Using EMT in concrete slab for ground?

[Nick Toth]

Hi!
In and older (1974) concrete block house built on concrete slab, no green-wire ground was pulled through EMT that emerges from the slab under the attached patio up into a metal box in the outside of the block wall of the house. The EMT originates from a metal box in a different block wall inside the house, which also has no green-wire ground in it. That box is fed by EMT but no way to know exactly where that EMT originates other than that it must come through the concrete slab somehow from the main breaker box which is on the other end of the house.

Is it safe to use the EMT/metal box on the patio-side of the block house wall as a ground for outside lights and outlets on the patio?

If a standard outlet tester (with the lights on it) shows correct wiring when plugged into an outlet using the metal box as ground, is that enough to be sure that the ground is sufficiently safe for the outside outlet and associated lighting?

Thanks for your help! Nick

 

[petersonra]

Hi!
In and older (1974) concrete block house built on concrete slab, no green-wire ground was pulled through EMT that emerges from the slab under the attached patio up into a metal box in the outside of the block wall of the house. The EMT originates from a metal box in a different block wall inside the house, which also has no green-wire ground in it. That box is fed by EMT but no way to know exactly where that EMT originates other than that it must come through the concrete slab somehow from the main breaker box which is on the other end of the house.

Is it safe to use the EMT/metal box on the patio-side of the block house wall as a ground for outside lights and outlets on the patio?

If a standard outlet tester (with the lights on it) shows correct wiring when plugged into an outlet using the metal box as ground, is that enough to be sure that the ground is sufficiently safe for the outside outlet and associated lighting?

Thanks for your help! Nick

I don't think a standard outlet tester is adequate to test continuity of EGC from box to box.

If there is continuity, the EMT is generally an acceptable EGC.

having said that, there is no telling how something that old was actually installed.

 

[Nick Toth]

Thanks for your reply Bob!
Is there any accepted way to test for continuity of ground through the EMT?

 

[petersonra]

Thanks for your reply Bob!
Is there any accepted way to test for continuity of ground through the EMT?

not that I know of.

you could run a wire from the main panel connected to the ground bar there and see if you have continuity to the metal box using an Ohmmeter.

I don't think I would trust it if you got a "good" reading, but if it showed open that will tell you it is not good.

it is not real clear just how you plan to run your new circuits out there. You would need to run whatever ground you need with the new circuits.

 

[Smart $]

I don't think a standard outlet tester is adequate to test continuity of EGC from box to box.

It'll probably verify continuity (provided the receptacle was wired correctly, that is ), but it will not verify integrity.

I doubt there is any safe way to test integrity...

 

[GoldDigger]

It'll probably verify continuity (provided the receptacle was wired correctly, that is ), but it will not verify integrity.

I doubt there is any safe way to test integrity...

A battery powered high-current-pulse low range ohmmeter will give a much better idea about EGC continuity and should be safe to use, but will still tell you nothing about mechanical integrity of the raceway at couplings, etc.
At the least it will give you a better indication of whether the EGC, in its exact present condition, could do its job.

 

[Smart $]

A battery powered high-current-pulse low range ohmmeter will give a much better idea about EGC continuity and should be safe to use, but will still tell you nothing about mechanical integrity of the raceway at couplings, etc.
At the least it will give you a better indication of whether the EGC, in its exact present condition, could do its job.

Perhaps... but what if there is a direct short between the conduit and the grounded conductor? You'd have to monitor the grounded conductor for current at the panel during the test *or isolate it and check it for continuity (or megger) to ground separately*. Also, several high-current pulses will be necessary to verify the previous pulse did not actually cause a short.

 

[GoldDigger]

Perhaps... but what if there is a direct short between the conduit and the grounded conductor? You'd have to monitor the grounded conductor for current at the panel during the test *or isolate it and check it for continuity (or megger) to ground separately*. Also, several high-current pulses will be necessary to verify the previous pulse did not actually cause a short.

True. If you cannot see it and don't even know what was installed, it is hard to verify it completely. There could be a low resistance EGC path which results from a bootleg connection to some other grounded/bonded metal along the way which would be impossible to detect without the test you describe of matching currents at both ends.
But even that could fail even with a good raceway EGC since, AFAIK, there is no requirement to keep two raceways from touching each other.