty much every house in the US is wired like that. (I'm not making > this stuff up. :-) ) > > Now, if you don't want to do that inside the panel for some reason > (which I might have missed), that may be a different story. > I'm just saying that is standard practice in the US and on the European > machines I have worked on as well. > > There are a lot of good reasons to tie one leg the transformer to ground > besides to establish the safety ground and neutral as is common on the US. > Intermittent faults to ground, with an ungrounded system, can cause the > secondary of the transformer to fly way above absolute ground causing > connected devices, or the transformer to suffer from insulation > breakdowns. That's the extreme, but it can happen. Strongly agree! Do not float transformer secondary and have a PC connected to it! This is not about an isolation transformer, it's about stepdown transformer which is way different. Primary side of this step down transformer needs to float in this case. In ideal situation, CT on the primary side would be neutral and connected to local ground but it is better to not use it; transformer is not ideal. A PC or anything else for that matter, that is not grounded is against all safety rules based on physics, never mind the code. You don't want your mouse or the monitor electrically float in relation to the metal machine! For comparison, I used _isolation_ transformer (that is 1:1) to repair switching power supplies in early 90's when it was still profitable. Oscilloscope ground was connected to a point on the "primary side" of switching power supply and 1MOhm probe was good for hundreds of volts. Of course, I was well isolated from the "house ground" and made sure I didn't touch grounds on scope or outlet ground to be safe. Touching outlet and scope ground could of course be fatal! One of magazines [1] in the 70's suggested to use a neutral wire if you don't have an antenna for the radio. Experiment for my first AM and SW radio detector worked fine until one day I accidentally reversed the plug, yes you can do that in one of STUPID EU power outlet designs, DIN code in my case I believe. I mention this to support my thesis that "electric codes" don't necessarily cover all aspects of safety. Based on that experience I lost my interest in electricity and experiments for a week which surprised my mom knowing that building a radio detector with 3 or 5 transistors, handmade coils, all on PCB with traces cut with a knife was fun. Since then I always used only one hand to work on live mainframe power supplies and such. That experience also confirmed my interests in "low voltage" side of electric phenomena: radios, TV, computers ;-) There were exceptions to my work, like TVs and radios with vacuum tubes ... [1] I don't think they write such articles anymore. I hope not. > One of way too many references on the web. > http://ecmweb.com/bonding-amp-grounding/basics-bonding-and-grounding-transformers > > Dave I hope that nobody gets zapped before this highly charged and perhaps the long(est) thread is over. -- Rafael ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
