Article 250 Violation?

[dneff]

I have a piece of electrical test equipment in a small network rack. It has things like oscilloscopes, mulitmeters, waveform generators in it. I think the requirements below violate NEC Article 250. Could someone please verify for me?
1. The chassis shall have a redundant ground through the AC power facility (ground wire in the AC power cable) and a low impedance strap to the facility earth ground.
2. The grounding pin of the AC power plug shall be connected to the reference plane at one point.
3. Each primary power source shall have its neutral or return side electrically connected to the ground plane at one point only.

I don't think the neutral should be connected to the same ground plane that the grounding pin is connected.
I also think by connecting this same grounding plane to a facility earth ground via a low impedance strap is a violation because shouldn't the neutral only be connected to the ground at one point in main breaker panel?

I appreciate any help I can get with this one.

 

[LarryFine]

To me, they're merely over-describing normal wiring practices.

As I read it, 1. says merely supplement the cord's normal ground with a jumper from the equipment chassis to a local grounding strip. The rack itself may provide this.

Again, 2. and 3. describe normal wiring, and expect you to confirm that the normal neutral-ground bonding at the service is the only place the neutral is bonded.

 

[jap]

This type of electronic jargon makes it difficult to interpret.

But ,I too feel this is simply describing normal wiring practices that are already required by the NEC.

JAP>

 

[zbang]

All that and a bit of "blame it on dirty power if something doesn't work".

 

[hbiss]

My test bench just has a Wiremold strip along the wall where everything plugs into. Conforms to everything you mention. Never saw anything different.

On the other hand, it's customary sometimes to lift the grounds on test equipment line cords to eliminate ground loops and other problems when more than one piece of equipment is connected to the same piece of equipment under test. This is because the test lead grounds are connected to the test equipment chassis which is connected to the line cord ground.

Then, of course, God forbid you are working on a old radio or TV with a hot chassis without an isolation transformer powering it.

-Hal

 

[dneff]

Thank you all very much, I appreciate your responses.

 

[PaulMmn]

My test bench just has a Wiremold strip along the wall where everything plugs into. Conforms to everything you mention. Never saw anything different.
...
-Hal

You'll find it handy to also install a plugmold strip along the front edge of the workbench. Much nicer to plug in the soldering iron there, rather than having the cord draped across your work area!

Ditto the phone-- mount it below the top of the table!

 

[iceworm]

"phone"? as in "phone jack"? What's that?
Seriously - the only rj-11 jack is where the Uverse comes in for Ethernet. No more incoming coax, no landline phones.

It felt great when I started ripping out the visible coax. There were miles of it. (Okay, "miles" is an exaggeration.)

 

[tom baker]

Then, of course, God forbid you are working on a old radio or TV with a hot chassis without an isolation transformer powering it.

-Hal

Yes. You had to be very careful of the plug and receptacle polarity! We had isolation transformers for just that use.

 

[iceworm]

...Then, of course, God forbid you are working on a old radio or TV with a hot chassis without an isolation transformer powering it.

Yes. You had to be very careful of the plug and receptacle polarity! We had isolation transformers for just that use.

I have not seen a working All-American 5 in what - 40 years.
And I don't ever remember having an isolation transformer.

Still, a really good idea.

 

[LarryFine]

The tube chassis is what started the polarized plug.

 

[iceworm]

The tube chassis is what started the polarized plug.

I did not know that. I though it was the Edison base screw shell lamp holders.
No references. Somehow just stuck in my head.

I suspect you are correct. Edison base showed up way before All-American 5, which showed up before the polarized plugs.

 

[LarryFine]

I suspect you are correct.

I'm sure.

When I was around 10 (circa 1965), we went on vacation and stayed at a small hotel. We brought a small TV (B&W, around 19") that had a polarized plug, and wouldn't fit any receptacles. The office found some extension cords for us to try, and my dad was able to force the plug into one of them.

 

[tom baker]

I have not seen a working All-American 5 in what - 40 years.
And I don't ever remember having an isolation transformer.

Still, a really good idea.

All all american 5 is a 5 tube radio? I built a few of those from kits. And I remember if the radio got a hum, it was the power supply electrolytic cap going bad.

 

[retirede]

I have not seen a working All-American 5 in what - 40 years.
And I don't ever remember having an isolation transformer.

Still, a really good idea.

The radio had no isolation transformer if that’s what you mean.

Most every repair shop had an isolation transformer on the bench to make them safer to work on.

 

[LarryFine]

All all american 5 is a 5 tube radio? I built a few of those from kits. And I remember if the radio got a hum, it was the power supply electrolytic cap going bad.

Yes, but do you remember radios so old that the speaker's field magnet was a coil that doubled as a power-supply "choke"?

Or, was it the power supply choke doing double-duty as the speaker's electromagnetic field magnet?

 

[hbiss]

Either/or. That was before they invented Alnico permanent magnets that were powerful enough for decent speakers.

You'll find it handy to also install a plugmold strip along the front edge of the workbench. Much nicer to plug in the soldering iron there, rather than having the cord draped across your work area!

Ditto the phone-- mount it below the top of the table!

Yes, I agree. But I have a soldering station so the controller has to sit towards the back on the bench with the cord from the iron plugged into it. So can't avoid that. The line cord from the controller plugs into the strip just above it.

Phone is on the wall to the right of the bench.

-Hal

 

[iceworm]

The radio had no isolation transformer if that’s what you mean.

Most every repair shop had an isolation transformer on the bench to make them safer to work on.

After school job in high school, 1965 - 1966, fixing radios and some TVs. I don't recall any isolation transformer on the bench.
Looking back, that would have been a good idea

 

[synchro]

Yes, but do you remember radios so old that the speaker's field magnet was a coil that doubled as a power-supply "choke"?

Or, was it the power supply choke doing double-duty as the speaker's electromagnetic field magnet?

I remember dragging home a few old radio consoles with this type of speaker that had been thrown out when I was a kid. They were usually called "electrodynamic" speakers, although today that's often a more generic term that also includes permanent magnet speakers.
The best speakers of that type included a "humbucking" coil that cancelled out the effect of any AC magnetic field from the electromagnet/choke:

https://www.radiomuseum.org/forumdata/users/133/archive2019/Hum_Bucking_k.png

I also remember finding an FM broadcast radio that operated in the early 42-50MHz band (where at one point TV channel 1 resided before it eventually completely disappeared).

 

[hbiss]

After school job in high school, 1965 - 1966, fixing radios and some TVs. I don't recall any isolation transformer on the bench.
Looking back, that would have been a good idea

Same here. Back in those days we knew what not to touch and about high voltages in tube equipment. We knew about hot chassis. That's what we grew up with.

Kids today would be dead if their mothers let them near anything but an iPhone.

-Hal