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"Wiring standars?"

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M. Jamil - (18 posts) Nov-02-00, 00:09 AM (EST)

"Wiring standars?"

My confusions is, is it necessary to keep the -0V separate for 24V, 110V, 220V, etc? and is it necessary to keep the +ve common or -ve common

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Table of contents

RE: Wiring standars?, Terry Woods, Nov-02-00, (1)
RE: Wiring standars?, Tom Jenkins, Nov-02-00, (2)
RE: Wiring standars?, M. Jamil, Nov-03-00, (3)
RE: Wiring standars?, Jim Dungar, Nov-03-00, (4)
RE: Wiring standars?, Terry Woods, Nov-03-00, (5)
RE: Wiring standars?, M Jamil, Nov-04-00, (7)
RE: Wiring standars?, Terry Woods, Nov-04-00, (9)
RE: Wiring standars?, Terry Woods, Nov-03-00, (6)
RE: Wiring standars?, M. Jamil, Nov-04-00, (8)
RE: Wiring standars?, Todd Belville, Nov-06-00, (10)
RE: Wiring standars?, Terry Woods, Nov-06-00, (11)
RE: Wiring standars?, mjamil, Nov-07-00, (12)
RE: Wiring standars?, Terry Woods, Nov-07-00, (15)
RE: Wiring standards?, Rick Densing, Nov-07-00, (13)
RE: Wiring standards?, Terry Woods, Nov-07-00, (16)
RE: Wiring standars?, Shawn, Nov-07-00, (14)

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Terry Woods - (545 posts) Nov-02-00, 03:23 AM (EST)

1. "RE: Wiring standars?"

Within each of the systems you listed, it is better to keep all commons separate. However, eventually, at least the primary side of those systems do use a common ground.
For example, If you have a 24VDC power supply you would do well to keep the 24V common or 24V ground isolated from house ground and instead bring it back directly to the common or ground terminal on the power supply. That is, maintain a ground plane, by wire only, and bring it back to the generating device. The generating device should provide the necessary filtering and ultimate connection back to the house ground. The same applies to the other low-voltage sources (less than 120VAC).
Ultimately, in all cases (I believe) the source side of any power supply needs to be tied to house ground because 120/220 are typically "grounded".
As a general rule, the more isolated you can keep the load side of a system, the cleaner the signals. Isolated means bringing +V and/or -V, and COM out by isolated wire. Shielded is even better.
Speaking of shielded, be sure that your shielding is grounded at only one end; the source end. Otherwise, if you ground at both ends (maybe thinking more ground connections are better) you create a transformer in the ground line - NOT A GOOD THING TO DO! Grounding at the load end only is also not so good. This is because the load end usually does not provide for ground filtering whereas low-voltage source usally do.
In 120/220VAC this rule doesn't apply because of the shear size of the voltages. In this case grounding at both ends is the norm.
I must also add, grounding is still a SPOOKY SCIENCE! Even for Electrical Engineers! This has been a debate ever since Edison and Tesla! Each school has their valid points. And the debate still rages on! We haven't quite figured it out yet.
But, in general, the rules apply as I have described, at least up until this time.

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Tom Jenkins - (546 posts) Nov-02-00, 11:06 AM (EST)

2. "RE: Wiring standars?"

One other note to add to Terry's thorough explanation - don't confuse ground, 0V, and neutral.
Ground, which should ultimately be an earth ground, is there for safety. It is not intended to be a current carrying conductor!
Neutral on AC circuits should be tied to ground at the distribution point, but not at panels or individual devices. It IS intended to be a current carrying conductor.
On US 220 VAC single phase systems (most common for 220) the 220 volt is from line to line, and voltage line to neutral or ground will be 110 VAC.
0 VDC is like neutral on AC, and as Terry says the 0 VDC is usually tied to ground through the power supply internals. In common parlance this is often referred to as "-" to differentiate it from + VDC, but some electronics and some power supplies have a + VDC and a - VDC which are opposite polarity when measured from 0 VDC!

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M. Jamil - (18 posts) Nov-03-00, 00:04 AM (EST)

3. "RE: Wiring standars?"

Thanks. here's another one.
On a machine, I was working (rather Re-designing), The Neutral of 110Vac/220Vac were SHORTED....(thats not enuf..) not only to themselves but also to the EARTH - GROUND, similarly the Neutral I would say but I think since its the -ve of 24VDC supply (dont know what to call it, anyway) well this ... -ve or 0Vdc is again shorted to the EARTH GROUND. The onyl reason I could find (for the 0VDC and earth shorted) was that the machine was gounded and it had mechanical sensors,(which would just make contact with the machines body when required) so obviously the output was EARTH GROUND these were used as PLC inputs.. is this right? or any precautions to be taken,
Keeping it in mind, as per Terry's advise its a SCARY thing, even for Elec. Engineers, so well I'm not even and Elec. Engineer, I just do the Programming part, but yes am interested, it always helps to have the basics of eveything... dont you think so.

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Jim Dungar - (121 posts) Nov-03-00, 10:13 AM (EST)

4. "RE: Wiring standars?"

All systems also consist of at least two wires which are often called "hot and neutral" in AC systems and "+/-V and 0V" in DC systems. For safety purposes, the voltage system must be connected to a common reference point, which is usually called ground. This connection should be done with "shorting" straps located only at the source of the voltage. Ground is not intended to normally carry current that is why we only have one connection point. For proper noise reduction cable shields should also follow this concept of "single point grounding".
Grounding is too important for safety to ever be called scary, confusing yes, but scary no. As machine control designers we need to know the basics of power systems, because improper connections can cause hours of troubleshooting problems not to mention failure of equipement.

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Terry Woods - (545 posts) Nov-03-00, 07:05 PM (EST)

5. "RE: Wiring standars?"

LAST EDITED ON Nov-03-00 AT 07:18 PM (EST)
I said grounding was SPOOKY not scary. SPOOKY as in Strange, Weird, X-Files kind of stuff! (Well, maybe not X-Files kinda stuff, 'cause some of that IS scary!)
You can always apply too much grounding and still be safe. If you don't have enough grounding you might not be safe. The problem with putting in too much grounding is that it sometimes de-grades your signal, especially in low-voltage systems.
As you experiment trying to find the best results, some connections provide better results than others. Why one connection should be any better than another similar connection is not very well understood. That is the SPOOKY part.
Now that I reconsider a little, there is an aspect of grounding that is SPOOKY like X-Files. That is, how is that sometimes, when you think that you've taken all of the grounding precautions that you can think of, sometimes someone still gets bitten or worse?
The scary part is that electricity is on the job constantly trying to find that short-circuit to ground. It would prefer to find a path that does not go through a load, but it will take any path it can find - even through you!

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M Jamil - (2 posts) Nov-04-00, 01:49 AM (EST)

7. "RE: Wiring standars?"

O.K. O.K. ..... Not Scary --- Spooky.. SATISFIED....
(But then again, if you think what can happen in most cases, there is a SCARY part)

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Terry Woods - (545 posts) Nov-04-00, 03:34 AM (EST)

9. "RE: Wiring standars?"

Well, like I said, there is a scary part, like when it tries to go through you!

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Terry Woods - (545 posts) Nov-03-00, 08:38 PM (EST)

6. "RE: Wiring standars?"

Hey MJ,
It appears that whoever designed or built the machine decided that ALL AC Neutrals and All DC Commons should be tied to the same ground plane.
This arrangement works in some technologies (radios and TV's) because of built-in filtering. The entire chassis of the radio or TV is designed to accomodate the AC and DC. However, in the very small low-voltage portions of the chassis, the voltages are still isolated! (because of noise!)
Now, the machine you are talking about is neither a radio nor a TV. It was not designed from the ground up taking into consideration what grounding may or may not be acceptable.
It's usually the case that grounding is the last thing thought about when building industrial machinery. Technicians that are adding parts and throwing things together are looking for a desired result. It isn't until they find that they don't get the desired result that they start looking for reasons. And even then, a grounding problem is the last thing they think of. (I'm not talking about machinery designers, I'm talking about those people that are designing and building machines in the factory on the fly.)
Sometimes, early in the process they simply decide, "We ain't gonna have that ground problem this time, so lets bring ALL AC Neutrals and ALL DC Commons to the same ground plane." They think that Common and Neutral are the same thing! The fact is, it IS and it AIN'T! Depends on how you look at it. (Light? is it a steam of particles or is it a set of waves? The answer is, BOTH!)
Anyway, back to your issue...
Like I said earlier, the supply side of the low-voltage sources should be tied to ground, but the load side does not need to be tied to the main ground plane. A 24VDC Power Supply will not hurt you. You'll just start to feel a tickle at around 48VDC.
Grounding the 24VDC Common does nothing to improve the safety. It only introduces the possibility of electrical noise on the low-voltage signal which could cause erroneous readings at the PLC input.
A low-voltage DC Supply will perform better if you have an isolated return path (Common) to the load side of the supply.
So, since Grounding the Common can lead to small signal noise problems, and since you really don't need a ground plane (as in safety ground) for the load side of the DC Supply, why do it?
In the case you described where the DC Supply provides power to mechanical switches, if the switch shorts to ground (Earth Ground) there will be minimal effect on the supply. That is, there is no short circuit across the DC Output of the supply (see NOTE).
NOTE:===========================================================
Unless you just so happen to have a short between V+ to ground, and another short from 0V or V- to ground, in which case your supply might fuse-out. You could have any number of shorts between the Common side of any load and ground without any serious effects.
================================================================
Your input to the PLC might not work (maybe), but you won't have a dangerous situation and your supply won't fuse-out.
At the PLC, the inputs are optically isolated from the innards. All of the terminals on the input card can and should be wired in such a fashion as to maintain isolation between the DC Voltages in the sensors and the AC Voltages in the PLC.
I'm open to all comments.

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M. Jamil - (18 posts) Nov-04-00, 02:22 AM (EST)

8. "RE: Wiring standars?"

Thanks - Now I have more things to think about....just kidding.
Using Siemens S7216 /226, In my case, the whole machine is Ground (EARTH), now I attach the +24V output from the PLC to the Input common (Module common) and the other wire comes from the machine (Mechanical sensors) in other words its the EARTH or GROUND, (keeping in mind that the EARTH and the -v coming out from the PLC supply output are shorted) this is used to trigger the inputs.
What I wanted to know since the whole machine is a sensor it self, different parts giving ON/OFF to the PLC what happens if a LIVE wire is dropped on the machine? or since the same ground is used, say a 3Phase motor is running on the machine, obviously the metal body of the motor is using the machine as ground, wont this create problem if the EARTH is weak. or should I use DIODEs at the PLC inputs - Not much clear but would appreciate all the help.

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Todd Belville - (3 posts) Nov-06-00, 07:00 AM (EST)

10. "RE: Wiring standars?"

MJ

What exactly do you mean by mechanical sensors r u talking about micro-switches enclosed in a metal body with a actuator on them? Regardless if u have a power supply on your plc, 24+and24- or whatever DC voltage is your PLC`s power supply should be isolated from the machines ground.Normally when I build a machine
I use a external 24v power supply, and filter the 110vac going to my PLC as well as my power supply these filters are relatively inexspensive.But I always keep 24+ and 24- separate from any AC voltages.

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Terry Woods - (545 posts) Nov-06-00, 08:58 PM (EST)

11. "RE: Wiring standars?"

LAST EDITED ON Nov-06-00 AT 09:07 PM (EST)
MJ....
What part of "KEEP THE DC SUPPLY ISOLATED" don't you understand?
Do NOT tie the DC Common or DC Ground or whatever you want to call it, to the FRAME! Simply use DC+ out to all of your DC sensors and return them individually to the various Inputs!
If you have a jumper that ties the DC-(or COM) or DC+ to FRAME, remove it!
S7 DC connection
<-from CPU Module
CPU DC+ <-------+----------+-------+--- ... ----+
CPU DC- <---+ | | | |
| | / / /
(-) (+) | | |
IN1 IN2 ... INX
This arrangement should give you reliable signal and minimal potential for damage caused by AC Hot to frame!
BTW
I smoke Parliament 100's, and I haven't seen any in the mail lately.

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mjamil - (44 posts) Nov-07-00, 00:05 AM (EST)

12. "RE: Wiring standars?"

Thanks Terry,
There is one thing (important) I forgot to mention, (though it is easy for one to suggest a solution, if the problem is describe well).
The mechanical Sensor, I meant that they just conveyed the Machine's (EARTH) to the PLC. that means that they were actually not using the +ve of any supply. like a sensor would ground itself to the body if there was no packet!!! the two wires coming out of (what I referred to as Mechanical sensor - oooops!) one was instantly grounded the other was carried to the PLC!!! thats how it has been done in most of the machines - I've seen.
The only alternate to this was to remove all the old mech. sensors and install (Isolated) DC sensor, but that would be very difficult as most of the Sensors would not only be custom designed but hell of EXPENSIVE, for instance the first sensor checks if the Cigarete has a Filter (and was not misplace or broken in the machine) this Old sensor was just a plunger with 20 brass pins which would GROUND the sensor if any of them failed to be pushed inside when bought in contact with the CIGARETTes, now to replace them with a typical sensor, this would mean I would need atleast 20Proximity (non-metal type) whose Dia would have to be (obviously) less than that of a Cigarette i.e., <9mm... or design a PCB with then sensor who share the same circuit, but than again, I'f you're not talking quantity here, I think we are on the right track..
Now back to the topic, (I think we never left the Topic anyway) what I do is The +ve of the PLC (OUTPUT SUPPLY) is connected to the common of the PLC module (1M and 2M in this case of S7-216) now the (so called) mech. sensor when shorted give the MACHINE EARTH to the inputs of the PLC, please do keep in mind (or advise) This EARTH is (or should not) shorted with the negative supply coming from the PLC!!! - Comments awaited.. if so...
Then there is another 12V and another 110V device whose +ve has been connected to them directly. again to activate them I use this MACHINE EARTH on the other contact. Now I know that for lower end (DC voltages) would be fine, but for AC 110 or 220 using the MACHINE EARTH as a "NEUTRAL" (I know its not right) but if that is the system, (for some unknown reason) and I do not want to rip all the wiring, and just want to save my piece of equipment only, what should I do?

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Terry Woods - (545 posts) Nov-07-00, 08:45 PM (EST)

15. "RE: Wiring standars?"

OK, I think I'm finally waking up and seeing the picture.
Sounds like you are sitting on a home-made kludge! Which of course is not unusual for older industries such as yours.
Even if the machine was built out of house (by some machine manufacturer), it was built using non-standard parts.
Seems you are saying that your machine has a bunch of custom (homemade, or at least very special) sensors where the part of the sensor that is connected to the frame is designed to carry DC current.
If this is so, then I can see your problem. I don't know what your sensor devices look like nor how they are connected to frame ground, but it seems you might be able to insert insulating material (sheet and bushings) to isolate the sensor from the chassis. You would then need to find a way to connected the newly isolated portion to the DC circuit.
This would isolate the AC system from the DC system as you where seeking in the original posting. It would prevent any spurious AC signals from harming or interferring with the DC devices.
But, at this point, understanding more about what the situation is (I think?)... I don't know if it would be worth the effort. You would know better!
Maybe you need to look at surge suppression on your AC supply.
As far as how to prevent AC spikes from entering your DC system because of ground faults, well, your supply should be internally protected and the PLC inputs should be optically isolated so I don't think the PLC would be harmed. Although, the PLC might read the AC Spike as a DC input signal. In that case, the program needs to be written in such a manner that the input signal is not accepted as being real unless the program is expecting that signal to occur at a particular point in the process.
Are we almost on the same page?

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Rick Densing - (90 posts) Nov-07-00, 10:32 AM (EST)

13. "RE: Wiring standards?"

I have almost always grounded the negative side of a 24VDC power supply and never had a noise problem even with servos and VFD's in the system. It all comes down to good wiring techniques. All 24VDC circuits are isolated from the higher voltages. The 0V line is then wired white with a blue stripe. This makes it easy to follow and troubleshoot.
The problem with not gounding the negative side of your power supply is that a short on, say a limit switch, may not cause a fault. It could just put your negative side to -24VDC in respect to ground, but still affect your PLC input, especially if your plc supply is different that the I/O supply. A grounded 0VDC line will result in a blown fuse, and with proper system design will be a safer machine.

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Terry Woods - (545 posts) Nov-07-00, 09:00 PM (EST)

16. "RE: Wiring standards?"

From a safety point of view, leaving the DC system floating is not a problem. 24VDC never hurt anyone.
From the PLC's point of view you might have a problem but not because you have spurious AC signals causing false triggering of DC inputs.
In a perfect world, grounding all would be fine but the AC can raise hell with the DC sometimes.

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Shawn - (19 posts) Nov-07-00, 07:55 PM (EST)

14. "RE: Wiring standars?"

Wow, you guys are going nuts about this grounding path! Cool. Keep it up, gives me something to read during office hours. Lots of good info.

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M. Jamil - (18 posts)	Nov-02-00, 00:09 AM (EST)
"Wiring standars?"
My confusions is, is it necessary to keep the -0V separate for 24V, 110V, 220V, etc? and is it necessary to keep the +ve common or -ve common
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Terry Woods - (545 posts)	Nov-02-00, 03:23 AM (EST)
1. "RE: Wiring standars?"
Within each of the systems you listed, it is better to keep all commons separate. However, eventually, at least the primary side of those systems do use a common ground. For example, If you have a 24VDC power supply you would do well to keep the 24V common or 24V ground isolated from house ground and instead bring it back directly to the common or ground terminal on the power supply. That is, maintain a ground plane, by wire only, and bring it back to the generating device. The generating device should provide the necessary filtering and ultimate connection back to the house ground. The same applies to the other low-voltage sources (less than 120VAC). Ultimately, in all cases (I believe) the source side of any power supply needs to be tied to house ground because 120/220 are typically "grounded". As a general rule, the more isolated you can keep the load side of a system, the cleaner the signals. Isolated means bringing +V and/or -V, and COM out by isolated wire. Shielded is even better. Speaking of shielded, be sure that your shielding is grounded at only one end; the source end. Otherwise, if you ground at both ends (maybe thinking more ground connections are better) you create a transformer in the ground line - NOT A GOOD THING TO DO! Grounding at the load end only is also not so good. This is because the load end usually does not provide for ground filtering whereas low-voltage source usally do. In 120/220VAC this rule doesn't apply because of the shear size of the voltages. In this case grounding at both ends is the norm. I must also add, grounding is still a SPOOKY SCIENCE! Even for Electrical Engineers! This has been a debate ever since Edison and Tesla! Each school has their valid points. And the debate still rages on! We haven't quite figured it out yet. But, in general, the rules apply as I have described, at least up until this time.
	Top

Tom Jenkins - (546 posts)	Nov-02-00, 11:06 AM (EST)
2. "RE: Wiring standars?"
One other note to add to Terry's thorough explanation - don't confuse ground, 0V, and neutral. Ground, which should ultimately be an earth ground, is there for safety. It is not intended to be a current carrying conductor! Neutral on AC circuits should be tied to ground at the distribution point, but not at panels or individual devices. It IS intended to be a current carrying conductor. On US 220 VAC single phase systems (most common for 220) the 220 volt is from line to line, and voltage line to neutral or ground will be 110 VAC. 0 VDC is like neutral on AC, and as Terry says the 0 VDC is usually tied to ground through the power supply internals. In common parlance this is often referred to as "-" to differentiate it from + VDC, but some electronics and some power supplies have a + VDC and a - VDC which are opposite polarity when measured from 0 VDC!
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	M. Jamil - (18 posts)	Nov-03-00, 00:04 AM (EST)
	3. "RE: Wiring standars?"
	Thanks. here's another one. On a machine, I was working (rather Re-designing), The Neutral of 110Vac/220Vac were SHORTED....(thats not enuf..) not only to themselves but also to the EARTH - GROUND, similarly the Neutral I would say but I think since its the -ve of 24VDC supply (dont know what to call it, anyway) well this ... -ve or 0Vdc is again shorted to the EARTH GROUND. The onyl reason I could find (for the 0VDC and earth shorted) was that the machine was gounded and it had mechanical sensors,(which would just make contact with the machines body when required) so obviously the output was EARTH GROUND these were used as PLC inputs.. is this right? or any precautions to be taken, Keeping it in mind, as per Terry's advise its a SCARY thing, even for Elec. Engineers, so well I'm not even and Elec. Engineer, I just do the Programming part, but yes am interested, it always helps to have the basics of eveything... dont you think so.
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	Jim Dungar - (121 posts)	Nov-03-00, 10:13 AM (EST)
	4. "RE: Wiring standars?"
	All systems also consist of at least two wires which are often called "hot and neutral" in AC systems and "+/-V and 0V" in DC systems. For safety purposes, the voltage system must be connected to a common reference point, which is usually called ground. This connection should be done with "shorting" straps located only at the source of the voltage. Ground is not intended to normally carry current that is why we only have one connection point. For proper noise reduction cable shields should also follow this concept of "single point grounding". Grounding is too important for safety to ever be called scary, confusing yes, but scary no. As machine control designers we need to know the basics of power systems, because improper connections can cause hours of troubleshooting problems not to mention failure of equipement.
		Top


	Terry Woods - (545 posts)	Nov-03-00, 07:05 PM (EST)
	5. "RE: Wiring standars?"
	LAST EDITED ON Nov-03-00 AT 07:18 PM (EST) I said grounding was SPOOKY not scary. SPOOKY as in Strange, Weird, X-Files kind of stuff! (Well, maybe not X-Files kinda stuff, 'cause some of that IS scary!) You can always apply too much grounding and still be safe. If you don't have enough grounding you might not be safe. The problem with putting in too much grounding is that it sometimes de-grades your signal, especially in low-voltage systems. As you experiment trying to find the best results, some connections provide better results than others. Why one connection should be any better than another similar connection is not very well understood. That is the SPOOKY part. Now that I reconsider a little, there is an aspect of grounding that is SPOOKY like X-Files. That is, how is that sometimes, when you think that you've taken all of the grounding precautions that you can think of, sometimes someone still gets bitten or worse? The scary part is that electricity is on the job constantly trying to find that short-circuit to ground. It would prefer to find a path that does not go through a load, but it will take any path it can find - even through you!
		Top


	M Jamil - (2 posts)	Nov-04-00, 01:49 AM (EST)
	7. "RE: Wiring standars?"
	O.K. O.K. ..... Not Scary --- Spooky.. SATISFIED.... (But then again, if you think what can happen in most cases, there is a SCARY part)
		Top


	Terry Woods - (545 posts)	Nov-04-00, 03:34 AM (EST)
	9. "RE: Wiring standars?"
	Well, like I said, there is a scary part, like when it tries to go through you!
		Top


	Terry Woods - (545 posts)	Nov-03-00, 08:38 PM (EST)
	6. "RE: Wiring standars?"
	Hey MJ, It appears that whoever designed or built the machine decided that ALL AC Neutrals and All DC Commons should be tied to the same ground plane. This arrangement works in some technologies (radios and TV's) because of built-in filtering. The entire chassis of the radio or TV is designed to accomodate the AC and DC. However, in the very small low-voltage portions of the chassis, the voltages are still isolated! (because of noise!) Now, the machine you are talking about is neither a radio nor a TV. It was not designed from the ground up taking into consideration what grounding may or may not be acceptable. It's usually the case that grounding is the last thing thought about when building industrial machinery. Technicians that are adding parts and throwing things together are looking for a desired result. It isn't until they find that they don't get the desired result that they start looking for reasons. And even then, a grounding problem is the last thing they think of. (I'm not talking about machinery designers, I'm talking about those people that are designing and building machines in the factory on the fly.) Sometimes, early in the process they simply decide, "We ain't gonna have that ground problem this time, so lets bring ALL AC Neutrals and ALL DC Commons to the same ground plane." They think that Common and Neutral are the same thing! The fact is, it IS and it AIN'T! Depends on how you look at it. (Light? is it a steam of particles or is it a set of waves? The answer is, BOTH!) Anyway, back to your issue... Like I said earlier, the supply side of the low-voltage sources should be tied to ground, but the load side does not need to be tied to the main ground plane. A 24VDC Power Supply will not hurt you. You'll just start to feel a tickle at around 48VDC. Grounding the 24VDC Common does nothing to improve the safety. It only introduces the possibility of electrical noise on the low-voltage signal which could cause erroneous readings at the PLC input. A low-voltage DC Supply will perform better if you have an isolated return path (Common) to the load side of the supply. So, since Grounding the Common can lead to small signal noise problems, and since you really don't need a ground plane (as in safety ground) for the load side of the DC Supply, why do it? In the case you described where the DC Supply provides power to mechanical switches, if the switch shorts to ground (Earth Ground) there will be minimal effect on the supply. That is, there is no short circuit across the DC Output of the supply (see NOTE). NOTE:=========================================================== Unless you just so happen to have a short between V+ to ground, and another short from 0V or V- to ground, in which case your supply might fuse-out. You could have any number of shorts between the Common side of any load and ground without any serious effects. ================================================================ Your input to the PLC might not work (maybe), but you won't have a dangerous situation and your supply won't fuse-out. At the PLC, the inputs are optically isolated from the innards. All of the terminals on the input card can and should be wired in such a fashion as to maintain isolation between the DC Voltages in the sensors and the AC Voltages in the PLC. I'm open to all comments.
		Top


	M. Jamil - (18 posts)	Nov-04-00, 02:22 AM (EST)
	8. "RE: Wiring standars?"
	Thanks - Now I have more things to think about....just kidding. Using Siemens S7216 /226, In my case, the whole machine is Ground (EARTH), now I attach the +24V output from the PLC to the Input common (Module common) and the other wire comes from the machine (Mechanical sensors) in other words its the EARTH or GROUND, (keeping in mind that the EARTH and the -v coming out from the PLC supply output are shorted) this is used to trigger the inputs. What I wanted to know since the whole machine is a sensor it self, different parts giving ON/OFF to the PLC what happens if a LIVE wire is dropped on the machine? or since the same ground is used, say a 3Phase motor is running on the machine, obviously the metal body of the motor is using the machine as ground, wont this create problem if the EARTH is weak. or should I use DIODEs at the PLC inputs - Not much clear but would appreciate all the help.
		Top


	Todd Belville - (3 posts)	Nov-06-00, 07:00 AM (EST)
	10. "RE: Wiring standars?"
	MJ What exactly do you mean by mechanical sensors r u talking about micro-switches enclosed in a metal body with a actuator on them? Regardless if u have a power supply on your plc, 24+and24- or whatever DC voltage is your PLC`s power supply should be isolated from the machines ground.Normally when I build a machine I use a external 24v power supply, and filter the 110vac going to my PLC as well as my power supply these filters are relatively inexspensive.But I always keep 24+ and 24- separate from any AC voltages.
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	Terry Woods - (545 posts)	Nov-06-00, 08:58 PM (EST)
	11. "RE: Wiring standars?"
	LAST EDITED ON Nov-06-00 AT 09:07 PM (EST) MJ.... What part of "KEEP THE DC SUPPLY ISOLATED" don't you understand? Do NOT tie the DC Common or DC Ground or whatever you want to call it, to the FRAME! Simply use DC+ out to all of your DC sensors and return them individually to the various Inputs! If you have a jumper that ties the DC-(or COM) or DC+ to FRAME, remove it! S7 DC connection <-from CPU Module CPU DC+ <-------+----------+-------+--- ... ----+ CPU DC- <---+ \| \| \| \| \| \| / / / (-) (+) \| \| \| IN1 IN2 ... INX This arrangement should give you reliable signal and minimal potential for damage caused by AC Hot to frame! BTW I smoke Parliament 100's, and I haven't seen any in the mail lately.
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	mjamil - (44 posts)	Nov-07-00, 00:05 AM (EST)
	12. "RE: Wiring standars?"
	Thanks Terry, There is one thing (important) I forgot to mention, (though it is easy for one to suggest a solution, if the problem is describe well). The mechanical Sensor, I meant that they just conveyed the Machine's (EARTH) to the PLC. that means that they were actually not using the +ve of any supply. like a sensor would ground itself to the body if there was no packet!!! the two wires coming out of (what I referred to as Mechanical sensor - oooops!) one was instantly grounded the other was carried to the PLC!!! thats how it has been done in most of the machines - I've seen. The only alternate to this was to remove all the old mech. sensors and install (Isolated) DC sensor, but that would be very difficult as most of the Sensors would not only be custom designed but hell of EXPENSIVE, for instance the first sensor checks if the Cigarete has a Filter (and was not misplace or broken in the machine) this Old sensor was just a plunger with 20 brass pins which would GROUND the sensor if any of them failed to be pushed inside when bought in contact with the CIGARETTes, now to replace them with a typical sensor, this would mean I would need atleast 20Proximity (non-metal type) whose Dia would have to be (obviously) less than that of a Cigarette i.e., <9mm... or design a PCB with then sensor who share the same circuit, but than again, I'f you're not talking quantity here, I think we are on the right track.. Now back to the topic, (I think we never left the Topic anyway) what I do is The +ve of the PLC (OUTPUT SUPPLY) is connected to the common of the PLC module (1M and 2M in this case of S7-216) now the (so called) mech. sensor when shorted give the MACHINE EARTH to the inputs of the PLC, please do keep in mind (or advise) This EARTH is (or should not) shorted with the negative supply coming from the PLC!!! - Comments awaited.. if so... Then there is another 12V and another 110V device whose +ve has been connected to them directly. again to activate them I use this MACHINE EARTH on the other contact. Now I know that for lower end (DC voltages) would be fine, but for AC 110 or 220 using the MACHINE EARTH as a "NEUTRAL" (I know its not right) but if that is the system, (for some unknown reason) and I do not want to rip all the wiring, and just want to save my piece of equipment only, what should I do?
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	Terry Woods - (545 posts)	Nov-07-00, 08:45 PM (EST)
	15. "RE: Wiring standars?"
	OK, I think I'm finally waking up and seeing the picture. Sounds like you are sitting on a home-made kludge! Which of course is not unusual for older industries such as yours. Even if the machine was built out of house (by some machine manufacturer), it was built using non-standard parts. Seems you are saying that your machine has a bunch of custom (homemade, or at least very special) sensors where the part of the sensor that is connected to the frame is designed to carry DC current. If this is so, then I can see your problem. I don't know what your sensor devices look like nor how they are connected to frame ground, but it seems you might be able to insert insulating material (sheet and bushings) to isolate the sensor from the chassis. You would then need to find a way to connected the newly isolated portion to the DC circuit. This would isolate the AC system from the DC system as you where seeking in the original posting. It would prevent any spurious AC signals from harming or interferring with the DC devices. But, at this point, understanding more about what the situation is (I think?)... I don't know if it would be worth the effort. You would know better! Maybe you need to look at surge suppression on your AC supply. As far as how to prevent AC spikes from entering your DC system because of ground faults, well, your supply should be internally protected and the PLC inputs should be optically isolated so I don't think the PLC would be harmed. Although, the PLC might read the AC Spike as a DC input signal. In that case, the program needs to be written in such a manner that the input signal is not accepted as being real unless the program is expecting that signal to occur at a particular point in the process. Are we almost on the same page?
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Rick Densing - (90 posts)	Nov-07-00, 10:32 AM (EST)
13. "RE: Wiring standards?"
I have almost always grounded the negative side of a 24VDC power supply and never had a noise problem even with servos and VFD's in the system. It all comes down to good wiring techniques. All 24VDC circuits are isolated from the higher voltages. The 0V line is then wired white with a blue stripe. This makes it easy to follow and troubleshoot. The problem with not gounding the negative side of your power supply is that a short on, say a limit switch, may not cause a fault. It could just put your negative side to -24VDC in respect to ground, but still affect your PLC input, especially if your plc supply is different that the I/O supply. A grounded 0VDC line will result in a blown fuse, and with proper system design will be a safer machine.
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	Terry Woods - (545 posts)	Nov-07-00, 09:00 PM (EST)
	16. "RE: Wiring standards?"
	From a safety point of view, leaving the DC system floating is not a problem. 24VDC never hurt anyone. From the PLC's point of view you might have a problem but not because you have spurious AC signals causing false triggering of DC inputs. In a perfect world, grounding all would be fine but the AC can raise hell with the DC sometimes.
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Shawn - (19 posts)	Nov-07-00, 07:55 PM (EST)
14. "RE: Wiring standars?"
Wow, you guys are going nuts about this grounding path! Cool. Keep it up, gives me something to read during office hours. Lots of good info.
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