24VDC grounding

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lquadros

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We have lots of control panels in various departments. They have different grounding systems. Some are positive ground (Japanese), some are negative ground (European)and some floating (communication circuits like D-net) These panels have been installed over a period of 15 years. Is it possible to make a standard grounding system? What is the best bet?
Thank you all for the replies, in advance.
 
lquadros said:
We have lots of control panels in various departments. They have different grounding systems. Some are positive ground (Japanese), some are negative ground (European)and some floating (communication circuits like D-net) These panels have been installed over a period of 15 years. Is it possible to make a standard grounding system? What is the best bet?
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Why do you care one way or the other?

The convention of grounding the DC makes it convenient to do trouble shooting because you can use any accessible grounded metal structure as your common.

Negative ground seems to be easier for most people to understand. Maybe because cars are done that way so it is familiar.

Most communications circuits these days are ungrounded.

If you want a ground fault to trip the OCPD, then you use a grounded control circuit. If you prefer that a ground fault not trip the OCPD, then use ungrounded control circuits.

I am inclined to use negative ground DC control circuits because that is the most common thing I run across. The next most common thing is ungrounded. I cannot recall ever having seen a postive grounded control circuit, even in the Japanese control panels I have run across. I have seen some cases where the postive side was common, but they were ungrounded.
 
It shouldn't matter

It shouldn't matter

It really shouldn't matter whether it is positive or negative ground, as long as you measure the other side of the equipment you are working so you know. (Always do this anyway as it can get confusing sometimes!) I have worked in the telecommunications industry for about 25 years now and most everything we have is (+) ground. (with an occasional (-) supply sometimes for special things). The real issue to be sure about is that the site has a good "single point" ground for lightning protection done properly. Many telecom circuits that run into phone companies, require that you use "Positron" (or the like) isolation equipment as well, and that's where the "single point" really is important that you have done properly! I don't think trying to make everything the same ground (+ or -) would really be worth it financially (for sure) especially since some equipment is just not made with the other polarity. We have a few sites that have both and it really doesn't cause a problem. Good luck.
 
lquadros said:
Is it possible to make a standard grounding system? What is the best bet?
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The simple answer is no. I would bet you lunch that the equipment is designed to have a certain polarity grounded like one polarity is bonded to the chassis. If you reverse the polarity to be grounded, you would put a fault on the bus.

From the FWIW department which polarity is grounded is not a matter of country origination, simply a design choice. Typically in the communications industry all 48 VDC systems have the positive polarity grounded, 24 VDC systems have the negative polarity grounded, and 12 VDC is either way or floating. There is no code requirement for either polarity to be grounded or floating. They all have advantages.
 
Nfpa 79

Nfpa 79

8.3 "Control Circuits. Control circuits shall be permitted to be grounded or ungrounded. Where grounding is provided, that side of the circuit common to the coils shall be grounded at the control transformer if alternating current or at the power supply terminal if direct current."

It seems pretty clear that

#1. Grounding is NOT required.

#2. If grounded, N24 should be grounded in PNP systems.

and

#3. If Grounded, P24 should be grounded in NPN systems.

Note: At a recent jobsite, I moved the ground from the N terminal to the P terminal on a AB power supply. The supply faulted. There were no other grounds. This seems to indicate that the power supply was not designed for NPN systems with grounding.

Anyone see a problem with this?
 
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carlvant2004 said:
8.3 "Control Circuits. Control circuits shall be permitted to be grounded or ungrounded. Where grounding is provided, that side of the circuit common to the coils shall be grounded at the control transformer if alternating current or at the power supply terminal if direct current."

#2. If grounded, N24 should be grounded in PNP systems.

and

#3. If Grounded, P24 should be grounded in NPN systems.

Note: At a recent jobsite, I moved the ground from the N terminal to the P terminal on a AB power supply. The supply faulted. There were no other grounds. This seems to indicate that the power supply was not designed for NPN systems with grounding.

Anyone see a problem with this?
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I am inclined to agree except that it is not unusual to have a mixture of PNP and NPN logic.

Some power supplies have an internal reference to ground, so if you ground the wrong terminal, it will indeed fault.
 
Mixed Systems

Mixed Systems

I think the NFPA recommendation is clear.

If the designer has mixed NPN and PNP devices, and is using the same supply for both, he should not ground the supply.

I don't know why he would do such a thing.

If he does, this leads to a single point of failure and "Control Reliability" issues.

e.g.

If you ground the N side of a NPN system and a ground fault occurs on the N side of an actuator coil, the actuator will actuate.

If you ground the P side of a NPN system and a ground fault occurs on the N side of an actuator coil, the actuator will NOT actuate.
 
You don't always get a choice. it is not unusual to get some kind of device in that has things prewired and you have to live with it unless you are willing to redo something.

I see this pretty regularly. If I have a choice, it is all one way.
 
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