clamp ohm meter

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ceb58

Senior Member
Location
Raeford, NC
Looking for answer. Electrical system 120/208 3ph on a tower site. Service comes into disconnect. At this point the only thing running on the system is the tower lights 3 amps. I was checking the ohm reading on the GEC and getting 5.68 ohm. However the meter would start beeping and flashing NOISE. The manual with the meter doesn't give possible causes for the indication. I turned of the disconnect and removed the GEC from the disconnect and checked again and got the same NOISE message. Clamped around the neutral at the whether head and got the same message with no load on system. Could the problem be on the poco side? what would/or could be the cause?
 

richxtlc

Senior Member
Location
Tampa Florida
First things first, you shouldn't be taking ohm readings with current on the line. Ohm reading need to be taken with the circuit de-energized. The beeping is a safety feature on the clamp-on telling you the circuit is energized. Did you take any voltage readings when this happened? Is the tower near any power lines as part of the problem may be induced voltages?
 

drbond24

Senior Member
I've never heard of that either. Just so I'm sure, you do mean that the word 'noise' is on the display and not that the meter is making a noise...?

Please tell us what type of meter you are using.
 

zog

Senior Member
Location
Charlotte, NC
Clamp on ground readings are not very reliable and are not as easy as most people think, proper training and use of this method is required. FOP readings are more reliable, clamp on method is not recognized by any testing standard.

1. How it works: It sets up a current in the ground circuit by magnetic induction. It then measures the current and voltage drop to determine the resistance. Ideally the ground being measured is a ground rod in a system of many other ground rods.

2. Pitfalls and Tips: Like any other instrument it has limitations. Please read the manual. There are a few real important items to notice like...

a. There must be at least 2 earth rods (ground rods) in the ground circuit for it to work properly. If you try to measure one ground rod, it will read OL (out of range). The meter's limit is 1200 OHMS, after that it will read OL. OL means that the ground circuit is open or the ground resistance is over 1200 ohms. A tactical ground should be far less than 1200 ohms, more like 25 ohms.

b. Also, the fewer ground rods in the circuit, the less accurate it is. This is because a clamp on ground tester uses other ground rods in the system (added in parallel) as a reference. If there are only two ground rods, the reading you are getting is the SUM of the two resistances. It is not possible to tell (with this instrument) if one ground rod has a greater resistance than the other when there are only two ground rods in the circuit.

c. There is a warning on the meter that reads R<1 OHM. If this is on, the reading is inaccurate and you are measuring a closed circuit conductive loop.

d. There is a warning on the meter that reads "NOISE". If this is on, there is over 50V or over 5A on the ground circuit. There is probably an electrical fault condition if this is true - power down the system and render it safe until the problem can be corrected.

e. Another pitfall is that the conductive contacts in the jaws get dirty. Wipe them frequently with a SOFT lint-free cloth. If the contacts are dirty, the meter will indicate "jaws open" or possibly read OL
 

ceb58

Senior Member
Location
Raeford, NC
First things first, you shouldn't be taking ohm readings with current on the line. Ohm reading need to be taken with the circuit de-energized. The beeping is a safety feature on the clamp-on telling you the circuit is energized. Did you take any voltage readings when this happened? Is the tower near any power lines as part of the problem may be induced voltages

As I said in the OP power was off and GEC was completely removed from disconnect. I did not check volt reading but I checked amp reading and got 361mA. The tower is about fifty feet from the transformers no lines run by the tower.

I've never heard of that either. Just so I'm sure, you do mean that the word 'noise' is on the display and not that the meter is making a noise...?

Please tell us what type of meter you are using.

Yes the word NOISE is on the display. The meter is a Megger brand 3710.
The only thing the instructions say about this indication is that there is ether 5amps or 50volts on the ground. My only conclusion is the poco ground-neutral tie at the transformers has a problem. The tower house is the only thing fed from these transformers
 

ceb58

Senior Member
Location
Raeford, NC
It could be excess current on the wire. Read the meter instructions.

Yes, Larry post #2 counts.
The instructions are vague and do not give troubleshooting guides. I'm just trying to come up with a answer to where is the current coming from with the GEC completely separated from the power source.
 

drbond24

Senior Member
Aww. Post #2 doesn't get any credit? :cool:

I suppose since there wasn't supposed to be ANY current on the wire and there was some, you could call that excess current. :)

I still like post #3 more, but you can have a cookie if it makes you feel better.

cookie.jpg
 
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