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Thread: Is an equipment grounding conductor required on 24VDC devices with a metal case?

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    Is an equipment grounding conductor required on 24VDC devices with a metal case?

    Hello,
    I am from a commercial background (15 years), and have moved to industrial controls. Many questions I never had to previously consider, I am now faced with on a daily basis. One of them which has recently come up, I cannot come to a solid conclusion on yet. If you could please reference relevant code sections with your answer it would be greatly appreciated. Here is my question:
    In an industrial control panel we have a class one power source supplying 24VDC source. The 24V source is then used to power a three wire level probe. The level probe is enclosed in a metal enclosure that is supplied with the device. There is a grounding terminal in the device, but for some reason the contractor has only run 3 wire cable with shield. I asked why he didn't ground the case and he said it isn't required for 24VDC devices. I asked for a reference, and he told me when he got some fee time he would send it over. I am a bit confused, and have spent quite a bit of time looking for an answer. One place I found another forum where a person suggested (without a code reference) is that with DC sources at 24V you have the option to attach the negative DC terminal to ground, or leave it floating. They said if you attach it to ground then all your devices must have an equipment grounding conductor. This kind of lines up with what our contractor said, but I can't find a definite answer in the code yet, so I was hoping someone more experienced with this topic could point me in the right direction. Thanks in advace!

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    Quote Originally Posted by chrisplusian View Post
    Hello,
    I am from a commercial background (15 years), and have moved to industrial controls. Many questions I never had to previously consider, I am now faced with on a daily basis. One of them which has recently come up, I cannot come to a solid conclusion on yet. If you could please reference relevant code sections with your answer it would be greatly appreciated. Here is my question:
    In an industrial control panel we have a class one power source supplying 24VDC source. The 24V source is then used to power a three wire level probe. The level probe is enclosed in a metal enclosure that is supplied with the device. There is a grounding terminal in the device, but for some reason the contractor has only run 3 wire cable with shield. I asked why he didn't ground the case and he said it isn't required for 24VDC devices. I asked for a reference, and he told me when he got some fee time he would send it over. I am a bit confused, and have spent quite a bit of time looking for an answer. One place I found another forum where a person suggested (without a code reference) is that with DC sources at 24V you have the option to attach the negative DC terminal to ground, or leave it floating. They said if you attach it to ground then all your devices must have an equipment grounding conductor. This kind of lines up with what our contractor said, but I can't find a definite answer in the code yet, so I was hoping someone more experienced with this topic could point me in the right direction. Thanks in advace!
    Still not sure on this and hoping someone has some insight.

  3. #3
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    the shield could be run to a grounding point and so continue a equipment ground that way. But, i DC the negative wire is in fact ground usually, so the unit is grounded, unlike an AC system. The difference is in a three wire DC system where one is positive, one is negative volts and one is ground so still grounded by the ground...

    Remember that in DC we normally misname the ground wire as negative, but use black for it, red for positive...
    Student of electrical codes. Please Take others advice first.

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    Quote Originally Posted by Adamjamma View Post
    the shield could be run to a grounding point and so continue a equipment ground that way. But, i DC the negative wire is in fact ground usually, so the unit is grounded, unlike an AC system. The difference is in a three wire DC system where one is positive, one is negative volts and one is ground so still grounded by the ground...

    Remember that in DC we normally misname the ground wire as negative, but use black for it, red for positive...

    I disagree with that. A DC common, or a DC negative is not always at ground potential. There are in fact many places that use a floating DC system where I have measured up to 12VDC to ground on the DC common side. If you run a DC common to the metal housing of a sensor, and that common is not at ground potential, you will create a potential difference between the housing and anything grounded. I would also have to argue saying "In DC we normally misname the ground wire as negative". UL 508 requires that the secondary of a DC power source be grounded inside a panel, BECAUSE the DC negative typically is not at ground potential. My company only uses UL listed 508 panel shops. However, My question is in regards to an ECG for DC field devices. The shield of the cable should be isolated from ground on the device side for EMI purposes.

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    Have you read Part VIII of article 250, especially 250.162(B)?

    Roger
    Moderator

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    Quote Originally Posted by roger View Post
    Have you read Part VIII of article 250, especially 250.162(B)?

    Roger
    I have, and I had to consider it. In all honesty I have never seen a 2 wire dc system with a neutral, other than a DC power supply I own for trouble shooting Electronic systems like operational amplifiers for load cells (which require +18VDC and -18VDC with a zero reference.

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    Quote Originally Posted by roger View Post
    Have you read Part VIII of article 250, especially 250.162(B)?

    Roger
    I don't think that has anything to do with whether an EGC is required. It is about whether the system has to be grounded or not.
    Bob

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    Quote Originally Posted by roger View Post
    Have you read Part VIII of article 250, especially 250.162(B)?

    Roger
    Here is what I have found, and there is one part that is left somewhat confusing:

    250.112(I) Specifically says that Equipment supplied by class 1,2,and 3 signaling circuits shall be grounded where "system Grounding is Required by part II or part VIII of this article"

    Part VIII specifically 250.162 allows for 24VDC 2 wire circuits to not be grounded, but 250.167(A) requires that ground fault detection systems are required if the system is not grounded. So it seems to me Part VIII is giving the ultimatum: Ground or Ground Fault protection. Either one would suffice to open the circuit if a ground fault condition occurs. In the one situation is a ground fault detection system... and that's the easy one: If current goes out and does not return the circuit will open. If the option is chose to ground the system, then it should be easy... ground the secondary of the DC source, and "IF" the DC sensor is properly bonded then if the DC current carrying conductors short to the metal enclosure of the Sensor... POP goes the fuse. But what if they don't ground the sensor? The case is left floating and when the DC current carrying conductor shorts to the metal case of the sensor if the metal housing is not grounded, then no fuse pops and the housing becomes a live wire. So it seems simple: ECG should be required. However, the wording of 250.112(I) make this unclear. It simply says ECG is the housing of a sensor is only required to be grounded when the system IS REQUIRED to be grounded. Because 250.162 allows grounding or ground fault detection, technically the system is not required to be grounded, and thereby implies an ECG is not required. I think this may be a place where the intent is clear, but the wording in a section allows something that wasn't intended. Maybe I am wrong and this is why I am asking the question here. My point is that if you say system grounding is not required, and therefore an ECG is not required to be connected to the metal housing of a sensor, then you leave the chance that the housing could become a live conductor since it wouldn't pop a fuse when the current carrying conductor touches the metal housing. But Part VIII made sure to cover the situation where someone elects not to have the system grounded by requiring ground fault detection. The catch is in doing so they made the case where someone does ground the secondary of the DC source seem as though it does not then require an ECG on the metal housing of a device associated with that equipment.

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