earth rod readings.

[BTMAN]

I work for a large telecoms company in the UK, and we have a/c power that is supplied by local dno to our street cabinets. part of the stipulation is the earth rod reading should be below 150 ohms. these rods are tested and fitted by contractors, then their electrician does the electrical testing and certification, but recently we've had an earth rod that was tested in the normal way and has a reading below 150 ohms.but when tested by a megger to get a RA.reading it's way over the 150 ohms, nearly double the value. we've tried. condecrete, earth mats, extra earth rods and it doesn't lower the reading, i've never come across this before. there's other cabinets in the area , same type of soil but these were installed with no problems.
any idea's as to why there should be such a difference, and a solution if any
thanks

 

[Darkwood]

Welcome to the Forum BTMAN

 

[Marvo]

The Ra readings of an earth rod will change over time and also seasonally, over time they generally improve as the earth compacts around the rod but seasonally they could get worse as the water table changes etc. If the Ra is too high then I'd suggest coupling a new rod on top of the existing one and whacking it in a couple of meters deeper if possible.

 

[davesparks]

Just to clarify, you have an earth electrode which had an Ra below 150 when installed but now it has a reading above 150?

How long between the two measurements of Ra and what were the exact figures?

 

[PEG]

I would also ask,by what method was it tested,and was it in isolation,ie where was it tested?

 

[ackbarthestar]

I work for a large telecoms company in the UK, and we have a/c power that is supplied by local dno to our street cabinets. part of the stipulation is the earth rod reading should be below 150 ohms. these rods are tested and fitted by contractors, then their electrician does the electrical testing and certification, but recently we've had an earth rod that was tested in the normal way and has a reading below 150 ohms.but when tested by a megger to get a RA.reading it's way over the 150 ohms, nearly double the value. we've tried. condecrete, earth mats, extra earth rods and it doesn't lower the reading, i've never come across this before. there's other cabinets in the area , same type of soil but these were installed with no problems.
any idea's as to why there should be such a difference, and a solution if any
thanks

Some thoughts

1/ It maybe that the previous tester was having a bad day

2/ What type of concrete were you using?

3/There maybe a line of rock or deep gravel across the path of the Ra back to the Tx

 

[BTMAN]

The measurements were taken at the same time, one with a ground tester used by the contractors, the other with a megger , this reading was taken at the cut out , so it seems when an Ra test is done using 240 power the reading goes up by over 100 ohms.

 

[davesparks]

The measurements were taken at the same time, one with a ground tester used by the contractors, the other with a megger , this reading was taken at the cut out , so it seems when an Ra test is done using 240 power the reading goes up by over 100 ohms.

A megger what? Megger is a company who make all manner of test equipment. Megger is also the common name for an insulation resistance and continuity tester.

All earth rod tests will measure Ra. Ra is the symbol used to represent the resistance to earth of an earth electrode, it is not a type of test or test method.

Presumably what you are trying to say is that the rod was tested by a three wire fall of potential test and also the rough approximation method of using an earth fault loop impedance tester. It is normal to get different results with these tests as the three wife test will give you Ra whereas using the earth fault loop impedance will give you Zs.

Can you confirm with a diagram exactly how the installation is set up and where the earth rod connects in to the system? It may be that by using the efli test you aren't testing the earth rod at all,but are in fact testing the suppliers installation (with a fault in it)

 

[PEG]

A megger what? Megger is a company who make all manner of test equipment. Megger is also the common name for an insulation resistance and continuity tester.

All earth rod tests will measure Ra. Ra is the symbol used to represent the resistance to earth of an earth electrode, it is not a type of test or test method.

Presumably what you are trying to say is that the rod was tested by a three wire fall of potential test and also the rough approximation method of using an earth fault loop impedance tester. It is normal to get different results with these tests as the three wife test will give you Ra whereas using the earth fault loop impedance will give you Zs.

Can you confirm with a diagram exactly how the installation is set up and where the earth rod connects in to the system? It may be that by using the efli test you aren't testing the earth rod at all,but are in fact testing the suppliers installation (with a fault in it)

I would change that,fella,or the missus might avoid you,when you have a test machine handy...

Total agreement over the remainder :tounge_smile:

 

[davesparks]

I would change that,fella,or the missus might avoid you,when you have a test machine handy...

Total agreement over the remainder :tounge_smile:

Hahaha, oops.

Nah I'll leave it in for comic effect I think