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Old June 19th, 2017, 07:38 AM   #46
g.mccormick
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For those worried about fault bringing down supply and pic, why not use separate supple for pic/module power only. Separate that from the field io power supply so that the plc stays up.
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Old June 19th, 2017, 07:44 AM   #47
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I tend to do that, except I use a mains powered PLC that means I also gain a separate supply for 4-20mA signals. All of my jobs fit within the block type PLC.
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Old June 19th, 2017, 11:57 AM   #48
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We always float our 24V and 120V DC systems but the battery chargers we use (MDS power) have a voltage divider with the centre grounded. If there is 0V across the top resistor there is a positive ground fault, if there is 0V across the bottom resistor there is a negative ground fault. These conditions alarm on the HMI.

Recently the operators got fed up with the negative ground fault alarm coming up intermittently and actually traced the negative ground faults to two Bosch proportional valves, which incidentally were running twenty degrees hotter than the other ones.

While these are greenfield power plants with a copper ground grid under the building and substation and all rebar and metal in the building bonded to the ground grid, we have no problems with noise.

If we do encounter a device that internally grounds 0VDC we purchase a 24VDC-DC converter and feed it from its own isolated power supply to maintain the floating system. Parker proportional valves and brake pad wear detectors are the usual culprits.

The control panels have 24 DC, 24AC, 120DC, 120 AC, and 208 AC, but there are 600V motor disconnects with status contacts so we would have 600V and 24 in the same box in some places too. I imagine if 120AC or higher found its way on to the 24 system some devices would pop so the problem wouldn't go undetected and persist as a safety issue waiting to zap me.

Phoenix power supplies are meant to have the oomph to blow fuses.
We have also run in to problems in extremely large buildings where there is just too much voltage drop to trip breakers fed on #14.
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Old June 20th, 2017, 02:01 AM   #49
BobB
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Safety transformer is double wound - isolated. I use the Legrand ones - they are good trannies, well priced and nicely packaged. I never use 240VAC powered PLCs - always 24VDC. Always 240VAC/24VDC Omron switch mode power supplies. If doing a generator job _ ground the negative from the control batteries and then feed the PLC and I/O through a 24VDC/24VDC Meanwell switch mode - the output floats. It is isolated. 2 benefits here - if the 24VDC from the batteries/battery charger goes to a higher level the switch mode brings it down to 24VDC and the PLC is happy. If the control battery voltage drops to say 19VDC the switch mode steps it up and the PLC is happy. Has anyone put a 240VAC positive onto a 24VDC input supplied from a floating supply? One day I will do it but suspect that the input will remain OK as there is no ground/neutral/phase reference to blow the input up. I suspect there would be no smoke!
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Old June 20th, 2017, 03:16 AM   #50
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Quote:
We always float our 24V and 120V DC systems
Out of interest do you know why it was designed like that? The only thing I think I know about hydro schemes is that you mustn't loose control of the turbine.
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Old June 20th, 2017, 03:53 AM   #51
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Quote:
Originally Posted by BobB View Post
Safety transformer is double wound - isolated.
A transformer that is used for safety, i.e. for isolation against electrical shock must be double wound and must not be tied to ground on the secondary side.
But not all double wound transformers are used mainly for isolation. Transformers that are normally used to generate a "control voltage" in control cabinets are used for usually 2 purposes:
To bring the voltage from a selection of possible supply voltages (i.e. 380/400/415/440/480V) via multiple tabs on the primary to a standard control voltage (i.e. 230V),
and to bring the short-circuit current down from an unmanagable value (f.ex. 25kA) to a managable value (f.ex. 5kA).

edit: The above is a continuation of an argument by me, that electrical isolation, despite desirable, is normally not an absolute requirement in many (if not most) applications.
Situations where it is a requirement are in showers, bathrooms, swimming pools and the like.
Maybe you also need isolation when you need to be able to do regular wash-downs. But I dont know that for sure.
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Last edited by JesperMP; June 20th, 2017 at 04:01 AM.
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Old June 20th, 2017, 12:12 PM   #52
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Out of interest do you know why it was designed like that?

Thoroughness in commissioning: We know if there are any loose strands touching ground anywhere in the plant since they manifest as ground fault alarms. If we couldn't detect them then all kinds of crappy connections made by others or on equipment supplied by others would go undetected. If it is a couple strands touching ground on the positive side probably it would just result in crappy voltage to the instrument and maybe an investigation in to the 'noise' in the instrument reading. If it was on the negative side then if it was a 4-20 mA we would loose some of the current and same thing would be investigating a misbehaving instrument.

Process uptime: We can have any number of positive ground faults and the process keeps running giving the operators time to troubleshoot until we also get a negative ground fault. Like the above example with two units having faults in the proportional valve but continuing to operate.

I have also worked on plants with the 0VDC grounded and everything worked fine, but again it was a greenfield plant so everything is done correctly from the get go.
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Old June 20th, 2017, 03:51 PM   #53
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Mine are mainly swimming pools and generator systems Jesper. Swimming pools isolation is a must and preferred on generators so they do not fall over - data centres, hospitals and the like.
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Old September 6th, 2017, 10:55 AM   #54
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I thought that I would share this pic. This is data from before and after grounding of power supplies.
This is showing a mv/v pressure transducer that is connected to Gantner Eblox analog input module. The pressure transducers are fed from a linear 10VDC power supply. The analog input module is power by a 24VDC switch mode power supply. When both supplies were floating, they could "drift" away from each other since they had no reference. Grounding all of hte supplies in this case solved noise issue.
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