Can an ac and a dc circuit share a ground point?

[Aleman]

It is not uncommon to find grounded DC systems. If you don't ground the DC you need to use fuses in both the positive and negative, if you ground one side,
you only need a fuse for the other leg.

Why is that? New one on me. We have a lot of controls with the DC isolated, and fused only on the positive. Why isolated? We have noisy grounds and I didn't want that noise in our control
voltage 24 VDC. That said, almost all of our panels that came with equipment have the DC negative tied to ground. I have been meaning to tie DC negative to ground but we have had zero
problems over 3-1/2 years.

 

[iwire]

the OP didnt have clarity. it quickly turned into "yes" and "no" depending on what DC system it was, and pros & cons depending on types of equip used on the DC side, more precisely, the gear that connects between DC and AC system, like an inverter.

the crude answer was yes. the more defined answer depends on many factors.

From another thread, I thought you might find it interesting.

I do a lot of 24 VDC control systems and radio systems, one side of the DC is always grounded, This is especially important with radio systems using RS232 or FSK comms, to keep the DC ground reference the same. "Ground is Good" as a very experienced radio tech once told me. This multiple grounding is not an issue and does not create ground loops
So grounding of one side of the DC is optional, but I typically see it on 24 VDC systems

 

[Aleman]

From another thread, I thought you might find it interesting.

Thanks for posting that over. 24VDC controls have very good immunity to noise in my experience. None of our systems DC grounded or not have
problems with noise. All of our analogs are 4-20mA which helps. But I do think grounded DC would be the better choice for all the reasons in
the post. So guess I'll get off my butt and do it. Really not much work there, but I always find other things to do.

 

[Johnnybob]

So, I have a lot of DC control. Some of it is 24VDC, some is 15VDC, 12 VDC and 5VDC. on aplications where i am utilizing multiple DC Voltages for an application (a linear transducer or hydrualic temposonic for example) I will have, for instance, +24DC and -24DC, +15DC and -15DC, +5DC and -5DC I'm pretty sure the neg side of these power supplies can't be tied together and grounded, or we would be frying power supplies and other sensitive electronics at a phenominal rate!
Okay, I just went out and looked in a cabinet. The power supplies are grounded on the AC side. Just +24VDC (or whatever, they're all wired the same) and -24VDC common. If they are grounded on the DC side, it's through the chassis' attatchment to the enclosure alone. There is nowhere for a gound on the DC output terminals! This goes for our older (solder only connections) and the brand new Allen-Bradleys I installed last year.

 

[Johnnybob]

I hope you can see them. On the input side x3 is clearly marked with an earth ground symbol. Not so on the output side. And only the +24 (and +5) are fused.

 

[gar]

160809-2144 EDT

Johnnybob:

I will have, for instance, +24DC and -24DC, +15DC and -15DC, +5DC and -5DC I'm pretty sure the neg side of these power supplies can't be tied together and grounded,

By +24DC and -24DC do you mean there is 48 V difference between the two terminals? I doubt it. Most likely this means 24 V between the two terminals and + and - define polarity. If this is simply a 24 V DC supply, and internally the supply is isolated from EGC, earth, chassis, or any other power supply, then either terminal could be tied to any desired reference point, including 1,000,000 V above earth if there were no voltage breakdown problems.

Continuing this thought: any one terminal of any one of the power supplies can be connected to any desired reference point so long as the connected loads are not interconnected in such a way that would produce a conflict.

In systems that I build I typically have +24 V, +16 V, -16 V, +12 V, -12 V, +8 V, and +5 V. Sometimes a -5 V. Each of these is referenced to a common that in turn is connected to the equipment chassis (possibly a 20,000 # machine), and that to the EGC.

+12 is derived from +16 via a series pass regulator. Same for the others. Combinatorial logic is powered from +5 V and Common, Operational amplifiers from +/-!2 V. Relays from +24 and Common. RS232 uses +/-12 V. RS232 signal output is referenced to Common at both ends of the communication path. But these are two different commons and there is likely an EGC voltage drop between the commons.

RS232 without isolation on its interconnection cable has serious problems from EGC noise and fault currents. The least significant problems are data errors. Large voltages like 80 V or more from fault currents can destroy RS232 components and possibly damage futher into the electronic equipment.

.

 

[Tony S]

160809-2144 EDT

Johnnybob:



By +24DC and -24DC do you mean there is 48 V difference between the two terminals? I doubt it. Most likely this means 24 V between the two terminals and + and - define polarity. If this is simply a 24 V DC supply, and internally the supply is isolated from EGC, earth, chassis, or any other power supply, then either terminal could be tied to any desired reference point, including 1,000,000 V above earth if there were no voltage breakdown problems.

.

Our early logic panels had +100V, +24V, -24V, all sharing a common grounded bus which ultimately shares the same grounding system as the supply.

If I were to measure the voltage between the various power pack outputs I would get 48V, 76V and 124V.

 

[ggunn]

160809-2144 EDT

Johnnybob:
By +24DC and -24DC do you mean there is 48 V difference between the two terminals? I doubt it.

That's exactly what it usually means. Between the + terminal of the +24V and the negative terminal of the -24V there is a 48V differential. The - terminal of the +24V and the + terminal of the -24V are common.

 

[gar]

1608010-0936 EDT

If a power supply has only two terminals and the terminals are marked +24 and -24, and there is no common terminal, then I would assume there is 24 V between the two terminals. The original post made no mention of a common terminal.

For various electronic instruments you need to know what is connected to EGC and what is not. Some examples: The Tektronix CFG250 function generator has a BNC shell to EGC resistance of about 0.2 ohms; Tek 130 LC meter the UHF shell to EGC is about 0.2 ohms; Tek 7603 scope about 3 ohms; Hewlett-Packard 200 CD oscillator has an output transformer with two output terminals that are isolated from ground, and a separate ground terminal connected to the chassis and EGC that with a supplied jumper bar can easily ground one output terminal; and a H-P 721A adjustable DC power supply has + and - output terminals that are isolated from ground and EGC.

The HP 721As can be cascaded together in any desired way so long as some voltage to EGC is not exceeded.

.

 

[ggunn]

1608010-0936 EDT

If a power supply has only two terminals and the terminals are marked +24 and -24, and there is no common terminal, then I would assume there is 24 V between the two terminals. The original post made no mention of a common terminal.

It would be unusual for terminals to be marked that way unless there is a common terminal. All voltages are shown in comparison to a reference, and it would be confusing to use Terminal A as a reference for Terminal B and then to show Terminal B as a reference for Terminal A. Dual power supplies with +V1 and -V2 referenced to a common V0 are very common, and the voltage differential between the two terminals that are not the common is V1 + V2. Op amps, for example, commonly use +5V (V1), -5V (V2) and 0V, and there is 10V between V1 and v2.

 

[jaggedben]

It would be unusual for terminals to be marked that way unless there is a common terminal. All voltages are shown in comparison to a reference, and it would be confusing to use Terminal A as a reference for Terminal B and then to show Terminal B as a reference for Terminal A. Dual power supplies with +V1 and -V2 referenced to a common V0 are very common, and the voltage differential between the two terminals that are not the common is V1 + V2. Op amps, for example, commonly use +5V (V1), -5V (V2) and 0V, and there is 10V between V1 and v2.

If you look at the pictures he posted, it appears they are 2 wire systems and the term 'common' is being used where probably 'negative' should be used instead.

 

[FionaZuppa]

So, I have a lot of DC control. Some of it is 24VDC, some is 15VDC, 12 VDC and 5VDC. on aplications where i am utilizing multiple DC Voltages for an application (a linear transducer or hydrualic temposonic for example) I will have, for instance, +24DC and -24DC, +15DC and -15DC, +5DC and -5DC I'm pretty sure the neg side of these power supplies can't be tied together and grounded, or we would be frying power supplies and other sensitive electronics at a phenominal rate!
Okay, I just went out and looked in a cabinet. The power supplies are grounded on the AC side. Just +24VDC (or whatever, they're all wired the same) and -24VDC common. If they are grounded on the DC side, it's through the chassis' attatchment to the enclosure alone. There is nowhere for a gound on the DC output terminals! This goes for our older (solder only connections) and the brand new Allen-Bradleys I installed last year.

i am 100% that you can share a common "bus" (common ground) with some power supplies. as example, i use Cotek DN sries of DIN mount industrial power supplies. you can tie all the neg's of the DC outputs together, and if DC is isolated you can then tie DC neg to a common chassis or bus "ground". but in many cases i would not take DC neg to earth ground, nor would i take DC neg to the input AC EGC. but in general if the AC input has a connection for AC EGC then i would not dare take DC neg to the same chassis. the downstream DC gear though could have their chassis tied to DC neg, but in many wiring setups that i have seen the AC EGC carries across the chassis of all items. as long as no DC gear ties DC neg to its own chassis (hence, chassis isloated) then i guess its fine to take AC EGC there, but you can introduce noise into the DC gear.

 

[gar]

160810-1518 EDT

Some photos:

http://www.wolfautomation.com/index...ingle-phase/?gclid=CJ2_kMPHt84CFZOIaQodDXUFNw
This is a single voltage 24 V DC supply with terminals marked +V and -V no common terminal. We have to assume that this supply is isolated from from its chassis, EGC, and earth.

Too hard to get other pictures that are readable. Mostly on DIN rail power supplies for single voltage units they are labeled +V and -V on the terminals. Unstated, but implied is that supplies are isolated from AC power, chassis, EGC, or earth.

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