60/120 volt ac single phase balanced power systems

[ken@backfence]

In the last few years there have a number of manufacturers in the custom audio/video world promoting the use of 60/120 vac balanced power systems for residential applications inspite of NEC 647.3 prohibition against such installations. Have others encountered these types of installations? If so how have you handled the issue of the separately derived grounded neutral? I am most concerned about when this grounded neutral comes into contact with the grounding conductor or neutral of the occupancy electrical system. Any insight would be greatly appreciated.

 

[hurk27]

Re: 60/120 volt ac single phase balanced power systems

I don't know who is promoting this But it is not allowed Per the NEC:647.3 (1)

647.3 General.
Use of a separately derived 120-volt single-phase 3-wire system with 60 volts on each of two ungrounded conductors to a grounded neutral conductor shall be permitted for the purpose of reducing objectionable noise in senstive electronic equipment locations provided that the following conditions apply:
(1) The system is installed only in commercial or industrial occupancies. (2) The system?s use is restricted to areas under close supervision by qualified personnel.
(3) All of the requirements in 647.4 through 647.8 are met.

 

[hurk27]

Re: 60/120 volt ac single phase balanced power systems

I should of said that if you know that a System is not allowed by the NEC then why are you trying to install one? if something were to go wrong it would be your butt as when we directly ignore rules we get people hurt. now as why this is not allowed in residential occupancies I don't know off hand but there must have been a reason. Maybe someone can tell us if the 2005 will allow it.

[ October 11, 2003, 02:38 AM: Message edited by: hurk27 ]

 

[dereckbc]

Re: 60/120 volt ac single phase balanced power systems

I work a lot with them. I use them in data centers and have one in my home for A/V equipment.

I think the answer to your question is the units made for home use are plug and cord units (black box) which do not fall under NEC, rather UL. Its just like a UPS or toaster oven.

IMO they are safer than your grounded single phase system. There is no grounded conductor (neutral), only two ungrounded conductors and one equipment ground.

Click here for more info www.equitech.com

[ October 11, 2003, 01:23 PM: Message edited by: dereckbc ]

 

[bennie]

Re: 60/120 volt ac single phase balanced power systems

Dereck: Good remarks. I like to see emphasis on equipment not being part of the premises wiring system. These units plug into the premises wiring system.

The 60 volts to ground also lowers the effects of capacitance coupling.

 

[ken@backfence]

Re: 60/120 volt ac single phase balanced power systems

I am not installing one. As part of my electical contracting business we do high end home theater and audio systems. Companies like Furman Sound, Monster Cable, Panamax, EquiTech and probably many others are promoting the use of the self contained cord connected units in residental theater installations. My own interpetation of 647.3 would exclude even the cord connected units due to the accessibilty by unqualified individuals. Where you have experience with one of these units in your own home how have you delt with the possibilty of the separtely derived grounded conductor comming in contact at some point with the building grounding conductor. Is this a potential problem? I am concerned about unwanted grounding pathways created by signal interconnections between equipment power from the balanced power system and equipment connected to the building branch circuits.

Thanks for the input already given.

 

[hurk27]

Re: 60/120 volt ac single phase balanced power systems

I think the one problem I can think of will be the coax from a cable feed as this would be an unwanted ground.
Maybe there's an isolation transformer for 75 ohm cable it would be easy to make with a few parts.

 

[dereckbc]

Re: 60/120 volt ac single phase balanced power systems

Originally posted by ken@backfence:
My own interpetation of 647.3 would exclude even the cord connected units due to the accessibilty by unqualified individuals. Where you have experience with one of these units in your own home how have you delt with the possibilty of the separtely derived grounded conductor comming in contact at some point with the building grounding conductor. Is this a potential problem?

647 pertains to premises wiring systems. The cord and plug units are not part of the premises wiring system. If you were talking about the units that use a seperate transformer integrated with premises wiring system 647 would apply.

Even though the plug and cord units are SDS they are not required to be connected to a ground electrode system. Many of the units do have an optional connection point for a supplemental ground if you wish to use it. IMO it serves no purpose, but no safety issues. Many people use this optional ground to connect their A/V equipment for a signal reference point.

 

[dereckbc]

Re: 60/120 volt ac single phase balanced power systems

Originally posted by hurk27:
I think the one problem I can think of will be the coax from a cable feed as this would be an unwanted ground.
Maybe there's an isolation transformer for 75 ohm cable it would be easy to make with a few parts.

Wayne, many of the units, including mine, have I/O ports such as CATV. Most of the upper end units contain some TVSS components including CATV. Basically it serves as a "surge reference equalizer". Or in other words it creates a isolated SPG so lightning or any other outside fault current paths cannot flow through the upstream devices that are plugged into the "black box".