That little A/C receptacle tester....

[Hoochie Coochie Man]

So, does UL require that a MI amp have a separate neutral that is not tied to the amp chassis ground?

The reason I ask:
I worked for a twin 24 track remote truck back in the day.
Standard master main residential panel with 2 legs of current and a neutral with an isolation transformer at the power entry. (transformer before panel)
We had a ground cable / ground rod set up but rarely used it.
Many times we were parked on the street or in a parking lot that would have required some demo and excavation for a low impedance ground. The assumption was that the rubber tires isolated the gear and staff from any shock hazards.

As I recall, the only time we were asked for a safety ground hook up was for a festival at the monument grounds in DC. which was easy since we were parked behind the stage on the grass.

Blues Bass 2, or agedhorse.

Maybe one of you can answer the question the post above raises.

I don't have a copy of NFPA 70 handy to refer to, and I was trying to figure out the reason why a remote truck being fed temporary power would, or would not require a grounding electrode.

I am asking this question purely out of curiosity.

I assume a truck would be considered a special occupancy.

Does Article 257 apply?

My ignorant analysis is that the panel in the truck is a sub panel and as such would not be required, (or maybe even permitted?) to have its own grounding electrode because the temporary feed to the truck would include an insulated equipment grounding conductor and a grounded (neutral) conductor that are bonded at the service equipment.

Is there an exception dependent on the length of the feed conductors/distance from the service equipment or the kVA rating of the equipment being fed that comes into play?

I don't think a grounding electrode would be required if the truck was fed by a portable generator.

Interesting, (to me at least) stuff.

Ultimately, whatever the authority having jurisdiction says is required is what will be allowed or required, which accounts for the comment sayng a grounding electrode was required at one location, and not required at others.

 

[agedhorse]

Article 250 is probably one of the most complicated sections in the code. The reason is that different interpretations trigger different requirements based on statistical risks.

For example, in the case of a remote truck, is this a portable system or considered a structure, a stage or theatre or broadcast installation, is is radio/TV/audio broadcast, does the inclusion of a listed isolated transformer change the installation? Look over articles 250-32 through 250-52 and you will see that the specific application can greatly vary what's necessary.

This is where an engineer makes their case, then the AHJ determines if the case is reasonable and fits what they (think) would like to see.

 

[Hoochie Coochie Man]

Thanks for the reply.

I originally referenced Article 250 in my post, but I edited it out because I decided that temporary power to a special occupancy was covered in Article 527. But, I don't have any code books handy, so I was clutching at straws trying to use logic in trying to interpret something I'm not familiar with.

I intentionally ignored anything about the transformer in my opinion because of my ignorance of how that might affect the way it should be done.

 

[agedhorse]

Not sure where article 257 is from...

While some applications require a grounding electrode (such as covered in article 250.32, some (such as those covered in articles 520, 525 and 530) do not. This is where a clear definition of the installation is critical.

Also, there is a requirement for a grounding electrode in many cases where under short circuit fault current the voltage to ground would otherwise rise to a level greater than 100V.(this is part of equi-potential safety).

The deeper you get into this, the less clear it appears to become

 

[Hoochie Coochie Man]

Not sure where article 257 is from...

While some applications require a grounding electrode (such as covered in article 250.32, some (such as those covered in articles 520, 525 and 530) do not. This is where a clear definition of the installation is critical.

Also, there is a requirement for a grounding electrode in many cases where under short circuit fault current the voltage to ground would otherwise rise to a level greater than 100V.(this is part of equi-potential safety).

The deeper you get into this, the less clear it appears to become

Oops...Article 527 Temporary Installations. (Not 257. I edited my post above).

Regarding Article 250, I have a book that addresses it as its sole subject.

(Off topic Andy, but as I type this, I'm watching a band set up where we have already set up our PA, and I see the bassist just brought in a Subway D800, and 2 112 cabinets. This will be the fourth time we have worked with your creation. Very nice stuff!)

 

[agedhorse]

Ok, article 527 has been discontinued, replaced (actually updated with a new number) by article 590. This is a tough one to argue for a stage and/or recording truck because it does place limitations for the use of the power (specifically for construction, holiday lighting, emergencies and testing) and must be GFCI protected (30A and under receptacles) and have an assured equipment grounding program in place for the entire site.

 

[Pops OB]

Since you guys, Hoochie Coochie Man and agedhorse, know what you are talking about, I have a question... This DIY ground loop eliminator is on the internet. It is five minutes long, but could either of you look at it and tell us, the ignorant, if 1) Will this do what it claims?, and 2) Is it safe or dangerous?

 

[agedhorse]

Absolutely illegal. In the event of a ground fault, the diodes will open and you will be left with an open ground. The amp's fuse is not going to protect against a ground fault in say the line cord, or in another piece of gear connected to the amp's chassis.

Learning electronics on the internet, just like learning how to do your on surgery, can be harmful to your health.

 

[Hoochie Coochie Man]

OK agedhorse. I'm mostly retired, and I only think about this stuff rather than actually do much anymore. I don't have a good reason keep up with the latest changes.

Also, I added an edit to my last post above regarding your most recent design.

Thanks for sharing your knowledge!

 

[Pops OB]

Absolutely illegal. In the event of a ground fault, the diodes will open and you will be left with an open ground. The amp's fuse is not going to protect against a ground fault in say the line cord, or in another piece of gear connected to the amp's chassis.

Learning electronics on the internet, just like learning how to do your on surgery, can be harmful to your health.

Thank You! That's why I asked.

 

[beans-on-toast]

Recording trucks these days come with built in generators. I toured a TV broadcast truck recently, their generator can deliver 100A @ 120VAC. The generator feeds uninterruptible power supplies that the equipment plugs into.

Sometimes they have to connect to equipment outsode the truck. If external power is tied in, they use isolation transformers. They carry a 4’ copper ground rod, a ground cable, and a sledge hammer.

 

[Old Garage-Bander]

Since you guys, Hoochie Coochie Man and agedhorse, know what you are talking about, I have a question... This DIY ground loop eliminator is on the internet. It is five minutes long, but could either of you look at it and tell us, the ignorant, if 1) Will this do what it claims?, and 2) Is it safe or dangerous?

 

[agedhorse]

Typically, with isolation transformers (and all transformers), it's classified as a derived system which requires a neutral to ground bond and/or grounding electrode after the transformer. It depends on the application and the specific listing of the transformer. The reason is to provide a path for the fault return current after the isolation that the transformer provides.

 

[Old Garage-Bander]

Since you guys, Hoochie Coochie Man and agedhorse, know what you are talking about, I have a question... This DIY ground loop eliminator is on the internet. It is five minutes long, but could either of you look at it and tell us, the ignorant, if 1) Will this do what it claims?, and 2) Is it safe or dangerous?

Done on an enclosed funny farm by professional morons. Do not try this at home.

Boys and girls. If you have ground loop problems, fix them on the audio side, not at the power connector.

 

[Old Garage-Bander]

Recording trucks these days come with built in generators. I toured a TV broadcast truck recently, their generator can deliver 100A @ 120VAC. The generator feeds uninterruptible power supplies that the equipment plugs into.

Sometimes they have to connect to equipment outsode the truck. If external power is tied in, they use isolation transformers. They carry a 4’ copper ground rod, a ground cable, and a sledge hammer.

Who's truck did you see?

 

[beans-on-toast]

Who's truck did you see?

Not allowed to say.

In the 50’s, a guy would show up with an Ampex suitcase recorder and capture a local show in mono. Things seem to be coming around, now you get someone with a DAW and some outboard equipment. The application is still there.

 

[Old Garage-Bander]

Not allowed to say.

In the 50’s, a guy would show up with an Ampex suitcase recorder and capture a local show in mono. Things seem to be coming around, now you get someone with a DAW and some outboard equipment. The application is still there.

Yeah. I've worked with 32 track consoles and 2" tape, 16 track recorders.
Now you can do it all with an iPad and a dongle. Amazing.

My next audio project coming up is to set up a Whiser Room voice-over booth.
I get all the fun jobs.

 

[agedhorse]

I've done a bunch of shows over the years where we split to a remote truck. It's always interesting, transformer isolated splits are your friend.

 

[beans-on-toast]

My next audio project coming up is to set up a Whiser Room voice-over booth.
I get all the fun jobs.

That should be fun. A smaller sized booth can get cramped when you add a stool, music stand, and mic stand.

The sound of silence can give you an odd sensation.

 

[beans-on-toast]

I've done a bunch of shows over the years where we split to a remote truck. It's always interesting, transformer isolated splits are your friend.

Tracking down sources of audio hum can drive you nuts.