They did that for the SCADA control signals but I don't think it was the case for the phone because they guys giving me the tour specifically said that this isolation transformer was to protect against faults which introduce ground potential differences. On Sat, Jul 21, 2012 at 3:43 PM, Bob Ammerman wrote= : > Could it possibly have been some scheme where the telephone signal > 'piggybacked' on the power line itself? > > -- Bob Ammerman > RAm Systems > > ----- Original Message ----- > From: "Sean Breheny" > To: "Microcontroller discussion list - Public." > Sent: Saturday, July 21, 2012 1:47 PM > Subject: Re: [OT] Lighting > > >> One more item - the box that they described as being an isolation >> transformer for the phone DID appear to be substantially just a >> transformer. It had a small number of terminals on it, and at least >> one side of it had the terminals "stood-off" on large ceramic >> insulators - I guess to maintain the insulation from the input side >> (i.e., the possibly HV side in a fault condition) to the metal chassis >> of the unit. >> >> Sean >> >> >> On Sat, Jul 21, 2012 at 1:45 PM, Sean Breheny wrote: >>> Hi Dave, >>> >>> I don't know whether it was a connection to the standard telephone >>> network or a dedicated line back to some other control point within >>> the power company. I saw it during a tour when I interviewed for a job >>> with a power utility (although I didn't end up working there). I >>> didn't have a lot of time to study it or ask questions about it so it >>> could be more than just literally an isolation transformer as you say. >>> They gave me the explanation that I paraphrased here - of course some >>> details were probably left out. >>> >>> Sean >>> >>> >>> On Fri, Jul 20, 2012 at 9:39 PM, Dave Tweed wrote: >>>> Sean Breheny wrote: >>>>> Electrical distribution substations often have (wired) telephones whi= ch >>>>> use >>>>> super high-voltage isolation transformers between the phone and the >>>>> phone >>>>> line. This is to protect the user of the phone in the event that ther= e >>>>> is >>>>> some kind of fault at the substation which produces a large differenc= e >>>>> between the local ground where the user is standing and the phone lin= e. >>>>> This >>>>> could be either contact between a power line and the phone line OR it >>>>> could >>>>> be a large ground potential difference between the ground reference o= f >>>>> the >>>>> phone line and the local ground where the user is standing. The >>>>> transformer >>>>> (at least the one I saw) is about the size of a large desktop PC so i= t >>>>> dwarfs the phone itself! >>>> >>>> It would have to be more than just a transformer, as POTS uses DC for >>>> both >>>> power and signaling, in addition to the AC voice and ringing. It would >>>> also >>>> need a relay or two, and possibly a local battery charged by some sort >>>> of >>>> high-isolation DC-DC converter. >>>> >>>> -- Dave Tweed >>>> -- >>>> http://www.piclist.com PIC/SX FAQ & list archive >>>> View/change your membership options at >>>> http://mailman.mit.edu/mailman/listinfo/piclist >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .