"Cord and Plug" Connected 120V Isolation Transformers

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Ground Loop Guru

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I teach AC Power and Grounding to students in the recording studio, home theater, and sound reinforcement industries. I'm an electronics engineer (analog circuit design) ... not an electrician, so I apologize up front if my terminology is unfamiliar. I will be teaching at an Audio Engineering Society regional meeting in Toronto next month. I'll be sharing the stage with a representative of a company that makes 120 VAC Isolation Transformers - of the same kind discussed in a June-July 2008 thread called "Isolation Transformer Grounding". I've maintained for years that the outlets on these devices must have their EGC connected to both the enclosure and to the incoming power (via cord and plug) EGC ("safety ground" as I most often refer to it. They claim their products eliminate that connection between incoming and outgoing EGC, so I expect I'll be taken to task to justify my objection. It's not a so-called "balanced power" unit that can use GFCIs at the output to comply with code.

All that being said, I believe there was a NEC section that specifically prohibited disconnection of the link tying input EGC to output (outlet) EGC. I used to have the reference (to NEC 2008 if memory serves) in one of my PowerPoint slides. But recent hacking into my computers has wiped virtually all of my reference material (a very sad story in itself but I'll spare everyone). My question to the forum is: Can anyone supply me with the NEC citation - to a current edition of code, if possible?

Bill Whitlock
Whitlock Consulting
Oxnard, CA
 
I'd start here.

250.4 General Requirements for Grounding and Bond-
ing. The following general requirements identify what
grounding and bonding of electrical systems are required to
accomplish. The prescriptive methods contained in Article
250 shall be followed to comply with the performance re-
quirements of this section.

(A) Grounded Systems

(3) Bonding of Electrical Equipment. Normally non–
current-carrying conductive materials enclosing electrical
conductors or equipment, or forming part of such equip-
ment, shall be connected together and to the electrical sup-
ply source in a manner that establishes an effective ground-
fault current path.

(4) Bonding of Electrically Conductive Materials and
Other Equipment. Normally non–current-carrying electri-
cally conductive materials that are likely to become ener-
gized shall be connected together and to the electrical sup-
ply source in a manner that establishes an effective ground-
fault current path.

(5) Effective Ground-Fault Current Path. Electrical
equipment and wiring and other electrically conductive ma-
terial likely to become energized shall be installed in a
manner that creates a low-impedance circuit facilitating the
operation of the overcurrent device or ground detector for
high-impedance grounded systems. It shall be capable of
safely carrying the maximum ground-fault current likely to
be imposed on it from any point on the wiring system
where a ground fault may occur to the electrical supply
source. The earth shall not be considered as an effective
ground-fault current path.
Click to expand...
 
And:

Article 640 Audio Signal Processing, Amplification, and Reproduction Equipment
640.7(B) Grounding

Separately Derived Systems with 60 Volts to Ground.
Grounding of separately derived systems with 60 volts to
ground shall be in accordance with 647.6.
Click to expand...


Article 647 Sensitive Electronic Equipment
647.6(B) Grounding Conductors Required

Permanently wired utilization equipment and receptacles shall be
grounded by means of an equipment grounding conductor
run with the circuit conductors to an equipment grounding
bus prominently marked “Technical Equipment Ground” in
the originating branch-circuit panelboard. The grounding
bus shall be connected to the grounded conductor on the line
side of the separately derived system’s disconnecting means.
The grounding conductor shall not be smaller than that specified
in Table 250.122 and run with the feeder conductors.
The technical equipment grounding bus need not be bonded
to the panelboard enclosure. Other grounding methods authorized
elsewhere in this Code shall be permitted where the
impedance of the grounding return path does not exceed the
impedance of equipment grounding conductors sized and installed
in accordance with this article.
Click to expand...

I know you said this wasn't a "balanced power" (120V center tapped secondary 60-0-60 transformer where you would ground the CT) but the requirements are the same.

-Hal
 
hbiss said:
And:

Article 640 Audio Signal Processing, Amplification, and Reproduction Equipment
640.7(B) Grounding




Article 647 Sensitive Electronic Equipment
647.6(B) Grounding Conductors Required



I know you said this wasn't a "balanced power" (120V center tapped secondary 60-0-60 transformer where you would ground the CT) but the requirements are the same.

-Hal
Click to expand...



I understand all the foregoing, which applies to "hard-wired" (permanently-installed) systems - or am I missing something here? The quote I remember (and referenced) specifically applied to "Cord and Plug" connected transformers. It was in an older edition ... perhaps the topic was deleted in later editions?
 
Bill, you fight the good fight.

A few disconnect thoughts--
I don't have a good cite on this, but generally this equipment would be covered by NRTL listings and not the NEC. Are they listed at all?

My usual "dime-store" explanation is that without a fault-current path nothing is going to trip the breaker when there's a fault; most people understand that. If "eliminate that connection between incoming and outgoing EGC" ask the maker about the fault current path.

If the equipment is powered by an external source (wall wart or rack-mount DC supply), it may not even have a hard connection from the PS to the main chassis, so it's already isolated.

Looking back at the 2008 thread... more audio engineers need to learn the joys of the 111C repeat coil :D; doesn't Jensen make an equivalent?
 
Care to share the name of the manufacturer? Way back we had isolation transformers used when servicing consumer electronics that had one side of the line connected to the chassis- but back then everything had a two conductor line cord on it.

If a "cord and plug" connected transformer has a 3 conductor line cord there is no way to get around not extending that ground to the secondary receptacle ground pins. Might as well just cut the ground pin off the plug.:happysad:

-Hal
 
Ground Loop Guru said:
I understand all the foregoing, which applies to "hard-wired" (permanently-installed) systems - or am I missing something here? The quote I remember (and referenced) specifically applied to "Cord and Plug" connected transformers. It was in an older edition ... perhaps the topic was deleted in later editions?
Click to expand...

I understand you to mean a portable plug-in isolation transformer and as such I'm not sure that the NEC would have any say in this. But I can't imagine any UL product standard allowing the secondary side of such a device to not have 1 end of the winding grounded to the case and to the supply cord EGC. In your OP I understood you to say that the EGC of secondary would have to connect to the supply cord EGC. I would agree but with the additional requirement that the secondary winding be bonded as I just mentioned.
What I'm trying to say is that you can't have a 120 volt non-grounded supply to power any type of equipment.
 
But I can't imagine any UL product standard allowing the secondary side of such a device to not have 1 end of the winding grounded to the case and to the supply cord EGC.
Click to expand...

A floating secondary might if the transformer were intended for test or temporary purposes by trained technicians. That's why I would like to see the spec sheet.

-Hal
 
zbang said:
...more audio engineers need to learn the joys of the 111C repeat coil :D; doesn't Jensen make an equivalent?
Click to expand...

I heard of someone thinking of throwing some of those out earlier this year. :jawdrop: I have a couple in the garage for emergencies. They can be great problem-solvers, especially in high-RF environments. Today's audio guys would probably poo-poo them, since it's old-school (mine were manufactured in the 50's) but Western Electric made 'em good...they were rated for a 30-15kHz frequency response, but someone has measured the high-end response to be only 2dB below reference at 65kHz!
 
hbiss said:
A floating secondary might if the transformer were intended for test or temporary purposes by trained technicians. That's why I would like to see the spec sheet.

-Hal
Click to expand...

I would agree. I was thinking along the lines of using such a device with a floating secondary being used out in the wild for everyday use. I just can't think of any compliant use.
And I had to chuckle over your comments about about the old TV chassis being bonded to one side of the circuit. Led to some interesting issues sometimes for the uninformed. Guess we're showing our age.
 
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