grounding 24vdc

[Michael Barabas]

Can someone help me on grounding 24vdc power. Some do and some don't, some say you should and some say shouldn't ground to an A/C ground.

[rdrast]

Here we generally tie all control commons to a single grounding bar. 24VDC, 120VAC Neutral, 48VDC, etc.

The only time we usually leave a supply floating is if it is driving 4-20mA transmitters (where due to different transmitter / receiver designs, common commons may cause problems), if it is driving a differential type analog signal source, or if the external device is not safely referenced to a low potential.

An example of the last are (much) older drives especially, where the drive common was often part of an armature circuit, and therefore at armature potential.

[rogerhollingsworth]

Michael
One of the main purposes for grounding a system is to provide a reference point that will help stabilize the voltage of the system.
According to N.E.C, in a two-wire dc or ac system, if the load across the two wires are balanced, there really isn't a need for a reference point especially when the voltages are fairly low in the 50 volt or less range, or if the control transformer is fed from a grounded voltage source.
None the less I am now a believer in grounding control circuits, they should be grounded to a common bus, DC together on a grounding strip, AC together on a separate grounding strip.
If you are using several transformers in a system of various analog type signals then the transformer commons should be tied together. This common reference is essential especially if the signals will be going to the same PLC analog module.

Roger

[okiebob]

Here it goes again folks, get your bricks and line up. The terminal the label SHOULD actually be - (minus) or COMMON. This is the point of Reference for the DC voltage. IT IS NOT ALWAYS GROUND. The potential on COMMON could be 24 VDC, between devices, and the devices operate at 12 VDC, so in the simple look the devices should be smoked, BUT as the COMMON has 24 VDC the device should not operate until 36 VDC is present e.g. 36 VDC - 24 VDC = 12 VDC. Another is -12VDC on the common for a 12 VDC circuit, it needs -24 VDC to operate. SIMPLY PUT BUT IT WORKS. The magnitude of the variation from COMMON is the VDC APPLIED. I Place my COMMONS at a COMMON point for all associated devices. Another point your laptop in one plug and the PLC that you are entering at another, both with diff L2's 100 yards apart both with 14 foot sticks and there is a good chance that your serial port can smoke. Remember Big bricks one at a time and little bricks all at once.

[rogerhollingsworth]

okiebob

This is a excerpt from a article discussing Grounding Techniques for Low Voltage Systems, Contained within Article 250 of the NEC.
It may give you some food for thought on your grounding practices.

This electrical reference point can be related to the zero reference point in our numbering system.
We subconsciously delete the zero in our numbering system when counting, both up, in a positive direction, such as +1, +2, +3,.....etc and down, in a negative direction, such as -1, -2, -3,..... The zero is placed in the sequence as a place holder between the positive numbers and the negative numbers for the same purpose as the ground is placed in the electrical system. This reference point will establish the place holder between positive voltage levels and negative voltage levels exactly the same as the zero was placed in our numbering system. If a 120 volt alternating current system is used, that means the voltage will go from zero to positive 120 volts on a half cycle and zero to negative 120 volts on the other half cycle.
The reference point between the two is the zero reference or ground. If the ground is not at zero or is fluctuating above and below the zero reference point then the positive and negative voltage levels or differences will vary. If the reference point is 10 volts above ground level or the normal zero point, then the positive voltage will only amount to 110 volts with the negative voltage level being 130 volts.

Even DC systems require a stabilized voltage difference to some reference point so that voltage fluctuation does not occur. By tying the voltage system to a certain reference point, circuits and equipment can be assured of the proper levels of working voltage.
Even if an ungrounded system is established or the system is not tied to the ground reference point, there is still an assumed ground that will be naturally established through the distributed properties of the system components. Distributed capacitance is the natural capacitance of an electrical system that is present because
there are two different voltages in the system separated by some sort of insulator such as air or conductor insulation. Likewise, there exists a distributed inductance between the conductor in the system and any grounded surface. These natural properties of distributed capacitance and distributed inductance will provide a reference
point and a resulting difference of potential between any two different points in the electrical system. The only way that the two points will ever become the same or at the same potential is if there is a bonding conductor of some sort connected between the two points.

If a totally ungrounded system is suddenly grounded, the result will
be a discharge of that distributed capacitance resulting in an arc that will last until the two points are at the same potential or bonded and grounded together.
An unstable or fluctuating reference point in an ungrounded system can be caused by imbalances in resistance in a dc (direct current) system or imbalances in the impedance in an ac system.
This fluctuating reference point will cause the voltage levels in the system to vary which can cause some problems in electrical equipment.

Roger

[Michael Barabas]

thanks to all that answered, Now have yall ever heard of krobar? (don't know if its spell correctly),Its where if a + vdc is grounded to a earth ground,PE ground, etc, like us we use alot of 24vdc sensors
for inputs to the PLC and if this happens your power supply is pulled down to a zero, trying to find where this point is can be a headache,
now if your dc power supply isn't try directly to a ground you lose your signal from your sensor that is shorted but still have your voltage, alot easier to troubleshoot. Now I believe in grounding ever thing but in a low voltage situation an isolated transformer makes a little sense to me, for the reason being that the plc is looking for the common to that power supply only, we use npn so if tied to a ground any ground can give you a false reading back to the input board.What yall think.
thanks
Michael

[Tom Jenkins]

The correct spelling is "crowbar" and it is named after the long pointy metal tool used to pry things apart.

[Vic]

A power supply "crowbar" circuit protects against high voltage, not grounds. Most 5 volt supplies have output over voltage protection built into them. If the output voltage of the supply rises above a set value, the over voltage protection circuit will apply a short (crowbar) across the output to protect the devices connected to the power supply from damage.

Power Supplies usually have current limiting built into them, which reduces the output voltage to a low value if the supply is required to output a current above the supply rating. This protects the supply if the output is shorted by either shorting between the output terminals or grounding both outputs. An ungrounded system will only save you from finding and repairing a ground problem if all grounds occur on the same side of the power supply output.

[DickDV]

In low voltage circuits where you are dealing with mostly signals as opposed to power distribution, I believe a common reference point for all power sources is essential for troubleshooting and voltage stability reasons. I prefer not to connect it to earth because that adds very little to the above two benefits and introduces far greater induced noise and RFI problems.

In short, as long as the voltages don't exceed 50V, use a common reference bus but let it float with respect to earth. Now, you can call this bus whatever you like: I've heard system ground, neutral, common system common, etc, etc. but it doesn't particularly matter to me as long as consistency prevails throughout the documentation.

I've also seen a few floating system reference bus's which are isolated for DC but have capacitance to ground for AC. Never understood the benefit of doing that!

[andydaoust]

What Michael Barabas could be referring to is a balanced DC Supply. From the negative side to the positive side there is 24vdc. To earth there is + and – 12vdc. The ground reference is made through two equal resistors and the center is taken to earth. Now if the +12vdc was to be grounded then the negative side would swing to negative 24vdc (crowbar?) there would be unequal current flow through the two resistors and generally a ground fault alarm would be generated. Using a balanced supply allows one free ground fault without tripping a breaker or fuse. In a 135VDC system it also keeps the voltages at +/-67.5VDC to earth without a ground fault and therefore safer.


+12vdc
|
|
16K
|
|___________GND
|
|
16K
|
|
-12vdc

[okiebob]

The key is the MAGNATUDE, take a circuit that operates at +24VDC with the common at +24VDC. Now apply +12VDC to this circuit and read across COMMON and + and you will read -12VDC. This is the Magnatude of the potential and the polarity based on the COMMON reference point. Very simplistic but the point is DON'T USE COMMON, GROUND, NUETRAL as equals they are not and NEVER should be used interchangebly (sp). Search threads and see some really cool things and VERY DANGEROUS things. ROGER I am still digesting your info, BTW I was not refering to any code just the safety and common mistakes made when using DC circuits.

[rogerhollingsworth]

OKIEBOB

I hear ya.

Just thought I would include my source so you wouldn't think I pulled it out of my *** (HAT)

ROGER

[okiebob]

Roger your write up is ANOTHER good ref to maintain in the forefront of my slushy grey matter. Things like this are what keep me reading and learning from this site (Thanks Phil), In my complete career I have NEVER zapped or caused another person to be zapped, so I try to pay attention and learn from others. Right now I am dealing with a whole MESS of XP and IS stuff from multiple vendors and VERY few willing to stand up and QUALIFY thier devices. So it is time to re-learn the circuit analysis (i.e. capacitance and inductance and the effects on a circuit due to inhierant properties). So keep kicking the info out as IT will be used.

[rogerhollingsworth]

Ok

[shooter]

ground a 24 v power supply with a resistor of 1 megohm to ground. this will prevent from floating all over the place.
When checking just remove the resistor and check with a voltmeter the plus and minus they both should go to zero.
at that time no leakage is present to ground.
same as meggering a motor.