Grounding and bonding of motors

[philly]

We have a PM in place here at our plant where we semi-annually go around to each motor with a microhmeter and check the resistance between the motor case and the surrounding steel structure. The purpose of this test is to prove that the motors or equipment are adequately grounded. We refer to these tests and "ground continuity tests" Almost all of the time, the readings that we get are less than an ohm.

The problem that I have with this test, is that all we are really doing is measuring the resistance of the bonded steel between the motor case and whatever part of the stell building we are taking our reading. This proves that there is continuity between the motor case and building structure. This does not prove however there is a low resistance path back to the source for clearing faults. This is obviously the purpose of the EGC. Also what is to say that the building structure has a low resistance path back to the source, or any path for that matter. Aside from that the NEC prohibits any part of a building or structure to be used as an EGC.

We perform these checks in order to provide to MSHA should they ask during an inspection which sometimes they do.

My question is, by performing these checks the only thing that I can see we are doing, it proving that the motor is bonded to the surrounded building to eliminate any touch potentials if the motor was faulted.

We are not proving at all that the motor has an low resistance path back to the ground bus in the form of an EGC. This would be a hard test to prove I think. I have heard in some cases where plants pull an extra conductor with their feeds and use this extra conductor for performing continuity checks on the EGC.

What do you guys think of this PM we are performing and its validity?

 

[Mr. Wizard]

I worked for a shop that performed MSHA grounding tests, but we measured resistance from the ground at the motor (usually in the peckerhead) all the way to the EGC. That way we verified continuity from the frame to the earth, as you suggested. It takes a little bit longer, labor wise, but the results are more comprehensive.

 

[weressl]

We have a PM in place here at our plant where we semi-annually go around to each motor with a microhmeter and check the resistance between the motor case and the surrounding steel structure. The purpose of this test is to prove that the motors or equipment are adequately grounded. We refer to these tests and "ground continuity tests" Almost all of the time, the readings that we get are less than an ohm.

The problem that I have with this test, is that all we are really doing is measuring the resistance of the bonded steel between the motor case and whatever part of the stell building we are taking our reading. This proves that there is continuity between the motor case and building structure. This does not prove however there is a low resistance path back to the source for clearing faults. This is obviously the purpose of the EGC. Also what is to say that the building structure has a low resistance path back to the source, or any path for that matter. Aside from that the NEC prohibits any part of a building or structure to be used as an EGC.

We perform these checks in order to provide to MSHA should they ask during an inspection which sometimes they do.

My question is, by performing these checks the only thing that I can see we are doing, it proving that the motor is bonded to the surrounded building to eliminate any touch potentials if the motor was faulted.

We are not proving at all that the motor has an low resistance path back to the ground bus in the form of an EGC. This would be a hard test to prove I think. I have heard in some cases where plants pull an extra conductor with their feeds and use this extra conductor for performing continuity checks on the EGC.

What do you guys think of this PM we are performing and its validity?

What you are assuring with this test is not that the OCPD will operate when a ground fault develops, but that the resistance is low enough so in case of a ground fault -even if the EGC is discontinous to the power source bonding point- you can not develop touch voltages that would be hazardous to personell.

 

[philly]

What you are assuring with this test is not that the OCPD will operate when a ground fault develops, but that the resistance is low enough so in case of a ground fault -even if the EGC is discontinous to the power source bonding point- you can not develop touch voltages that would be hazardous to personell.

O.k. that makes sense that we are ensuring that there are no touch potentials by performing these checks.

Has anyone ever heard or done any checks to test the continuity of an EGC? I would assume that the only way to do this was either with an extra wire.

As far as bonding goes If there is a motor the sits on a conveyor which is attached to a metal building is that usually sufficient for bonding and ensuring that there are no touch potentials. I have someone here who is constantly wanting to run around on every conveyor and other equipment and install grounds between the motor base and metal structure. Not that I am opposed to an extra level of safety but I explain to him that these grounds or jumpers are not doing anything for clearing a fault, and therefore would only be used to eliminate touch potential. However if the metal connections are all metal this would act as a sufficient bond. If there is an area in question, could you measure the resistance with a microhmeter to determine if there needs to be an extra level of bonding?

 

[Doug S.]

Has anyone ever heard or done any checks to test the continuity of an EGC? I would assume that the only way to do this was either with an extra wire.

I would assume this would be done indirectly? The few times I've been involved with such shinangins we checked the EGC at the feed panel (or MCC, or whatever) to bldg. steel first. Once this is established, I'm led to believe you can assume the rest of the bldg is bonded? (Steel stick construction?)
If this is something you plan to do regularly you could make a point of installing points that are for such testing. The bldg I'm in now has such points, where there are a couple of semi-accessible buss bars tied the our grounding mess, err... uuh... grid. I'm sure they cost a mint, but it's handy.

Regardless of whether what I mentioned works for you or not there are other, simple, indirect ways to accomplish a EGC connection verification w/out dragging around a spool of wire... be imaginative. :grin:

My 2?
Doug S.

 

[philly]

If I have for example a 150hp 480V motor which sits on a motor base and that motor base is connected to the building steel and the surrounding grating is there a need to add any additional bonding conductors to the motor or motor base? I am having a hard time visualizing when enough bonding is in place and explaining this need to others.

In a similar example there is a long conveyor we have in a quarry area. Several people belive that there need to be ground rods installed up and down the length of the conveyor and have these ground rods tied to the conveyor. I explained that these ground rods do nothing towards clearing a fault, and that the conveyor components were alreday bonded through its interconnecting steel.

However at the same time I give this explanation I have seen conveory structures like this with these ground rods in several places. I'm not sure if this is correct or not, or what purpoose these rods serve if any?

 

[weressl]

O.k. that makes sense that we are ensuring that there are no touch potentials by performing these checks.

Has anyone ever heard or done any checks to test the continuity of an EGC? I would assume that the only way to do this was either with an extra wire.

As far as bonding goes If there is a motor the sits on a conveyor which is attached to a metal building is that usually sufficient for bonding and ensuring that there are no touch potentials. I have someone here who is constantly wanting to run around on every conveyor and other equipment and install grounds between the motor base and metal structure. Not that I am opposed to an extra level of safety but I explain to him that these grounds or jumpers are not doing anything for clearing a fault, and therefore would only be used to eliminate touch potential. However if the metal connections are all metal this would act as a sufficient bond. If there is an area in question, could you measure the resistance with a microhmeter to determine if there needs to be an extra level of bonding?

Why would you care if they do or do not take part in clearing the fault? The objective is to protect people from getting killed.

Why doubling up on grounding became a practice? I think you halfway answered that question by looking at the difficulty of testing the EGC's integrity. Grounding and bonding is a part of the installation that performs a passive function. What I mean that it's integrity is not necessary to operate the subject machincery. When you loose a phase connection, the results are either immediately or soon become evident. When you loose a grounding connection you will only know when somebody comes in touch with the energized part, eg. touch voltage will be present. So doubling up assures you that even though you may not clear the fault, but the touch voltage is limited to a harmless - hopefuly - level. (There are active, on-line devices available that do monitor the integrity of the grounding, but their cost is yet prohibitive to have widespread use.) http://www.bender.org/continuity_relays.aspx

 

[weressl]

If I have for example a 150hp 480V motor which sits on a motor base and that motor base is connected to the building steel and the surrounding grating is there a need to add any additional bonding conductors to the motor or motor base? I am having a hard time visualizing when enough bonding is in place and explaining this need to others.

In a similar example there is a long conveyor we have in a quarry area. Several people belive that there need to be ground rods installed up and down the length of the conveyor and have these ground rods tied to the conveyor. I explained that these ground rods do nothing towards clearing a fault, and that the conveyor components were alreday bonded through its interconnecting steel.

However at the same time I give this explanation I have seen conveory structures like this with these ground rods in several places. I'm not sure if this is correct or not, or what purpoose these rods serve if any?

In industries where corrosion is an ever-advancing enemy, the resistance of the structural connections could deteriorate. Painted surfaces, isolated mountings and a miriad of other potential installation and maintenance factors exist that could jeopardize the integrity of the grounding, so it became a practice to just directly bond the elelctrical equipment rather than make individual judgment if the installation consist of "good" bonding tot he surrounding metal objects or not and will it retain its integrity under any circumstances in the future. Who would make the judgment? Would you require the person's name to be recorded for legal purposes? Big can of worms........

 

[philly]

Why would you care if they do or do not take part in clearing the fault? The objective is to protect people from getting killed.

Why doubling up on grounding became a practice? I think you halfway answered that question by looking at the difficulty of testing the EGC's integrity. Grounding and bonding is a part of the installation that performs a passive function. What I mean that it's integrity is not necessary to operate the subject machincery. When you loose a phase connection, the results are either immediately or soon become evident. When you loose a grounding connection you will only know when somebody comes in touch with the energized part, eg. touch voltage will be present. So doubling up assures you that even though you may not clear the fault, but the touch voltage is limited to a harmless - hopefuly - level. (There are active, on-line devices available that do monitor the integrity of the grounding, but their cost is yet prohibitive to have widespread use.) http://www.bender.org/continuity_relays.aspx

I have seen these monitors used in mining applications. They have been referred to as "Ground Check Monitors" and they ensure that in a mining appliation where above ground mining cable is used that your EGC is intact. This is probably much more critical to ensure in a mining envirornment where conduit or other metal structures do not serve as a backup to the EGC ( even know by code other metal structures cannot serve as an EGC)

I guess the same device could be used throughout the plant on each different motor or service to ensure EGC, however as you pointed out, this would be rather expensive for several hundred motors.