Mike, I agree with you if there are surge arrestors to ground on the other lines.
But consider this common situation - a port with isolation transformers and surge suppressors only between tip and ring pairs. The three lines surged have high impedance to ground, so all 100A will pass through the grounded resistor. Regards, David. ----- Original Message ----- From: Mike Hopkins To: 'David Gelfand' ; [email protected] Sent: Tuesday, September 05, 2000 5:28 PM Subject: RE: IEC 61000-4-5 Surge Immunity Question Here's how it works: The combination of the four resistors determines the total surge current. If the voltage is 4kV and each resistor is 160 ohms (1.2/50us generator), the total source impedance becomes 42 ohms (2 ohms internal to the generator in series with the 4, 160 ohm parallel resistors), and the total current is 4kV/40 ohms, or 100A. Grounding the output side of one resistor as shown, is the equivalent of one line being a very low impedance or short, while the other lines are all surged. 25A is available to each line if that line were connected to a low impedance surge arrestor or were shorted. To answer your question, if one of the R's is removed, the total surge current changes because the source impedance is now changed. Instead of 25A in each line, there is 33A now available in each line. With the resistor in place and grounded, that resistor takes the current that is expected through that line, 25A, even though it is now effectively disconnected from the unit under test. Mike Hopkins KeyTek -----Original Message----- From: David Gelfand [mailto:[email protected]] Sent: Tuesday, September 05, 2000 4:09 PM To: [email protected] Subject: IEC 61000-4-5 Surge Immunity Question Group, When earthing a line as in Figure 12, shouldn't the resistor to that line be removed? Otherwise the surge will simply pass through that resistor to ground. Opinions? Regards, David. David Gelfand Regulatory Approvals Group Leader Memotec Communications Inc. Montreal Canada
