nick

question for anyone which tester is better ,meaning a earth resistance test with a clamp on rod tester or a old time vibroground tester? we have noticed that in most job specs the electrical engineers still state that they perfer the old three point tester why ? comments and which one does your company still use ?take care best to ya

brian john

We use both plus a hybrid of the two, I still think the 3-point tester gives a more realistic reading. With a clamp-on there can be error readings due to a variety of issues. But in the right circumstance when properly applied it gives a reading.

nick

We see mostly in the job 1600 specs the engineers prefer the three point ,so i was going to see if anyone had info why ,the one we use is the old megger vibroground type ,and its a different reading from the clamp on ,its the old vibrator reed type with 3 resistance knobs,and galvanometer i know you have seen these Brian but iam going to fine out why ? one reason is we know the clamp gets dirt on it and the core must be clean , and i was told the clamp on is not good for total checks like whole system or grounding matt ? we check the rods before we connect up the copper each rod first if we get 5 ohms or less than we cadweld . in florida we go 40 foot on most jobs but i was told the three point tester can test the whole grid ? thanks Brian take care best to ya

brian john

If you are testing a single rod, not connected to the utility through the neutral ground bond with a clamp on the readings will not be accurate.

MDShunk

Ah-hah. So, it wouldn't be any good for a remote antenna ground or a lightning system electrode, I'm guessing.

I've never used a clamp on ground tester, and probably won't anytime soon. I'm just generally suspicious of them, and I'm not sure I know enough to know if I'm using it right or not.

__________________

nick

Well MD SHUNK, It actual works on lighting ground rods meaning down rods in earth and you could use it on a antenna ground system if you like to or anything meaning metal, pipe, probe, metal ground plates , grid screens, anything in earth , it measures the earth soil ground resistance to test set . it uses three rods one grd rod under test and two others test rods probes one is a [voltage rod ]an one is a [current rod] ,the fall of potential method gives the voltage drop in the earth to the ground rod under test kind of like old voltage divider circuit with resistors kind of simple . it has a dc battery inside but uses two transformers one auto type/ one standard centered tap changes dc to ac output with vibrator reed switches a galvanometer and some restats to adjust null of amp meter it tests with a ac signal at 98hz or the newer ones 545hz to elimimnate line 60 cycle noise to get a clean test ,you run test leads out one at 90 foot current probe one at 50 foot voltage probe then you test back to the ground rod your testing ,only one rod is tested at one time not the whole system . you must get 5 ohms or less on our jobs but the code says 25 or less. if your rod tested bad high resistance you add another to that one deeper until you get 5 ohms then you can cadweld your system ground electrode conductor to each rod which is three rods in a delta tri angle arrangement .lots of fun theres more to it then that but i dont want to go into magnetic field of the earth to the rods overlapping testing distance to rod stuff. youll go to sleep on me . but the old type are the best ones not the new clamp ons . take care best to ya

brian john

Nick, I believe Marc was asking about the clamp on tester.

nick

Well BRIAN and MD SHUNK after looking into it results == i wrote another post just for the new ground clamp meter . iam not a expert after using both ill say there is a difference , the jaws on the clamp part on one side has a current or field injection type coil of high feq signal 500hz or above it induces a current at high freq and at a set voltage with a regulator circuit into the rod under test, now the other side of the clamp jaws has a current sensing coil a pick up coil which a returning callapsing magnetic field signal via earth soil field induces the original sent expanding magnetic field signal back into the rod from soil /earth , this test circuit now calculates the resistance at a[ fixed voltage] key word fixed, i dont like this it should not be a set voltage to be accurate . how can one determine the correct ohms of the earth to rod i know it makes the field expand out into the earth soil or ground around the rod but to me its limited by just a small area i feel its not a accurate way to test ? as the old way we use current thur soil or earth thats the way to measure and it sees the real voltage drop. not a fixed one from a regulator circuit . lots of stuff that can effect the field and actually make it stronger or weaker to the earths soil different soil is to me a issue ? thats what i think and i maybe out of line ? comments best to ya

brian john

This device was designed in conjunction with the utility companies so they could verify the proper grounding of their distribution equipment without disconnecting the electrode under test. For this application it works fine, for testing complex electrode systems. IMO the 3-point tester the equipment of choice.

nick

Yes , Brian thats what i found in reading more on this you are correct and some say thats there main purpose.Thats why the Engineers like the old three point or four point tester on a new install to test one rod at a time . now i found out why thought someone would like to know the reason == the clamp method needs two rods to give a earth soil test one rod alone can not give this , Why? current must flow thur the earth to test rod so if your just clamped around one rod its not giving the earths resistance . its designed circuit works as i stated above by example . But i found that there is one important method one must do , when you clamp this tester on a rod you must be below the grounding electrode conductor tap connection on that rod or you will include other paths for current flow meaning you really want a test on that rod to the other rod or multiple rods. which does not give this if your above the conductor tap , you can test above if you just want the whole system which will not be a accurate rod ohm test on the earth part system resistance ,but what you said about that bonding jumper to neutral, Brian you were correct in that statement also .i guess you have been around a few years Brain, most electricians i work with dont care about how things work but i do its nice to talk with a person on such subjects and get good info ,take care Brian ,geez i bet you cant wait for my next question ?

mdcorreia

Repairing or replacing 5 ft of damaged k & t close to the light fixture does not seem to be easy if you want it to be code compliance! The reason is that the neutral is switched and the switch is in a remote unaccessible area. Any suggestions?

Pierre Belarge

Brian
This is well stated.
For simple grounding electrode testing the clamp on is about 8% off in total measurement.

For more complex systems, it really is not accurate at all.