Using ground for current carrying on a 24VDC system

[Jraef]

I don't have a lot of experience with limited energy building wiring systems and codes, so to be honest I'm just looking to short-cut on some code research here.

I have a client who has a 24VDC control circuit for Building Automation. They wired some basic switches to manually override some control contactors and only ran the +24VDC to and through the switches. Now they decided they want an indicator light at the switch to indicate when the switch is left On, but they have no common conductor in the box. They have been told by others that they can just connect the other side of the lamp to ground and that's OK, because the 24VDC power supply is grounded on that side (-) anyway. I know in regular 120V building wiring that is not allowed, but being that it's "Limited Energy" 24VDC, are there different code rules that allow Ground to be a current path?

 

[G._S._Ohm]

I have a client who has a 24VDC control circuit for Building Automation.
They have been told by others that they can just connect the other side of the lamp to ground and that's OK, because the 24VDC power supply is grounded on that side (-) anyway. I know in regular 120V building wiring that is not allowed, but being that it's "Limited Energy" 24VDC, are there different code rules that allow Ground to be a current path?

The ground wire current will be a few mADC at most?

 

[jumper]

I cannot find anything in 725 that says this is okay.

 

[dbuckley]

I'm dubious.

Putting control circuit and other signal currents on ground is common, but then the ground is called a "functional ground", which is different to (and should at one point be bonded to) "protective ground".

You cant just arbirtraily decide that you're going to use a protective ground as a functional ground. Perhaps especially if there is a boilted fault then the protective ground changes potential, and thet can munt the equipment misusing the protective ground.

 

[hurk27]

Very bad idea, first fault of line (120v) side will place high voltage across the 24 volt circuit, remember a fault in a 120 volt circuit could be as high as a few hundred amps, and this can produce a voltage differential between two points in the grounding system, this voltage could be high enough to fry the 24 volt loads.

 

[nhfire77]

That is intentionally causing a ground fault. That's a huge problem. If there are other systems IE fire alarm, and they have a ground fault, now you have a ground loop.

That is NOT a good idea.

 

[tom baker]

The cable ground can also be called a shield or drain, its for noise reduction. The 18-2 sheilded twisted pair I run (a lot for instrumentation) has the drain bonded at one end only. Its not a ground as in equipment grounding conductor.
The wiring method needs to be a listed cable, which has insulated conductors for each function. Here are few code sections for your argument
110.7 wiring integrity
110. wiring methods
300.3(C) insulation - equal to max circuit voltage.

 

[tom baker]

Sorry I misread the OP. I thought it was using a ground shield wire not building ground. But the code sections I stated are still applicable.

 

[GMc]

I don't have a lot of experience with limited energy building wiring systems and codes, so to be honest I'm just looking to short-cut on some code research here.

I have a client who has a 24VDC control circuit for Building Automation. They wired some basic switches to manually override some control contactors and only ran the +24VDC to and through the switches. Now they decided they want an indicator light at the switch to indicate when the switch is left On, but they have no common conductor in the box. They have been told by others that they can just connect the other side of the lamp to ground and that's OK, because the 24VDC power supply is grounded on that side (-) anyway. I know in regular 120V building wiring that is not allowed, but being that it's "Limited Energy" 24VDC, are there different code rules that allow Ground to be a current path?

I had a dicussion on this forum awhile back about some TS-400 switches we use at our facility. The switches are rated for 120/277 and it uses the ground as a current carrying conductor for the LCD display on the switch. I guess the only difference is that these are UL listed..

 

[esobocinski]

... which is really just a lower-current version of the ground return for clothes dryer lights until the dryer ground/neutral separation in the '90s.

And that brings us back to hurk27's comment. If something is UL listed for 120v ground return, at least there's some expectation that it's been reviewed/tested to know that a ground-fault elsewhere won't fry it and cause a bigger problem. Low voltage DC electronics often don't cope with their ground reference varying at a significant AC fault voltage. I've relearned this a few times, usually expensively.

 

[stevenj76]

I do alot of lowvoltage, some of the stuff is 24VDC.

I would never ever even contemplate using building steel as a return path.

That's just nuts, but it's okay in the world of cars.

 

[hurk27]

but it's okay in the world of cars.

Not really, at least since the addition of computers in cars go, Chrysler tried this way back with the lean burn computer system and they had nothing but problems which eventually turned out to be the shared grounding paths with the computer signal path, if you have notice all cars today have stopped using the chassis as a return path for 12 volt loads, now each load has its own negative conductor, the loads/supply's such as the starter and alternator on the engine is kept out of the loop by using a single point bond from the engine block to the battery.

I get a kick out of these kids who hook up these big amps and try to use the body as a return path, and wonder why the computer goes nuts why they pump up the volume, actually had one that would kill the engine, the louder he turned it up, the slower the car went, he had it to 3 shops who couldn't figure that one out.

 

[LarryFine]

Back when I was in jr. hi and high school (back when they were the same thing), I was a member of the Radio Club. Among the other things we did (mostly with school permission), we ran stereo audio from the club room to the three cafeterias for lunchtime music and personal announcements and song dedications.

We ran two #10 wires along the sidewalk canopies and simply paralleled the three speakers on each channel, allowing wire resistance to protect the stereo amp. We used the electrical EGC system as the common return for both channels. While 50w per channel wasn't a lot, it was adequate, and everyone loved it.

 

[LarryFine]

... but it's okay in the world of cars.

Not really, ...

Same with using building steel as the return with structural welding. Strange things have surfaced because of voltage gradients.

 

[hurk27]

Back when I was in jr. hi and high school (back when they were the same thing), I was a member of the Radio Club. Among the other things we did (mostly with school permission), we ran stereo audio from the club room to the three cafeterias for lunchtime music and personal announcements and song dedications.

We ran two #10 wires along the sidewalk canopies and simply paralleled the three speakers on each channel, allowing wire resistance to protect the stereo amp. We used the electrical EGC system as the common return for both channels. While 50w per channel wasn't a lot, it was adequate, and everyone loved it.

And this was very common in cars back pre IC amps days, but once the output amp became an IC chip it was a no no to common the speaker negatives as there was 7.2 volts between the left and right channels and connecting them together would pop the output IC chip like a fuse, most all car and home stereo amps are IC now days, so no more 3 wire speaker feeds, while many pro PA amps such as Crown, QSC, Carvin and even Pevy are built the old way and can be run with 3-wire, don't try it with a Radio Shack PA amp, as they are IC out, and all the amps above are of a push pull design finals except Pevy which is a parallel output finals design with a crowbar impedance protection.

 

[hurk27]

Wonder what happened to Jraef, he normally responds to threads he has created?

 

[Open Neutral]

I do alot of lowvoltage, some of the stuff is 24VDC.

I would never ever even contemplate using building steel as a return path.

That's just nuts, but it's okay in the world of cars.

My BinL owns a Morgan +2, and there it would be a big fail. Morgans have a sheetmetal skin over an ash body. (The frame is box steel..) Yes, it squeaks...

But then it is British, so the electrical system doesn't work anyhow...

 

[K8MHZ]

Not really, at least since the addition of computers in cars go, Chrysler tried this way back with the lean burn computer system and they had nothing but problems which eventually turned out to be the shared grounding paths with the computer signal path, if you have notice all cars today have stopped using the chassis as a return path for 12 volt loads, now each load has its own negative conductor, the loads/supply's such as the starter and alternator on the engine is kept out of the loop by using a single point bond from the engine block to the battery.

I get a kick out of these kids who hook up these big amps and try to use the body as a return path, and wonder why the computer goes nuts why they pump up the volume, actually had one that would kill the engine, the louder he turned it up, the slower the car went, he had it to 3 shops who couldn't figure that one out.

Oh, really?

I worked for Chrysler in the 90's and they used the body for the negative conductor of all the components. If there is only one cable to the negative battery terminal and that cable is connected to the engine, they are using the chassis as a negative conductor. The motor will be bonded to the body with flexible conductors. The components will be star configured and attached together at one or more points under the dash.

Commercial radio installations (like for police cars) always use the shortest conductor for the negative and it is always connected to the chassis as close to the radio as possible.

The chassis is used to reduce voltage drop and the amount of wire used in the vehicle.

Big car audio systems typically use more instantaneous power than the vehicle's alternator and battery can provide. That is why you see huge capacitors used for reserve. Without a big cap, the entire voltage of the car will drop drastically, then surge upon recovery. With the big caps to stabilize the voltage, now you are dealing with a reactive load that can affect the computerized controllers in the car.

The addition of a cap, an accessory battery or an additional alternator will boost the power and a direct connection to the accesory power supply will reduce or eliminate the voltage issues for the rest of the car. Simply connecting the existing system directly to the battery with no other changes made will likely have no positive effect.

 

[S'mise]

And this was very common in cars back pre IC amps days, but once the output amp became an IC chip it was a no no to common the speaker negatives as there was 7.2 volts between the left and right channels and connecting them together would pop the output IC chip like a fuse, most all car and home stereo amps are IC now days, so no more 3 wire speaker feeds, while many pro PA amps such as Crown, QSC, Carvin and even Pevy are built the old way and can be run with 3-wire, don't try it with a Radio Shack PA amp, as they are IC out, and all the amps above are of a push pull design finals except Pevy which is a parallel output finals design with a crowbar impedance protection.

Thanks Hurk27, Explains how I fried my dennon amp years ago.
I still have my McIntosh:grin:

 

[stevenj76]

In other words... even in cars, it's a good idea to pull a return conductor....

When I think 24VDC, I think two gel cel back up batteries. Never seen where the negative terminal is simply bonded to the tub and call it good.