Grounding Electrical Equipment

[ProjectDelta]

I’ve been working in an auto plant for quite some time and the design of the systems I work with (robot cells, paint booths, etc.) call for individual grounds from the equipment to a ground block that is bonded to building steel (which is called out as a Grounding Electrode on the prints). For example a 480V 100A PDP, which contains its own transformers to step down voltage to 120v but also utilizes 480v, has an equipment ground with the feed. On top of that, a #6 ground wire is run to a ground block which is bonded to building steel. Most of the equipment like PLCs, Junction Gate Boxes, Ethernet switches, also have their own #8 grounds run separately to said ground block. Like I said, these grounds are in addition to the equipment grounds that run with the conductors that power the equipment. I’ve asked around as to why the equipment is bonded/grounded in this way. I receive varying answers. Some say it Carrie’s fault current (which makes no sense if it’s bonded to a GE). Some say it’s to reduce electrical noise in the signal circuits. Has anyone worked on a design similar to this and definitely know it’s intent?

 

[ron]

That goes to the difference between grounding and bonding of equipment. On the surface, both of those conductors (EGC and GEC) seem to have similar function, but in reality, one provides a path for ground fault current and the other for a path to establish system grounding to earth which may stabilize voltage during normal operation along with other benefits.

See interesting definitions and code info below:

Grounding Conductor, Equipment (EGC).
A conductive path(s) that is part of an effective ground-fault current path and connects normally non–current-carrying metal parts of equipment together and to the system grounded conductor or to the ground
electrode conductor, or both.
Grounding Electrode Conductor.
A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.
250.4(A)(1) Electrical System Grounding.
Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize voltage to earth during normal operation.
Informational Note No. 1: An important consideration for limiting the imposed voltage is the routing of bonding and grounding electrode conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided.

 

[jim dungar]

Some say it’s to reduce electrical noise in the signal circuits. Has anyone worked on a design similar to this and definitely know it’s intent?

In many cases it is done this way because grandpa did it that way.

For decades automation equipment has liked to specify 'single point' grounding (actually bonding) for their remote panels. This gives a known dedicated grounding path rather than the 'daisy chain' multipoint found using paths like raceways and industrial piping. A common reason was to minimize potential differences on the ground path, which could interfere with the remote I/O communications.

 

[ProjectDelta]

In many cases it is done this way because grandpa did it that way.

For decades automation equipment has liked to specify 'single point' grounding (actually bonding) for their remote panels. This gives a known dedicated grounding path rather than the 'daisy chain' multipoint found using paths like raceways and industrial piping. A common reason was to minimize potential differences on the ground path, which could interfere with the remote I/O communications.

Quick follow up question. What would cause potential differences in equipment other than perhaps a close lightning strike?

 

[jim dungar]

Quick follow up question. What would cause potential differences in equipment other than perhaps a close lightning strike?

Almost every ground conductor has some amount of current flowing it, in large industrial facilities even tens of amps is not unheard of.

 

[Michele]

Hello to the Forum. I am hoping I can hop on the grounding/bonding conversation to request your feedback.
I work at a industrial facility construction site where the electrical contractor is pushing back adding a visible ground connection to the electrical equipment that contains terminations. The push back statement is that the box is connected to the Unistrut which is connected to the pipe rack steel, which is grounded to the grid. My question to the Forum: Is the box connection to the steel a sufficient ground or should there be a bonding connection from the electrical box to the steel ? Any reference to the NEC will be helpful. Thank you.

 

[Sea Nile]

250.4 (A)(3) - in a manner that establishes an effective ground fault current path.

ground fault current path is defined in 100.

so what is considered effective?
250.4 (A)(5) capable of carrying maximum ground current likely from a fault.

I would guess it wouldn't hurt to have a a bonding that is listed to carry that current. If there is nothing in the manufacturers instructions saying how it can be attached? I don't know.

I guess an approved method of bonding would ensure that there is no question and eliminate the debate on whether it's acceptable or not.

Don't believe anything I say though, I am not an electrician yet.

 

[kwired]

In many cases it is done this way because grandpa did it that way.

For decades automation equipment has liked to specify 'single point' grounding (actually bonding) for their remote panels. This gives a known dedicated grounding path rather than the 'daisy chain' multipoint found using paths like raceways and industrial piping. A common reason was to minimize potential differences on the ground path, which could interfere with the remote I/O communications.

I think that is the case with 99.99% of them.

Problem is when they want an earth ground but no other grounding conductors of any kind. That sets you up for some serious issues should an ungrounded conductor fault to the non current carrying frames and such of said equipment.

If you have both a supplemental ground and the regular EGC it doesn't hurt anything and gives those requiring said supplemental ground that warm fuzzy feeling they are looking for. reality is unless one path fails all you are doing is divide the path whenever there is any current flow on it.

You may have natural paths occurring via piping, metal structures, etc. anyway.

 

[Strathead]

I think that is the case with 99.99% of them.

Problem is when they want an earth ground but no other grounding conductors of any kind. That sets you up for some serious issues should an ungrounded conductor fault to the non current carrying frames and such of said equipment.

If you have both a supplemental ground and the regular EGC it doesn't hurt anything and gives those requiring said supplemental ground that warm fuzzy feeling they are looking for. reality is unless one path fails all you are doing is divide the path whenever there is any current flow on it.

You may have natural paths occurring via piping, metal structures, etc. anyway.

Kind of like driving a ground rod at each light pole base, especially ones that are 8 feet x 2 feet of conduit with rebar directly in the ground.

 

[petersonra]

I can remember back when the big engineering companies used to spec 3 seperate ground bars in each cabinet.

One for power that was connected to the cabinet and the incoming power EGC.

Another was for IS circuit shields, and another for shields for non-IS circuits. These had to be isolated from the panel and were fed back to the ground grid separately.

But as often as not, someone in the field would just jumper all three together.

 

[Michele]

Thank you all for your feedback.