When a copper wire gets a fault in its insulation and it is exposed to the environment (rain, mud etc), you want the wire to be negative with respect to earth. That allows conductive ions to flow to the wire.
If the wire was positive with respect to the earth, then copper ions would flow from the wire to the earth and the wire would slowly disappear. Phone companies learned this years ago and went totally –48 because of this. From: Scott Vander Dussen Sent: Thursday, December 31, 2015 2:56 PM To: [email protected] Subject: Re: [AFMUG] 48v Rectifier Recommendations Not to be a total noob, but what exactly is -48, and why does it exist instead of just +48? I read up on it sometime ago I think it had its origins with the telecom industry frying stuff or something? But I don't understand what the fundamental difference between -48/+48 is. Thanks, 'S Sent mobile! On Dec 31, 2015, at 10:26, Ken Hohhof <[email protected]> wrote: Either of those should be good. I have some Eltek gear, but people have said nice things about Emerson also. One other option would be this: http://www.alpha.ca/web2/products/standard-systems/dc-power-solutions/item/48vdc-cordex-hp-300w-2 I haven’t used that product, but I do have some of their FXM series UPS and they have been very solid. Watch out that some of the rectifier products only work for one polarity, like –48VDC, so if you primarily need +48 that might be a problem. From: Josh Baird Sent: Thursday, December 31, 2015 11:52 AM To: [email protected] Subject: Re: [AFMUG] 48v Rectifier Recommendations Check Emerson Netsure (211) or Eltek for rectifier solutions. On Dec 31, 2015, at 12:45 PM, David Milholen <[email protected]> wrote: I throw this name out there alot but dont know if anyone ever bites because the Cheaper solutions prevail. When it comes to power I do not want to keep going back to revise or figure out what doesnt work. � Have a look at these babies from Duracomm. The price is right on these to. We are having to update a majority of our sites since the release of pmp450i We also invested in the dc -dc converters and isolators for the -48 needed for ptp microwave gear. http://www.duracomm.com/product-category/rack-mount-power-supplies/he1u-mu-series/ � On 12/31/2015 11:35 AM, Scott Vander Dussen wrote: I need to upgrade several sites from AC to DC. Usage is 15A or less at 48vdc. Desire a robust, rack mount, with SNMP solution. Any specific recommendations? -- <Davidmvcf.jpg>
