This is one of those areas where the term "isolation transformer" can take on a different meaning and conform to a different set of compliance standards, depending on whether it's located before or after a receptacle.
UL 1950 appears to allow for full isolation of the neutral on the isolation transformer's secondary where the iso-transformer is used after a receptacle. By contrast, isolation transformers used on feeder or branch circuits on the "NEC side" of power distribution require that neutral and ground be bonded at the transformer secondary. This is known as a "separately-derived power source." When such source is created by the iso-transformer, this is the only other instance allowed by the NEC where neutral and ground are bonded beyond the Xo ground at the service entrance panel. The IEEE Emerald Book repeatedly discusses a reduction in common-mode noise when the secondary's neutral and ground are bonded. This is my take after reading applicable portions of the NEC, UL 1950 and IEEE Emerald Book (2005 Sec. 9.9.5 and 10.4.2.1). I'll concede that my thoughts on this may not be correct. Paul, W9AC ----- Original Message ----- From: "Don Wilhelm" <w3...@embarqmail.com> To: "Jim Brown" <j...@audiosystemsgroup.com> Cc: "Elecraft List" <elecraft@mailman.qth.net> Sent: Saturday, March 27, 2010 4:23 PM Subject: Re: [Elecraft] (OT) Grounding Mat > But Jim, > > The secondary of an isolation transformer is fully isolated - i.e. > floating AC. The fact that the neutral of the transformer input side is > bonded to the green wire ground is quite a different thing. A proper > isolation transformer has no relationship to neutral on the secondary > side - only the safety ground and the voltage across the secondary > winding. > > Remember the old AC/DC receivers that had one side of the AC line tied > to the chassis! Every proper service bench had an isolation transformer > during that era. > > 73, > Don W3FPR > > Jim Brown wrote: >> On Sat, 27 Mar 2010 09:32:12 -0400, Don Wilhelm wrote: >> >> >>> An isolation transformer *does* isolate the neutral (and the hot), >>> >> >> NO! This is a summary of NEC (National Electric Code) requirements: The >> NEUTRAL of a transformer on the secondary side MUST be bonded to the >> equipment ground (steel conduit, the green wire, building structure, >> etc), >> and that green wire must be carried from the breaker panel to each outlet >> and >> to the transformer. The neutral conductor that feeds the primary side of >> the >> transformer must be bonded to ground at the service for the building >> (that >> is, the main breaker panel). And, as we all know, all groundes must be >> bonded >> together. Thus, an isolation transformer does NOT isolate either the >> neutral >> or the equipment ground. >> >> What an isolation transformer DOES do is reduce the voltage between >> neutral >> and ground to zero. It also shortens the return path for leakage currents >> on >> the green wire -- they now return to that transformer, not to the more >> distant breaker panel at the service entrance. This has the potential to >> reduce noise current on the shield of signal cables. BUT -- the simple >> bonding regimen outlined in my Ham Interfacing Power Point is a MUCH less >> expensive AND more effective solution. >> >> 73, >> >> Jim Brown K9YC >> >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
