Grounding Conductor Connection Monitor

Shaneyj

Senior Member
Location
Katy, Texas
Occupation
EET
Drive requires a disconnection of mains supply if ground breaks.
Any thoughts on how to achieve the monitoring of ground connection?
I've got a shunt trip to actuate opening circuit, just not sure how to go about detecting a broken ground connection.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Interesting. To monitor it, you'd have to intentionally run a small current through it, such as that of a normally-closed relay that is held open by the intact ground, that trips the shunt-trip if the ground is broken.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
No different than monitoring for ground faults at SE. Smaller scale and maybe TRMs metering.

I believe Franklin Pumps suggests that method for monitoring the EG of submersible pump motors used where corrosion is an issue.
The EG functions as it should, returning fault current until the connection is compromised.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
It monitors the connection while the equipment is in operation. Why would you care if the EG is not connected if you are not using it?
Sorry, still not following. During normal operation, current will flow on the phase/neutral conductors, nothing on the EGC. The CT can't tell whether the EGC is connected or not.

Yet the equipment wants an assured EGC, and a mechanism to kill power to it if the EGC is interrupted, even if there is no other fault. EVSEs, for example, often (always?) have a power-on EGC assurance test, which I assume consists of intentionally running a little current line to EGC to ensure it's there. [And that current should be well below the 4-6 ma trip setting of any upstream GFCI; the fact that for some older EVSEs it is not is I believe leading to difficulty using them with the new expanded requirements for GFCIs.]

Cheers, Wayne
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Drive requires a disconnection of mains supply if ground breaks.
Any thoughts on how to achieve the monitoring of ground connection?
I've got a shunt trip to actuate opening circuit, just not sure how to go about detecting a broken ground connection.

Is this a requirement on the continuity of the ground connection to the VFD unit itself, or to the motor that it drives (or both)?
For example, there was a thread some years ago where monitoring the ground connection to the motor was desired to indicate whether the cable to the motor was disconnected from the VFD, in which case they did not want a brake on a hoist to be released.
And so how and where the ground continuity should measured could depend on the specific requirements of the equipment and also how it's installed.

Is there a ground wire within metallic conduit to the drive and/or motor? That could affect how the issue should be addressed and what options are available.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Is this a requirement on the continuity of the ground connection to the VFD unit itself, or to the motor that it drives (or both)?
For example, there was a thread some years ago where monitoring the ground connection to the motor was desired to indicate whether the cable to the motor was disconnected from the VFD, in which case they did not want a brake on a hoist to be released.
And so how and where the ground continuity should measured could depend on the specific requirements of the equipment and also how it's installed.

Is there a ground wire within metallic conduit to the drive and/or motor? That could affect how the issue should be addressed and what options are available.
Yes, minor details.
Sorry, still not following. During normal operation, current will flow on the phase/neutral conductors, nothing on the EGC. The CT can't tell whether the EGC is connected or not.

Yet the equipment wants an assured EGC, and a mechanism to kill power to it if the EGC is interrupted, even if there is no other fault. EVSEs, for example, often (always?) have a power-on EGC assurance test, which I assume consists of intentionally running a little current line to EGC to ensure it's there. [And that current should be well below the 4-6 ma trip setting of any upstream GFCI; the fact that for some older EVSEs it is not is I believe leading to difficulty using them with the new expanded requirements for GFCIs.]

Cheers, Wayne
Correct.
What happens when there is a small current flow on the EG? Nothing, because the EG is connected and in the sensing window. The EG is acting as it should. (The drive will no doubt trip on GF it is not been disabled.)

Now remove the EG connection. A current imbalance will occur because the fault current must find a different path (assuming there is one) outside of the sensing window, thus proving the EG connection has been lost or compromised.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Now remove the EG connection. A current imbalance will occur because the fault current must find a different path (assuming there is one) outside of the sensing window, thus proving the EG connection has been lost or compromised.
Yes, proving it after there is also a fault. The goal is to kill power as soon as the EGC is compromised, not to wait until there is also a fault.

Cheers, Wayne
 

ron

Senior Member
If this is an existing installation, or if there is an existing condition that you can check, there may be some ground current on the EGC due to capacitive coupling or some other non-objectionable ground current that you could put a CT around the EGC, and just measure a small >0A current and if it falls to 0, then it is deemed open.
 

Shaneyj

Senior Member
Location
Katy, Texas
Occupation
EET
This requirement comes from the drive manual - with respect to the drive grounding conductor, not the motor egc.
Interesting idea @LarryFine.
But it is all moot - I found that I can bypass this by adding a secondary egc to the drive.
Thanks for the responses.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Probably can be done using a low volt relay that uses the egc as a return path. Power supply on line side of egc monitoring point, relay on load side, or drive egc input. Break in egc would drop relay out. Fault at the same time the egc is broken, would fry the relay, also opening the contact. Just a thought.
 
Top