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Old June 16th, 2017, 08:52 AM   #16
AutoMax
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Well, I'll illustrate something you may be unaware of in the UK as I believe even your homes single phase power outlets have two powered legs. In America, our standard power outlets have one energized leg and a "neutral" that is grounded at the service entrance and the utility transformer. It is also not uncommon to find this power structure in our control cabinets. Not only are the loads on these circuits a very common source of noise, if the primary ground to a panel like this becomes compromised everything formerly grounded becomes energized. This is certainly a dangerous situation, but American maintenance electricians (well qualified individuals, anyway) are aware of this. In such a situation the DC- would also begin carrying hazardous loads.
I'm just providing the scenario because you asked...
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Old June 16th, 2017, 09:02 AM   #17
dmargineau
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Quote:
Originally Posted by AutoMax View Post
I believe even your homes single phase power outlets have two powered legs.
I dunno about that...European 230 VAC Single-Phase is identical to the US one in each and every respect except the voltage level and cycle frequency; 230 VAC vs. 120 VAC and 50Hz vs. 60 Hz.

There are three 'legs'; one 'hot', one 'neutral' and one 'safety ground'; different colors (BR, BLU, YEL/GRN) however, electrically (except 'hot' voltage amplitude and frequency) identical.

Last edited by dmargineau; June 16th, 2017 at 09:05 AM. Reason: Added Hz differences.
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Old June 16th, 2017, 09:37 AM   #18
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Interesting... I've never worked on European residential systems, but when working in Europe with systems with 120v power destined for the US they did not ground one leg of the transformer, yielding a 60v/60v system rather than 120v/0v. I made the assumption (perhaps foolishly) that they wired their houses like that.
Also, I've serviced British two-phase machinery (nightmare.) Perhaps that contributed to my dumb assumptions.
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Old June 16th, 2017, 09:39 AM   #19
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Also, fwiw, the Japanese seem to always ground their DC-, not their 120v AC, and they use NPN input cards...
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Old June 16th, 2017, 09:47 AM   #20
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Quote:
Originally Posted by AutoMax View Post
Interesting... I've never worked on European residential systems, but when working in Europe with systems with 120v power destined for the US they did not ground one leg of the transformer, yielding a 60v/60v system rather than 120v/0v.
That's also very interesting to say the least...

There might've been some end user specifications especially if said 120 VAC was being used in control circuits; I have encountered some nightmare Indramat/Rexroth 24 VAC control circuitry derived from similar 2-Phase 120 VAC (obviously ungrounded) transformers...Not pretty and very dangerous when expanded with US produced 120 VAC devices...
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Old June 16th, 2017, 09:52 AM   #21
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Quote:
Originally Posted by AutoMax View Post
Also, fwiw, the Japanese seem to always ground their DC-, not their 120v AC, and they use NPN input cards...
That's a mishap waiting to happen...Any grounded signal wire will turn its associated Input ON...
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Old June 16th, 2017, 09:54 AM   #22
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Just to be clear, when I said "British two phase equipment", I meant equipment with legs 90 degrees out of phase instead of 180. That is a unique experience.
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Old June 16th, 2017, 11:27 AM   #23
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just one thing to think about with the new switched power supplies one leg of the output may be connected to one of the supply legs, every manufacture has there own design and you really should check with them before grounding them or even paralleling the outputs
you could end up causing a more problems.
I have seen a lot of problems with misconnected power supplies.
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Old June 16th, 2017, 11:53 AM   #24
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Quote:
Originally Posted by AutoMax View Post
Also, fwiw, the Japanese seem to always ground their DC-, not their 120v AC, and they use NPN input cards...
No, the Japanese ground their DC+, and they use PNP input cards with NPN input devices.

Been this way for the two Japanese plants I've worked in.

Jerry
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Old June 16th, 2017, 12:03 PM   #25
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Quote:
Originally Posted by AutoMax View Post
Also, fwiw, the Japanese seem to always ground their DC-, not their 120v AC, and they use NPN input cards...
Quote:
Originally Posted by dmargineau View Post
That's a mishap waiting to happen...Any grounded signal wire will turn its associated Input ON...
I know this is getting a little off topic but, when the DC- is grounded than you want NPN input cards. A grounded signal wire will not turn in input on. An NPN input card requires a voltage input to turn it on.

Jerry
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Old June 16th, 2017, 12:32 PM   #26
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Staying with the off topic, don't get too tied in to the grounding causing a false signal. I was reading some machine fault reports and a surprisingly large number were caused when an unexpected +24V was received by the a system. You think of wires touching earths but theses were mostly crushed multicore cables that then put supplies where there shouldn't be one.

Back on topic-ish. I am not trying to make anyone look foolish, or to put them down. As has been pointed out to me elsewhere recently I sometimes get a bit too impassioned when discussing subjects. If it happened here I apologise.

Back on topic. It is my understanding that in a normal mixed voltage panel it is safer to earth one side of the control supply. If someone can argue that point then I am happy to be corrected. If you accept that it is safer then all other discussions start with 'I choose the less safe option because:' For AutoMax it is 'I choose the less safe option because I was having problems with electrical noise'. I accept that electrical noise can be a problem but the fix makes me nervous. Yesterday I had a customers customer complaining that an over pressure shutdown kept tripping. Their fitter's way to fix the problem was to wire out the sensor. When I logged in they were running a 250bar system at 610bar. They should have fixed the problem and not the symptom.
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Old June 16th, 2017, 12:41 PM   #27
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When I logged in they were running a 250bar system at 610bar.
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Old June 16th, 2017, 01:02 PM   #28
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Quote:
Originally Posted by JerryH View Post
I know this is getting a little off topic but, when the DC- is grounded than you want NPN input cards. A grounded signal wire will not turn in input on. An NPN input card requires a voltage input to turn it on.

Jerry
In my previous post I was referring to NPN field Input type devices connected to their corresponding Input modules (NPN type sensor-Sourcing Input modules); a NPN sensor sends the (-) to the input module when changing states; if a NPN sensor system has its 24COM (-) grounded then a shorted NPN sensor Signal wire will falsely trigger its associated Input point.
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Old June 16th, 2017, 01:18 PM   #29
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Quote:
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Wondering what are the pros/cons of tying Power +24VDC supply Common to chassis (earth) ground. Is it OK to do so? I have seen some control panels where this is done and others without. I have also seen Dual polarity supplies +/-15VDC commons tied to chassis ground as well. What determines if a power supply common should or shouldn't be tied to a chassis ground?

Thanks
The default position that I take is to ground the negative of the power supply. This method enhances stability for analog signals and makes it easier to troubleshoot DC circuits, since they aren't floating. I'm sure that there are valid reasons to avoid earthing some DC power supplies, but those would be the exception to the rule.
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Old June 17th, 2017, 01:16 AM   #30
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Some 24VDC devices ground the common without it being obvious. I seem to remember many AB PanelViews being this way. So if you do intentionally ground the DC common at the power supply, you just created a ground loop and thus a possible noise problem.
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