Dear John, Varistors to Ground (earth) is a very old fashioned way for designing equipment to withstand transient voltages. A better and more sophisticated way is to use some better engineering principles; for example larger clearances/creepages (approx 6 mm) and good design of the power supply input, especially around the input chokes.
Nowadays, of the hundreds of products which we test for safety and emc, a varistor between line-to-ground or neutral-to-ground is almost never used. If you do employ a varistor to ground, not only is it a problem with some countries (mainly due to Leakage of Current over time), but a bigger headache is the conduct of your 100% Production Line Dielectric Voltage Withstand Test. Each unit which must be subjected to this routine test must first be opened, the varistor disconnected before conducting the Dielectric Voltage Withstand Test. I hope that this additional piece of information will help you all analyze the high cost of putting a varistor to ground, with or without a spark gap connected in series to it. Happy and Safe Holidays to All, > All: > > A couple of weeks back I posted a message regarding a CENELEC Decision that > impacted the use of varistors between mains conductors and ground. I have > appreciated the resulting messages / dialogues, but I still am unclear about > my initial questions: > > 1) Is this Decision only for pluggable equipment type A, or is the spark-gap > / fuse requirement in effect for pluggable equipment type B as well? > > 2) Will Denmark, UK, and Sweden accept varistors to ground if the circuit > contains a spark-gap and two fuses? > > If this Decision means DK, UK, SE will simply not accept varistors to > ground, then a lot of small pluggable type A equipment intended for sale in > DK, UK, SE will have to either: 1) discontinue using varistors, or 2) become > pluggable equipment type B (which would be onerous, given the IEC 309 plugs > I've seen). > > I appreciate your input. > > John Boucher > jpbouc...@lucent.com > > Decision text: > Varistors Transient suppressors between the mains and the protective earth > cannot be accepted by the following countries: > * pluggable equipment type A: > Austria, Belgium, Denmark, Finland, Norway, Sweden, UK > pluggable equipment type B: > Belgium, Denmark, Norway, UK > A combination of a varistor in series with a spark gap (gas tube,) complying > with basic insulation, and with a fuse will be accepted for: > a) pluggable equipment type B and permanently connected equipment: > * by all countries > b) pluggable equipment type A: > * by all countries except Denmark, UK, and Sweden > > For pluggable equipment type A two fuses are required. > (to ensure that even by non-polarized plugs a fuse is provided) > > Permanently connected equipment connected to protective earth: > * accepted by all countries > > > > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to majord...@ieee.org > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to ed.pr...@cubic.com, > j...@gwmail.monarch.com, ri...@sdd.hp.com, or > roger.volgst...@compaq.com (the list administrators). > PETER S. MERGUERIAN MANAGING DIRECTOR PRODUCT TESTING DIVISION I.T.L. (PRODUCT TESTING) LTD. HACHAROSHET 26, P.O.B. 211 OR YEHUDA 60251, ISRAEL TEL: 972-3-5339022 FAX: 972-3-5339019 E-MAIL: pe...@itl.co.il Visit our Website: http://www.itl.co.il --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
