Thanks guys, I was measuring the Extech... And yes the frequency could flow across the I= SOLATION transformer...... The advice given are well descriptive :) Here is what I am going to do in the future: 1) Identify the DC ground if it floats or tied to earth ground. 2) Each PSU or test gear like the scope I use *make sure it is safe* does n= ot complete the ground connection if the =A0=A0=A0 test circuit is tied to earth ground..... 3) Buy an isolation transformer and use a linear power supply..... Not on s= witch mode PSU.... In short for an isolation=20 =A0=A0=A0 transformer there are rules I need to follow and *think* before I= hook it up. 4) MARK THAT EXTECH PSU &(&*(&^^%&!=20 Thanks all, John Chung ----- Original Message ----- From: Sean Breheny To: Microcontroller discussion list - Public. Cc:=20 Sent: Monday, December 5, 2011 11:23 AM Subject: Re: [EE] Question of bench PSU safety. Hmmmm. I just looked at the datasheet and manual online and they do not specify whether=A0 the DC output is isolated or tied to AC mains ground. It is very suspicious that they talk about connecting several supplies in parallel but NOT in series. The strange thing is that there is also no explicit warning about trying to run the supply with the negative terminal connected to something other than ground. As others have said, most bench power supplies isolate the DC output from AC mains ground and explicitly state a maximum allowed voltage between AC mains ground and either output terminal. If such a statement is NOT made in the manual or on the power supply case, then you would have to check with a multimeter. External isolation transformers can be helpful but you have to know what you are doing. They can also create dangerous situations, such as running an oscilloscope from an isolation transformer and then connecting the probe ground clip to a high voltage point. Doing so would normally blow a fuse but in this case, it will act normally until you happen to touch the scope case and receive a shock. If you were to float this power supply, you absolutely must NOT do it by just disconnecting the AC cord ground pin. Switching power supplies like this one can have rather high leakage current to chassis and require the AC mains ground for safety. Using an isolation transformer MIGHT allow you to leave the ground disconnected because, while the leakage current would make the case "hot" with respect to one or both of the line inputs, it would still not be able to source any current to AC ground IN MOST CASES. It could still be that a high frequency current, especially in a fault condition, could flow across the parasitic capacitance of the isolation transformer and still harm you. In short, I would recommend NOT trying to run a power supply on an isolation transformer - if you need to float the output, you should use a power supply which was designed with an isolated output. Sean On Sun, Dec 4, 2011 at 9:12 PM, John Chung wrote: > Extech 382260....... > > > > > ----- Original Message ----- > From: Sean Breheny > To: Microcontroller discussion list - Public. > Cc: > Sent: Monday, December 5, 2011 4:47 AM > Subject: Re: [EE] Question of bench PSU safety. > > Hi John, > > What model of bench PS are you using? > > Sean > > > On Sun, Dec 4, 2011 at 11:41 AM, John Chung wrote: >> Hi guys, >> >> Had an eventful night...... Tried to power a circuit "which" I thought w= as a DC circuit ONLY to be greeted >> by a power trip and my bench power supply was smoking...... >> >> Needless to say I opened up the bench power supply to find that the tran= sformer leads to the >> banana jack had it's enamel coating melted... I found out later that the= DC ground was >> actually tied to the AC ground....... 145volts on the tested circuit and= the bench PSU banana jack was hooked to AC ground too....... >> There was some reactance between the bench banana jack to AC ground whic= h KIND of save the PSU life...... The >> bench PSU still powers up and works*despite the transformer's enamel bei= ng melted.* >> >> To NOT repeat history here are some questions: >> >> 1) Do I need to check ALL DC circuit to determine if it is tied to AC gr= ound? >> 2) When the DC circuit*tied to AC ground* is tied to ground hooking up t= o the PSU which is also tied to ground would ruin it.... But IF the >> =A0=A0 PSU is NOT tied to ground, would it be safe to use. Since it cann= ot be grounded since the PSU has not connection to AC ground. >> 3) Case scenario. The notebook DC ground has resistance of 1MOhm to AC g= round which is okay to hook up a scope probe BUT it was a reactance of 1Koh= m >> =A0=A0=A0 it would be hazardous? >> >> >> >> Thanks, >> John >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .