Is the ground resistance meter indispensable when grounding your home?

Question

I'm Brazilian and I need to ground my house, especially because of some expensive equipment I don't want to use without proper protection.

By "grounding my house" I mean: provide an alternative path for current to flow to, in case of an electrical discharge or if the neutral provided by the power company breaks, for example. Here all power outlets have a third terminal for that purpose, and it's done by driving metal rods into the ground and connecting those terminals to these rods.

The thing is, I live in as small town and all the electricians around seem not to care for appropriately measuring grounding systems appropriately, by using a ground resistance meter. None of them even had one, and when I expressed my concerns about it (even though I'm admittedly a layman) they all dismissed it as something unimportant.

One of them told me ground resistance meters are expensive and that he'd test using a multimeter. If the voltage between ground and phase was the same as the one between phase and neutral, then it was ok.

Other would said he'd test it with a lamp. If the ground and phase could turn the lamp on, the grounding was good.

The other one (one whom I briefly worked with and the one I trust the most) was the only one who had a different attitude. He first asked me what kind of load I'd have on my system, what I planned to have connected. Then he told me I should have a separate electrical system for each of my showers (or other potent devices), each with individual grounding, and then all the least potent equipment could be connected to the main system, with one grounding for them all. But he also said he didn't have a ground resistance meter, but he'd tackle that by having 'three rods 3 to 5 meters away from each other providing the grounding' and that it should suffice.

So I tried looking into some other info in the internet and everything seems conflicting. The one guy who made the most sense for me was an electrician who told it is indispensable to have a proper meter, but that if you're an electrician and don't have one, there are some tests that are somewhat reliable you could make to make sure the grounding is good.

Basically he said one should disconnect all equipment from the outlets in the house, then do the multimeter test I mentioned before. After that, if the voltages were the same between phase/ground and phase/neutral, he'd connect some potent equipment (he used a drill machine) to phase/neutral and measure the current with an amperimeter. After that he's do the same test while connecting it to the phase/ground and see if the current matches. If those matched, the grounding was good.

I'm now left wondering, I absolutely need good grounding in my house, but the way it seems, I will either have to buy the ground meter - and they're expensive, especially considering ill use it only once - myself or go for one those fishy 'solutions' above.

What are your thoughts on it?

Answer 1

If the utility neutral breaks you're up against the ground (i.e earth, dirt, soil, water, rocks) being a relatively terrible conductor as compared to a metal wire, so the various tests involving testing the ground with a heavy load between hot and ground are highly dubious.

In the USA, the NEC (our local code, presumably not what applies in Brazil) requires either a ground test (time consuming, expensive meter) or two ground rods 8 feet or more apart, minimum. Many places have also implemented code requiring that in new construction the concrete foundation should include a "Ufer" ground or basically have the reinforcing steel tied into the grounding system, which is a far more effective connection to earth ground than ground rods in almost all soil conditions.

As a result, few electricians here bother to own the expensive meter, and most services have two (or more) ground rods 8 (or more) feet apart; Or one foundation ground (concrete encased electrode) and one ground rod.

Answer 2

Ezequiel, most electricians do not have earth/ ground resistance meters. For example in the U.S. if you drive 2 each 8 foot rods 6’ apart no measurement is required. A single rod must be below 25 ohms but drive a second rod and who cares having more than 1 is all code requires.

I have a multi point tester and can tell you that I have seen up to 6 each rods driven and the resistance was still in the 30 ohm range, I have driven multi segment rods in the same area 30’ deep, my plant has a single point tester and it’s measurements are very close to my multi point although some may point to a single point it is not legal in the 3 states I work most often.

Is 25 Ohms a good value because the National electric code uses it? No it is not it is two high to do the job of providing a path to trip a circuit breaker of 15 or 20 amps. Infact for a direct fault to ground would require the resistance to be below 6 ohms (earth resistance meters are not straight dc Ohm meters to clarify an edit).

To expand on the resistance needed to clear a fault to ground it is a ~6 Ohms for a 20a breaker. To calculate the value amps= voltage/ resistance so a resistance above 6 ohms will not be low enough to trip a 20 amp breaker at 120v or 8 ohms for a 15 amp breaker , you can be electrocuted by 600 ma(less than 1 amp)

The National Electric Code used by most of the U.S. and some other countries requires any receptacle in a bathroom be protected by a ground fault circuit interrupter (GFCI) these are set to trip 6ma maximum (milli amps) also protection is required at any receptacle within 6’ from a sink or basin or any outside receptacle or out building.

On your side of the pond RCD is common on your main these trip at 30ma fault current but turn everything off so if you have an RCD you do have some protection.

Grounding / Earthing all depends on soil conditions the best grounding electrode is the metal casing for a well, after a well a UFER ground (a grounding system using the metal reenforcement in a buildings foundation) developed in WW2 for a low impedance.

a RCD or GFCI are better protection from getting a severe shock or electrocution because even if you had less than 1 ohm resistance the circuit breaker will not trip until long after you are dead with a 1 or 2 amp breaker, in the us 15 is a small one other areas 10 is a common one but there voltages are higher.

If one can be electrocuted even with 6 ohms how can I be safe? GFCI circuits trip when there is an imbalance in the Grounded conductor and the Hot of more than 6ma (the earth is not used at all) this is a level that will give a shock but is below the normal threshold of life threatening.