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Old June 17th, 2017, 01:17 PM   #31
BryanG
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I hadn't thought about a ground loop issue because my devices are always in the local box, or on a chassis made of very large section highly conductive steel box. The chance of there being a potential difference on the earth is slim. I can see it for remote devices around a factory. However the question then is what if you have two screens or two devices that internally connect ground to negative. And it ties in to what I mean when I say unless you check each device you are creating an unknown system rather than a floating system. And if you think about what happens when there is a fault, you can end up with a lot of current flowing through the grounding cable, would you rather have it be a cable that you designed to carry that current or the one that was just big enough to feed the HMI.
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Old June 17th, 2017, 02:15 PM   #32
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Quote:
Originally Posted by skyfox View Post
WOW! Seems 50/50. I was hoping for 90/10 sort of response. Thanks to all of you that replied. I guess more research is needed on the subject.

Cheers!!!!
I bet you're glad that you didn't ask a question that had three choices.
The responses might have been 50/50/50 (jk)
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Old June 17th, 2017, 04:58 PM   #33
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Couple of other issues that arise from grounding the 24VDC - if a 24VDC supply wire to the field is grounded the switch mode power supply turns off killing the PLC till the ground short is removed and if the field devices are switching negative grounding an input will turn it on. Generally I switch positive on all my inputs but sometimes have negative switching devices in the field then there is no choice.
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Old June 18th, 2017, 04:29 AM   #34
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Couple of other issues that arise from grounding the 24VDC - if a 24VDC supply wire to the field is grounded the switch mode power supply turns off killing the PLC .
That's what fuses are for, right? I fuse each block of I/O separately and monitor the 24V after the fuse. If it blows, raise an alarm.

Even without grounding of the 0V, it's still possible for a device to experience a fault to 0V, if that device is supplied from the same 24V supply.
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Old June 18th, 2017, 07:40 AM   #35
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As stated earlier some devices will internally ground the zero reference. If with a meter you end up having potential between your positive and the ground a device has broken your isolation and you probably should use a good conductor and properly tie it at the supply. You don't want a fault traveling around and finding that small link in a very expensive device.

I have seen techs land 120 on 24vdc IO before, and it would suck for it to find you very expensive fiber media converter, switch, HMI, or computer.
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Old June 18th, 2017, 03:41 PM   #36
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Switch mode will shut down before the fuse blows. The PLC shuts down as well. Have had it happen - generator system running hard during a power failure in a data centre - generators shut down when the PLC shut down - circuit breakers opened - not pretty! Have never grounded 0VDC ever since. Have never had an issue since. The above was not a system I built - it was a system built by someone else.
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Old June 19th, 2017, 01:31 AM   #37
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In the document System Design for Control of Electrical Noise
http://literature.rockwellautomation...m001_-en-p.pdf (pages A-8 and A-9)
Rockwell recomends also to ground 24- of switched mode power supplys, because of noise reduction.
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Old June 19th, 2017, 02:09 AM   #38
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Rockwell recomends also to ground 24- of switched mode power supplys, because of noise reduction.
Isolation is gone though - pluses and minuses.
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Old June 19th, 2017, 02:57 AM   #39
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Isolation is only important if you are designing it as a floating system, otherwise it doesn't matter two 'bits'. A floating system can be the safest of all but you have to design it from the outset with proper segregation of low and medium voltage circuits and with the correct supply system. Normal mixed voltage control panels must have the control supply earthed, it is in the regs in Europe and as the Aus/NZ regs are based on those I am pretty sure it will apply there, EN 60204 6.4 Protection by the use of PELV, paragraph b 'one side of the circuit or one point of the supply of that circuit shall be connected to the Protective Bonding Circuit'.

It also states that in a PELV system 24V d.c. shall only be used in dry small contact conditions and that otherwise it should be maximum 6V a.c. or 15V ripple free d.c. Not sure how that applies to SELV.

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generators shut down when the PLC shut down - circuit breakers opened - not pretty! Have never grounded 0VDC ever since.
Sorry Bob but you didn't fix the problem, you just took things out of your control. Not grounding a standard mixed voltage system means there can be a fault and unless you put in monitoring, you don't know there is a fault. So your generator might get a fault where the -DC goes to ground and you won't know until the system fails on the second fault. You go to site and unless you test for all faults you will just fix the second leaving a faulty system. If something is critical in nature then you need to be designing in redundancy, not fudging a PELV system in to an SELV.
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Old June 19th, 2017, 03:24 AM   #40
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Then what is the point of safety transformers? Double wound - ground the 0VAC and the isolation is gone.
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Old June 19th, 2017, 03:25 AM   #41
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By the way LV does not have to be grounded in Oz - flow meter - 24VDC - no grounding required. Sparkies do not even run an earth wire out to 24 volt equipment.
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Old June 19th, 2017, 04:37 AM   #42
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Completely agree on the fuses failing to blow. Have now gone to Siemens 'Selectivity Modules':
https://support.industry.siemens.com...fn=ps&lc=en-GB

They do react quick enough to stop the Switchmode sitting down.
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Old June 19th, 2017, 04:40 AM   #43
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Then what is the point of safety transformers? Double wound - ground the 0VAC and the isolation is gone.
Because you can get serious problems with autotransformers that just tap off a single winding, has to be double wound or in a fault the high voltage can appear on the low voltage.
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Old June 19th, 2017, 06:01 AM   #44
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Then what is the point of safety transformers? Double wound - ground the 0VAC and the isolation is gone.
Not sure what you mean by "safety transformer".
The ubiquitous control voltage transformer is primarily to take the 3-phase power supply and bring both voltage and short-circuit current to managable levels.
In that case it is typical to ground the neutral (N). This neutral is not a ground (PE) even though they are tied together at the transfomer.

If by "safety transformer" you mean a transformer to provide isolation in environments where there is extreme risk of contact with electrical voltage, i.e. bathrooms or showers, then the legs are both not grounded.
But that is a special case.

As for grounding the DC 0V, well we normally do ground this.
To my experience, problems with noise is greater without 0V grounded, but I am not religious about it.
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Old June 19th, 2017, 06:48 AM   #45
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To my experience, problems with noise is greater without 0V grounded, but I am not religious about it.
I'm not religious about it either, but that has also been my experience.
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