On Wednesday 06 January 2016 10:33:58 andy pugh wrote: > On 6 January 2016 at 14:43, Gene Heskett <ghesk...@wdtv.com> wrote: > >> It is _not_ rated to switch 300V DC, so I just ensure it is never > >> asked to switch when there is 300V across it. Or, in fact, more > >> than 5V. > > > > Is it big enough to not weld closed forever? > > It is rated at 6A, so has no problem with the current. But it is only > rated to switch 24V DC.
I expect it can withstand more than that as long as there isn't a big capacitative sink on the load side to generate a huge inrush when it makes contact. I'd be tempted to use any extra contacts as series wired so there would be more than one break. I have seen 3 and 4 pole 50 amp relays wired so that all 4 sets of contacts were in series in order to assure a break for decades of use. I had to replace a 30 yo coil once but that was it, it just worked. Most transmitter bleeders are wired thru a door switch. Our old GE had two layers to that switch, one to crowbar the 7200 volt supply thru a few kilowatts of resistors on the cabinet ceiling, and one set of contacts to crowbar the 3 phase power, tripping that cabinets main breaker, and cascading back to kill the AK-225 main plate contactor thru its undervoltage relay. Unknown to us, an adjuster bolt had rattled out so it couldn't knock the linkage knee out from under the closer latch, so when a new operator opened the door to see what a transmitter looked like, but closed the door when all hell broke loose. But the chain reaction was started because the dump switch maintained the arc when it was pushed apart, and the AK-225 remained closed. Cooked everything between the door interlock including a 4400 lb plate transformer, the 750mcm cables to the substation pole, tried to blow the fuse links in the flag switches on the pole, but the ceramic tubes that contain the fuse wire were so metalized from previously blown fuses that they didn't totally interrupt the 14.4kv lines, caught fire, and kept the lights on dim and flickering, and the dripping material from those flags caught fire and dripped into the grass, and I'm out there after I arrived on the scene, stomping out a smoldering grass fire that was by then about 15 feet in diameter. With no way in hell for a fire truck tanker/pumper to get to it. Stomp it out or let the whole mountain burn, 3 sq miles or more of it. You do what you have to do... We'd bought a spare transmitter just like ours, taken out of service at a station in Texas before I took the job in '84, so we were only off the air about 2 days total by having a fork lift from a scrap metal yard and its driver come up and move the transformers. Big fork lift, sides cleared the garage door about 4" on each side, and less than that after he was inside and had to skid steer 90 degrees to get turned to where the transformer was located. That driver was good, never touched a thing but what he was asked to do. Otherwise it would have been around 9-12 days for Peter C. Dahl to make us another, which due to the improvements in transformer steel in 40 years , would probably only weighed a ton. And would still have ran cooler than the original. In the process of the repairs after power was restored, I managed to get across the 3 phase line while looking for an interlock switch part I had dropped as I was running on pure coffee by then, gave me 2nd degree burns on both arms & my chest where I fell across it. But I managed to kick myself off it and I'm still here to pester you kind folks. The shock trauma did give me a case of the shingles that put me in bed for about 2 weeks, redefining my pain threshold upward several notches. But I'm a survivor, having been in similar situations before. But I did get it up and running before I went to see how many times the clock went around while I was catching up on sleep & food. Took about 3 days for the shingles pox to start showing & several months for them to heal. One of my several war stories. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
