Relative to IEC61010-1/UL61010-1 I am struggling with the design value of a grounded secondary to satisfy the double barrier concept of this standard. I have circuit like figure E1.8 where there are many secondary communicaiton interfaces coming out of this product. The figure says that the barrier should be double Basic. BUT there is a foot note saying that if the impedance to ground is low enough then the barrier could be single Basic. This is all based on rules in 6.3.2 to 6.6.1 that test what happens when there is a single point failure. How practical is it to ty to use this clause? I can see that in a Power Supply example that a failure of a SMC transformer could cause the logic and +V planes of my PCB to raise to a dangerouse level and so not be an adequate low impedance. Testing could show if this would happen but difficult to plan for. As a result I am desinging the power supply of my latest product to double basic. I have other examples that look like good arguments for single Basic. I have relay circuits - 120-240VAC, OVC3, Pollution degree 2. The relays themselves are qualified to double basic at those ratings so that covers the field relationship to the coil. But must I maintaind double basic to my PCB ground plane pour? IT is solidly bonded to the metal chassis in multiple points top and bottom pours are bonded with vias all around the board. A failure of the PCB from a tracking problem would have to traverse from the terminal block and hit the ground pour. This low impedance shield ought to satisfy the clause but again testing? Seems clear to me....... Your Thoughts? Thanks Chris Wells
