Grounding Between Cabinets

I am insatalling a new system which consists of 2 cabinets. One of them is built by anOEM and the other we did in housse.

Cabinet A from the OEM and cabinet B from us has 4 conduits between them and are about 170 feet apart. We have 2 ethernet cables between the 2 cabinets and a switch in each to comm between a compact logix in our cabinet and a micrologix 1400 in the OEM cabinet.

We have 24 volt dc wiring between them to interlock the estop system. Should I connect a ground between the 2 cabinets or not and why? the OEM panel is fed from an Iline panel borad and our cabinet is fed from a MCC bucket and the MCC is fed from the same Iline panel board as the OEM panel.

I am just not sure what needs to happen with grounding between the 2 systems.

I would think as long as the two panels are ultimately connected to the same grounding electrode in the building, you wouldn't need to run a ground between the two, especially since it's low current 24vdc. In fact, I'd think you'd probably want to avoid running excessive ground wires everywhere.

I'm not really an expert though so if someone can confirm this, that'd be great.

I always install a ground rod at each cabinet... ground is ground so if you have a rod installed they will be tied via the ground (pun intended), I do not run a ground wire from A to B unless you are using power from one to the other and thats a different story... if yes make sure you label the **** out of the cabinet, I hate when people pull in power from another source and dont label it.

You should also have both grounded just for lighting and safety reasons so by doing so they are connected

geniusintraining said:

I always install a ground rod at each cabinet...

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If you do install 2 ground rods beware of Ecm differences between the two grounds that could potentially cause ghost signals to servo drives. I experienced this on a new machine install for a feedline for an automatic stamping press when I worked in a factory.

The maker of the feedline couldn't figure it out & finally called in Allen Bradley techs who said to run a daisy chain of ground wires from every component to every other & to tie the 2 grounds together.

Aabeck said:

If you do install 2 ground rods beware of Ecm differences between the two grounds that could potentially cause ghost signals to servo drives. I experienced this on a new machine install for a feedline for an automatic stamping press when I worked in a factory.

The maker of the feedline couldn't figure it out & finally called in Allen Bradley techs who said to run a daisy chain of ground wires from every component to every other & to tie the 2 grounds together.

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This would be the most rare occasion. I would suspect that if this were the case that the ground rods were ineffective being too short for the type of soil that they were in. There are times in dry sandy areas where a standard 8 footer is too short, that is why they make couplings to extend them.

Aabeck said:

If you do install 2 ground rods beware of Ecm differences between the two grounds that could potentially cause ghost signals to servo drives. I experienced this on a new machine install for a feedline for an automatic stamping press when I worked in a factory.

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Did you install the rods? the only time I have seen something like that before is when a helper got tired of swinging the hammer and cut the rod short

If you use a 8ft rod and you are anywhere east of the Rockies you or north of California you should be fine, I have used a 12ft in AZ and still had issues

Each cabinet is well grounded at each point but with me using interlocks for estops which are 24 vdc should I run a ground cabinet to cabinet?

I am thinking no but wanted to be sure.

I would not worry about the ground, sounds fine, your estop should just be going to a NC contact and back so thats not a big deal either

interlocking wire should be yellow or orange per NFPA 79.
i'm not sure about the e-stop. it still shouls be one of those colors since power is going from one panel to the other.

regards,
james

This is much more difficult to answer than any of us would like. NEC rules require that the panels share a common ground: to eliminate shock hazard of ground potential differences and to ensure a low impedance ground fault return path to the power source so the overcurrent protection devices will work.

However, that common ground between the panels should be occurring at the architectural level of the plant: the electricians should have wired the plant such that the ground is distributed via the power wiring. So if you are dealing with ground issues by running extra wires, something is likely wrong. And potentially hazardous. And likely against code.

NEC also specifically disallows local ground rods as the sole means of equipment grounding. However, many plants have problems where the ground has become a current carrying conductor through improper wiring and therefore does not serve as a safe or effective earth reference.

There is a good article on ecmweb titled 'Understanding the Differences Between Bonding, Grounding, and Earthing' that helps explain these issues.

I also would strongly caution against the concept that 'ground is ground' when you run multiple grounding rods. Keep in mind: the earth is not at the same potential at each location for lots of reasons, especially if lightning occurs. Ohms law applies here: if you have just a few millivolts difference between ground rods you need to also consider the impedance between them. The impedance may be in the micro-ohms equivalent. That means you can have significant currents between the 'earths': hence the ground 'loops'.

So bottom line here is I would NOT run a new ground between the panels but I would ensure there is no possibility of ground loops. Follow the current flow. Make sure that E-Stop system is isolated when it is handled in the 'foreign' system: use extra contacts, make sure the power sources have no common references. And label accordingly for safety.

nwboson obviously knows what he is talking about so I would not go against what he is saying... but 'good old boys' in the south have a lot of lighting storms and I have found that a rod at each cabinet is the best way, this by no means replaces the ground going back to the main switch or MCC, so when I say ground is ground, I am talking earth ground is earth ground, if you have a good ground then you do not need to add others running from one cabinet to another.

I have replaced too many components because of storms here in the south, when I started at one place in SC their issues were so bad that if they had a line of storms coming they would shut the plant down for 30 min and let it pass then, after we made some changes (grounding rods being one) it got a LOT better.

Tim Ganz said:

I am insatalling a new system which consists of 2 cabinets. One of them is built by anOEM and the other we did in housse.

Cabinet A from the OEM and cabinet B from us has 4 conduits between them and are about 170 feet apart. We have 2 ethernet cables between the 2 cabinets and a switch in each to comm between a compact logix in our cabinet and a micrologix 1400 in the OEM cabinet.

We have 24 volt dc wiring between them to interlock the estop system. Should I connect a ground between the 2 cabinets or not and why? the OEM panel is fed from an Iline panel borad and our cabinet is fed from a MCC bucket and the MCC is fed from the same Iline panel board as the OEM panel.

I am just not sure what needs to happen with grounding between the 2 systems.

Click to expand...

Yes. I'd run a #10 ground wire between the two panels in the same conduit along with my communication cables.

I think ground rods are for lightning and to establish a low impedance low voltage connection between the neutral and the ground of the building wiring ..bonding that neutral and ground will allow fuses to work in the event of a ground fault, and establish the voltage potential with respect to the ground ... Where you and i are gonna stand

Seems to me the electrical system should be bonded from one box to the other with the power wiring..... And the power wiring should be grounded in the appropriate places. If it is not, i think that needs to be addressed and i think installing ground rods is not the answer.

I am assuming in all of this that u are talking about a grounding conductor and not the return for some low voltage power supply (ies).
Those power supplies need their returns installed and not depend on completing the circuit through the grounding conductor. Ie. conduits etc

GEEZ... I just read nwbosons post ..... What he said is what i would have liked to have said... If i were intelligent, educated , and articulate

Ground should be done normal at every panel, a ground wire between both panels is not wise as it would give groundloops (and they can be very heavy.
24 Volt system use isolated contacts so no problem.
Comm cables should be grounded on ONE side only and i would for sure use isolated converters in a cabinet.

Both cabinets should be properly bonded to the building grounding system with an equipment grounding cunductor(the green wire) run with the feeders from the power source in this case the I line panel and the MCC bucket. This will ensure the cabinets are at the same potential. You don't need a ground running between the two cabinets. Also you can't just drive a ground rod wherever you want and call it a ground. All ground rods in the system have to be bonded together with no less than #6 copper unbroken per NEC. You can not rely on the ground itself as a ground path, in fact this is dangerous because it can cause a ground loop, hence the reason to bond all the ground rods together.