Current on earth?

[ericmark]

Ohms law 47Ω 20A = 940 volt. Clearly some error.

In the main we have an earth rod only good enough to ensure enough current to safely operate a RCD. Where earth are better then some method is used to limit current, one does get neutral earthing resistors, but although worked on a site with a huge resistor I am not really up to speed on when they are fitted or why.

However I have had impossible readings using an AVO auto electricians multimeter due to RF in the area, it was so crazy 25 amp for a little transceiver with a 5 amp fuse which did not rupture it was clearly the meter up the creak.

Since the readings don't add up, I would suspect some measurement errors.

 

[Diver Fred]

Where earth are better then some method is used to limit current, one does get neutral earthing resistors, but although worked on a site with a huge resistor I am not really up to speed on when they are fitted or why.

In a previous life I worked in buildings containing floors of 'Digital' equipment in multiple racks - The racks would be bonded at several (>10) points on each framework. The intention being that the highest voltage difference across the whole earth plane did not exceed 0.15v.

That equipment 'earth plane' was isolated from the mains earth by a 'hugh' 20ohm resistor. The battery backed power supply units although mounted inside the equipment earth plane were isolated from that earth plane by use of insulated washers and sleeves on the fixing bolts.

 

[SUNRAY]

Sounds to me like the DNO earth is missing

 

[JohnW2]

It may be that the Ground rod's impedance to AC voltage is a lot lower than it's resistance to a DC test voltage. ... This difference between impedance (AC) and resistance (DC) is the result of electro-chemical reactions between the metal of the rod and the chemicals in the moisture in the ground.

Conceivably, but I seriously doubt that the difference between resistance and impedance would be anywhere near great enough to be compatible with what has been reported.

Kind Regards, John

 

[JohnW2]

Sounds to me like the DNO earth is missing

What do you mean by that- are you referring to the infamous 'lost PEN'?

If that were the case, one would expect there to be an appreciable measurable PD between the "DNOs 'earth' " (i.e. the neutral at cutout) and the earth rod - but apparently there isn't.

Kind Regards, John

 

[SUNRAY]

What do you mean by that- are you referring to the infamous 'lost PEN'?

If that were the case, one would expect there to be an appreciable measurable PD between the "DNOs 'earth' " (i.e. the neutral at cutout) and the earth rod - but apparently there isn't.

Kind Regards, John

I have several things going through my mind. Indeed yes I do expect to see a PD if there is a current, however I do know that sometimes it's easy to not be making the test one expects.
Having been in this position myself where DNO had lost their earth due to metal theft (albeit I didn't attempt to measure current,just voltage) it took me a whileto mske sense of the situation and even after the fix there was a struggle to work out exactly why I saw 200V between earthspike and DNO circuit.
Measuring the resistance of the earth rod is not the same as the impedance and I have certainly managed to trip a 63A OCPD with a fault to earthspike, which is extremely unlikely to measure <4Ω (not RCD trip).

 

[jelliottelectrical]

Well sorry to disappoint, but think the issue has been solved. I arrived on site today to no power (fortunately most of the site is still running off generators) the DNO swiftly arrived as the fault was affecting multiple houses. The have replaced a pole termination and the cable going up the pole and the issue is no longer there. so despite their lack of help yesterday, it has now been fixed

 

[JohnW2]

I have several things going through my mind. Indeed yes I do expect to see a PD if there is a current, however I do know that sometimes it's easy to not be making the test one expects.

Indeed - that's why I said that the OP "needs to do some more measuring"!

It's not as if we would be looking for a trivial PD, perhaps not large enough to be easily/accurately measurable. To get any appreciable current (let alone 20A) through an earth rod with a resistance anything remotely as high as 47Ω would require an 'easily measurable' PD to exist. Indeed, even if the impedance (per bernard's comment) were an order of magnitude less than the reported resistance (i.e. 4.7Ω), that would still require a PD of 94V to get 20A.

Kind Regards, John

 

[JohnW2]

Well sorry to disappoint, but think the issue has been solved. I arrived on site today to no power (fortunately most of the site is still running off generators) the DNO swiftly arrived as the fault was affecting multiple houses. The have replaced a pole termination and the cable going up the pole and the issue is no longer there. so despite their lack of help yesterday, it has now been fixed

Glad it's fixed!

However, it still doesn't explain why you could not measure a potential difference between the two 'earths' whilst the fault was present!!

Kind Regards, John

 

[jelliottelectrical]

I’m fairly confident I was testing correctly, but there’s always the chance I missed something obvious, was hoping to go back today to verify.

 

[JohnW2]

I’m fairly confident I was testing correctly, but there’s always the chance I missed something obvious, was hoping to go back today to verify.

Yes, in some senses it's frustrating that it has been fixed, since now you will never know. I can but presume that you did something 'wrong' (although by no means 'obviously' wrong/silly) when you measured the voltage (or lack of voltage!) - but that will have to remain one of life's mysteries!

Kind Regards, John

 

[SUNRAY]

One of the classic errors is things like using the DC range instead of AC.

One of the meters I had use of defaulted to DC on the auto volts range but AC on the current clamp range whereas my Tenma defaults to DC on both.
Flipping back and forth between them it's easy to forget to make the selection each time.

 

[JohnW2]

One of the classic errors is things like using the DC range instead of AC. ... One of the meters I had use of defaulted to DC on the auto volts range but AC on the current clamp range whereas my Tenma defaults to DC on both.

Yep, that's a definite possibility but, as I said, and unfortunately, we will never know in this case.

More generally, a situation in which what has been measured is "zero volts", even if the meter is on the correct range that opens up a number of other possibilities

In fact, I would probably be rather suspicious if a measurement between any two (not 'joined together') 'earths' showed a result of exactly 0.00V, particularly with AC.

Kind Regards, John

 

[SUNRAY]

Yep, that's a definite possibility but, as I said, and unfortunately, we will never know in this case.

More generally, a situation in which what has been measured is "zero volts", even if the meter is on the correct range that opens up a number of other possibilities

In fact, I would probably be rather suspicious if a measurement between any two (not 'joined together') 'earths' showed a result of exactly 0.00V, particularly with AC.

Kind Regards, John

I'd be ****ing my pants if 2 disassociated earths had exactly 0V.

 

[JohnW2]

I'd be ****ing my pants if 2 disassociated earths had exactly 0V.

Exactly my point. In the real world, it doesn't really happen - so "0.00V" generally indicates that something's not right about the measuring equipment and/or how it is being used!

Kind Regards, John