grounding/resistance tests on emt?

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steveng

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I am checking the grounding and bonding connections on some rooftop hvac units, they are 7 1/2 ton commercial units trane, when they were installed, emt was used as egc.

these are 480 3ph units /with gas heat/with non fused disconnect at unit.
min circuit amp 20.8, ocpd 30 brkr
feeeder #10

question :

most likely the conduit installation, installed 20yrs ago has loose fittings on coupling set screws, and locknuts,

the emt used as egc would be compromised, yes? (have high resistance)
unlikely to open the ocpd in the event of a fault,

so, can you resistance test emt? just like you would wire?

what ohm's reading would be acceptable for 50-60' of emt.

how do you resistance test emt?

gar, zog, brainjohn?

pulling in a egc would be costly and time consuming, on 9 units.

thanks
steve
 
1st. I assume these units are connected to some sort of steel supports, copper cooling pipes, HVAC ducts? If so the ground resistance of the readings of the EMT would not be valid. Any fault current would follow all paths.

2nd. What type of meter are you using? A DLRO/ductor/micro-ohm meter utilized to read across each coupling would be the best IMO.

3rd. I'll take readings in the morning with a ductor and let you know what a new connection would be on EMT.
 
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brian john said:
1st. I assume these units are connected to some sort of steel supports, copper cooling pipes, HVAC ducts? If so the ground resistance of the readings of the EMT would not be valid. Any fault current would follow all paths.

2nd. What type of meter are you using? A DLRO/ductor/micro-ohm meter utilized to read across each coupling would be the best IMO.

3rd. I'll take readings in the morning with a ductor and let you know what a new connection would be on EMT.
Click to expand...

brian, these roof units are mounted on sheetmetal curbs adapted to the roof
structure, and are bonded to the steel roof deck,

i have a fluke 1587, but must admit, its new, and im green on meggers,

i need help doing basic tests with this meter, do you know any megger training schcools?:D

thanks
steve
 
Steve,
There is information here that will show that properly installed EMT has a lower impedanace than a code sized copper EGC.
 
don_resqcapt19 said:
For a 30A OCPD?
Click to expand...

Sure. I'd rather use a #10 but I suspect that might be harder to pull in 3/4;
and a #12 is or should be adequate for grounding a 30A.


I'm beginning to wish I had a code book here now. ;)


edit:
Now I remember why I was ok to say 12. From OP: "min circuit amp 20.8, ocpd 30 brkr"
 
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Bryan,
Table 250.122 requires a #10 EGC for a 30 amp OCPD. The EGC size is based on the OCPD and not on the minimum circuit ampacity. For OCPDs 30 amps and less the code requires the EGC to be the same size as the circuit conductors.
 
don_resqcapt19 said:
Bryan,
Table 250.122 requires a #10 EGC for a 30 amp OCPD. The EGC size is based on the OCPD and not on the minimum circuit ampacity. For OCPDs 30 amps and less the code requires the EGC to be the same size as the circuit conductors.
Click to expand...

Thanks.
I'm familiar with the 1:1 on #10 and smaller and should have known better.
I was thinking it was limited to grounded/grounding and not the ground itself;
that there is (was?) some exception. Say la vee and Mea Culpa.
 
don_resqcapt19 said:
Steve,
There is information here that will show that properly installed EMT has a lower impedanace than a code sized copper EGC.
Click to expand...

thanks for the link don,

i work in maintenance, right now one of my main tasks is making sure these
units are properly grounded/bonded.

i have several bldgs to check also.

thanks again!
 
Sorry, late to the party.

Megger is not the right meter to use here, you would want to use a DLRO (mircohmeter). Per NETA spes any point to point reading >0.5 ohms should be investigated.

As far a schools go, you are in luck, Texas has the 2 best schools in the country (Now that I retired from training :) )

Shermco http://www.shermco.com/training.html
AVO http://www.avotraining.com/

I would choose Shermco over AVO because AVO tends to push sales of thier equipment (Megger), but both schools are good.
 
zog said:
Sorry, late to the party.

Megger is not the right meter to use here, you would want to use a DLRO (mircohmeter). Per NETA spes any point to point reading >0.5 ohms should be investigated.

As far a schools go, you are in luck, Texas has the 2 best schools in the country (Now that I retired from training :) )

Shermco http://www.shermco.com/training.html
AVO http://www.avotraining.com/

I would choose Shermco over AVO because AVO tends to push sales of thier equipment (Megger), but both schools are good.
Click to expand...

thanks Zog,

you comments and suggestions have been a big help, you da man!:D
 
steveng said:
i have a fluke 1587, but must admit, its new, and im green on meggers,
Click to expand...
Yeah, you'd want to use a DLRO and not a megger. Sort of totally opposite tools. A DLRO is an ohm meter that will check for the tiniest of resistances.
 
mdshunk said:
Yeah, you'd want to use a DLRO and not a megger. Sort of totally opposite tools. A DLRO is an ohm meter that will check for the tiniest of resistances.
Click to expand...


is a 1587 fluke a micro meter? DLRO?
what does DLRO stand for?
 
steveng said:
is a 1587 fluke a micro meter? DLRO?
what does DLRO stand for?
Click to expand...

No Steve, the 1587 as Marc pointed out is a megger and DMM combined with a low band pass feature for true readings on VFD outputs. The 1587 does not have the low-ohm's bond function such as the 1507 and the 1520 which it replaced.

EDIT: Should proofread!
 
76nemo said:
No Steve, the 1587 as Marc pointed out is a megger and DMM combined with a low band pass feature for true readings on VFD outputs. The 1587 does not have the low-ohm's bond function such as the 1507 and the 1520 which it replaced.

EDIT: Should proofread!
Click to expand...

thanks, 76nemo , ;)
 
mdshunk said:
No. A micrometer is typically used by a machinist to measure small dimensions to the 1 thousandths of an inch. The Fluke 1587 is essentially a megger built into a multi meter.
Click to expand...
Modern mics have added graduations that easily measure to .0001" or one ten thousandth of a inch.
 
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