In a message dated 11/30/00, Jim Lyons writes: > The gas tube would look pretty much like a short circuit once it starts to > conduct, and I would expect the Buss MDL2 fuse they used to blow. Their > failure criteria would seem to exclude the use of gas tubes directly > across the tip and ring lines. Is this a proper assumption?
Hi Jim: In general, you will need to have fuses or PTC devices in front of your gas tube. To pass the M1 test, your equipment must become an open circuit before the fuse in the test set opens. Since you have protection from tip to ground and ring to ground, you will need a fuse in tip and a fuse in ring. Otherwise you will fail the L-1 test. I generally use a 1.25 amp time lag fuse such as the Teccor F1250T (surface mount) or the Bussmann C515 (through hole). I also prefer Teccor sidactors over gas tubes, since they are faster and more precise. Some people use the Raychem TR600 Polyswitch instead of a fuse, but a well chosen conventional fuse will be a little more robust for lightning immunity. Whatever protection scheme you use, you need to consider how your circuit will respond to the M3 and M4 tests. These tests can force quite a bit of current through the circuit and overheat it, since they are designed to "sneak in" below the trigger thresholds of your fuses and overvoltage protection. Lastly, I should mention that there are some ways to avoid putting in fuses, but they are not often used. You will have to specify the use of a 26 gage or heavier line cord, and you may need to have a fire enclosure. Take a look at Figure 18b in UL 1950 to see a flow chart of possible compliance paths. This flow chart is not easy for the uninitiated to follow, so don't feel bad if it seems confusing. Your UL test lab should be able to walk you through it. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
