Is it OK to drive a ground rod at a machine if switch gear & buss plug have no ground

[derelectric]

Is it OK to drive a ground rod at a machine if switch gear & buss plug have no ground

Were in an older plant built in the 50's. The Switch Gear, Buss rail & Buss plugs are 3 wire 3 phase 208 V with no ground. There using the conduit & frame of the buss rail, buss plug and switch gear as ground. Question is it legal/safe to drive a ground rod at each machine to ground the machine since? Will this create a ground fault condition or other problems?

 

[Smart $]

Were in an older plant built in the 50's. The Switch Gear, Buss rail & Buss plugs are 3 wire 3 phase 208 V with no ground. There using the conduit & frame of the buss rail, buss plug and switch gear as ground. Question is it legal/safe to drive a ground rod at each machine to ground the machine since? Will this create a ground fault condition or other problems?

You can drive a ground rod and connect it to the machinery... but that cannot be the only means of grounding. You'll have to extend the grounding of the bus/bus plugs to the machinery with a wire type EGC... whether or not you install the ground rod.

 

[kwired]

You can drive a ground rod and connect it to the machinery... but that cannot be the only means of grounding. You'll have to extend the grounding of the bus/bus plugs to the machinery with a wire type EGC... whether or not you install the ground rod.

Metal raceways in general are acceptable equipment grounding conductors, main limitations is usually for flexible conduits, there may be bonding issues at concentric/eccentric KO's but still doesn't limit the use of the raceway itself as the EGC, just means extra attention is necessary at the concentric/eccentric KO's.

 

[stevenje]

Metal raceways in general are acceptable equipment grounding conductors, main limitations is usually for flexible conduits, there may be bonding issues at concentric/eccentric KO's but still doesn't limit the use of the raceway itself as the EGC, just means extra attention is necessary at the concentric/eccentric KO's.

What would be the purpose of the ground rod at the machinery location?

 

[kwired]

What would be the purpose of the ground rod at the machinery location?

Got me, it certainly is not for equipment grounding conductor purposes, as it will is very likely to not have low enough resistance to carry enough current to cause the overcurrent protection to open.

NEC allows supplemental grounding electrodes at pretty much any equipment, but still requires an equipment grounding conductor with the supply conductors to that equipment. Electronics people think driving ground rods fixes certain problems, in reality it generally just masks a problem and maybe even creates more problems.

 

[Smart $]

What would be the purpose of the ground rod at the machinery location?

The OP suggests it to be in lieu of providing compliant equipment grounding, or perhaps to supplement equipment grounding. It is neither an effective or compliant alternative. It could supplement compliant equipment grounding, but not to any substantial degree. There are also other applications, whether sound or not, such as reducing step potential, reducing EMI/RFI, and quite likely several others.

 

[stevenje]

Got me, it certainly is not for equipment grounding conductor purposes, as it will is very likely to not have low enough resistance to carry enough current to cause the overcurrent protection to open.

NEC allows supplemental grounding electrodes at pretty much any equipment, but still requires an equipment grounding conductor with the supply conductors to that equipment. Electronics people think driving ground rods fixes certain problems, in reality it generally just masks a problem and maybe even creates more problems.

My thoughts exactly. Over the years I have been "required" to put some pretty elaborate "grounding grids" under some rather large raised floor projects. I never really understood exactly why it was required or how it was suppose to work.

 

[derelectric]

What would be the purpose of the ground rod at the machinery location?

What would be the purpose of the ground rod at the machinery location?

I think many people think driving a ground rod next to a machine and connecting to it protects them. Not understanding that they need an EGC back to the breaker to open the circuit. Its my understanding that the ground rod at the machine would serve the purpose to get rid of higher than normal voltages that get on the lines, such as surges and possible help in some lightning.

 

[kwired]

I think many people think driving a ground rod next to a machine and connecting to it protects them. Not understanding that they need an EGC back to the breaker to open the circuit. Its my understanding that the ground rod at the machine would serve the purpose to get rid of higher than normal voltages that get on the lines, such as surges and possible help in some lightning.

So does an EGC. Ground rod at machine is usually the idea of someone that doesn't know any better, usually thinking it will bleed off noise/interference associated with data/communication lines or other electronic devices.

The other idea of people that don't know any better is when they think it can replace an EGC.

 

[Sahib]

Providing supplemental ground at the equipment may offer a degree of safety for a while in cases of high resistance ground faults when the equipment insulation is only partly damaged so that the fault current through the EGC is not high enough to operate the concerned OCPD quickly.

 

[kwired]

Providing supplemental ground at the equipment may offer a degree of safety for a while in cases of high resistance ground faults when the equipment insulation is only partly damaged so that the fault current through the EGC is not high enough to operate the concerned OCPD quickly.

For under 600 volts? I don't think so, not if that supplemental ground is an isolated ground rod. Even if the ground rod is directly below the piece of equipment, you will have significant voltage drop in very short distance so it will not even perform any effective equipotential bonding in the earth below the equipment. Someone touches that piece of equipment they are getting hammered if they are not insulated from grounded surfaces.

If you do have a good EGC as you are suggesting it is going to do the bulk of protecting you, a ground rod will just give you another path, one of higher resistance than the EGC, and will do little or nothing to protect people from being subjected to extraneous voltages that happen in this condition.

 

[dereckbc]

Providing supplemental ground at the equipment may offer a degree of safety for a while in cases of high resistance ground faults when the equipment insulation is only partly damaged so that the fault current through the EGC is not high enough to operate the concerned OCPD quickly.

Not even a remote chance of that. At the point of contact of the fault even with a properly bonded equipment you have a simple voltage divider circuit and at the fault point and is roughly equal to half the supply voltage. Well unless you are fortunate enough to be somewhere standing on a Equipotential Ground Plane where the return path impedance is significantly lower than the supply path.

 

[Sahib]

The equipment ground and neutral ground resistances at the service entrance form a parallel path with EGC resistance. So if ,say 60V is the ground fault voltage drop along the EGC, the same 60V also appears across the sum of the two ground resistances, the voltage drop across the individual resistance being proportional to the individual resistance value. So if the equipment ground resistance is reduced, less touch voltage at the equipment and so safer.

 

[kwired]

The equipment ground and neutral ground resistances at the service entrance form a parallel path with EGC resistance. So if ,say 60V is the ground fault voltage drop along the EGC, the same 60V also appears across the sum of the two ground resistances, the voltage drop across the individual resistance being proportional to the individual resistance value. So if the equipment ground resistance is reduced, less touch voltage at the equipment and so safer.

Just how often are we guaranteed a failure resistance of the EGC to be equal to the resistance between this supplemental electrode and the source? True the lower the resistance of the electrode the lower any touch voltages are likely to be, but there is still pretty good chance those voltages are going to be high enough to nasty, seems to me it is well worth more effort be spent to help ensure a good equipment grounding conductor than to waste any time at all on a grounding electrode for the application from the perspective of preventing a shock during a fault condition.

 

[Fulthrotl]

So does an EGC. Ground rod at machine is usually the idea of someone that doesn't know any better, usually thinking it will bleed off noise/interference associated with data/communication lines or other electronic devices.

The other idea of people that don't know any better is when they think it can replace an EGC.

what's optimum is everything that is grounded being attached to the same point, and that is grounded
with appropriate means.

two widely separate ground rods not part of a ground ring, grid, or whatever is just a ground loop in training.
they are batty about using ground rods everywhere in electronics to "dissipate" static electricity, but sometimes
they create more problems then they solve.

i've seen two ground rods 10' or so apart with a significant difference of potential between them, and a
fair bit of current flow, in amps, measured with a clamp on meter.

 

[Smart $]

Just how often are we guaranteed a failure resistance of the EGC to be equal to the resistance between this supplemental electrode and the source? True the lower the resistance of the electrode the lower any touch voltages are likely to be, but there is still pretty good chance those voltages are going to be high enough to nasty, seems to me it is well worth more effort be spent to help ensure a good equipment grounding conductor than to waste any time at all on a grounding electrode for the application from the perspective of preventing a shock during a fault condition.

The general theory of local ground rods/rings is the same as equipotential bonding. During a line-to-ground fault it raises the voltage of nearby earth to the voltage potential on the grounded side of the fault.

To use the voltage divider analogy mentioned earlier, grounded metal can be at 60V (one-half of a 120V circuit) to electrical ground at the service, but if the nearby earth is raised to that same 60V potential, shock potential is reduced significantly.

 

[Sahib]

Just how often are we guaranteed a failure resistance of the EGC to be equal to the resistance between this supplemental electrode and the source?

In parallel circuit, branch resistances do not matter for branch voltages to be equal to one another.

ensure a good equipment grounding conductor than to waste any time at all on a grounding electrode for the application from the perspective of preventing a shock during a fault condition.

The problem is when there is not sufficient ground fault current through the EGC due to partial equipment insulation breakdown, the concerned OCPD may take a long time to clear the fault. In such a condition the equipment ground may keep the touch voltage at the equipment at a safe level for a while.

 

[Sahib]

The general theory of local ground rods/rings is the same as equipotential bonding. During a line-to-ground fault it raises the voltage of nearby earth to the voltage potential on the grounded side of the fault.

To use the voltage divider analogy mentioned earlier, grounded metal can be at 60V (one-half of a 120V circuit) to electrical ground at the service, but if the nearby earth is raised to that same 60V potential, shock potential is reduced significantly.

Unlike a metal plane, the ground resistance has a resistance area beyond which it has insignificant resistance. So voltage drop across one ground resistance has no effect on the other if it lies beyond the resistance area of the latter.

 

[jxofaltrds]

What would be the purpose of the ground rod at the machinery location?

It provides individuals with a warm fuzzy feeling.

Edit for sp

 

[kwired]

In parallel circuit, branch resistances do not matter for branch voltages to be equal to one another.

The problem is when there is not sufficient ground fault current through the EGC due to partial equipment insulation breakdown, the concerned OCPD may take a long time to clear the fault. In such a condition the equipment ground may keep the touch voltage at the equipment at a safe level for a while.

That is correct, and kind of has been what I have been trying to say all along, the equipment grounding conductor is more important than some isolated grounding electrode ever will be. But something tells me that you unintentionally agreed with me here.

Current on a EGC will raise voltage from that EGC to ground, I agree. If that EGC is in good condition it would need to be pretty significant current to have 60 volt drop (on a 120 volt circuit). If that kind of current is flowing to produce that kind of voltage drop, hopefully it is well over the trip setting of the overcurrent device and all we are waiting for is the time needed according to the trip curve to open the overcurrent device.

That said a faster operating overcurrent device is probably going to limit touch voltages better than a ground rod will, either way still requires an EGC in good condition.