Below, Bruce Hunter assumes Hans Mellberg is using an air gap comprised of traces on a printed wiring board. When I first read Hans' message, I thought he was discussing discrete components. I have seen both used, but would like to ask you, Hans, to clarify your application, before we get off on a tangent.
Regards, Peter L. Tarver Nortel ptar...@nt.com >---------- >From: Bruce Hunter[SMTP:bru...@wormald.com.au] >Sent: Wednesday, April 23, 1997 1:51 PM > >On Wed, 23 Apr 1997 hans_mellb...@non-hp-santaclara-om4.om.hp.com wrote: > >> I have another question regarding class II equipment. My power supply >> intends to use a printed wiring spark-gap for transient/lightning >> protection. It's inherent design requirements are such that the air-gap >> spacing are less than those specified for double insulated (4mm). If the >> gap is increased to 4mm then this spark-gap will not work except at >> voltages too high to provide protection. > >> Is anyone familiar with exceptions to the clearance rules when using >> spark-gaps? I have heard that some European countries don't like using MOVs >> (an alternative to a spark-gap) but a spark-gap cannot fail in a short >> circuit like a MOV could. > >It is a dangerous assumption to make that spark gaps do not go short >circuit. With sufficient energy in the arc generated, the printed circuit >base material can carbonise and leave a conductive path. > >Additionally we have used spark gaps in low voltage circuits and to be >effective any conformal coating or screen must be left off in this area. >This can cause leakage due to dust and moisture build up. > >If the spark gap may bypass other protective insulation, then I believe >this is just as dangerous as an MOV in the performing the same function. > >My views only, >Bruce Hunter >
