Grounding - neutral wire or grounding pin

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Maciek Szymanski
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Grounding - neutral wire or grounding pin

Post by Maciek Szymanski »

I’m thinking about grounding the HV equipment at my garage. I’ve installed a copper bar with binding posts to be used common grounding point. I have two options to connect it: the neutral wire or a separate grounding pin.

The neutral wire is easier - I just need to connect the bar to the neutral point at the fuse box. The neutral wire is probably connected to the transformer neutral point and properly grounded. But this connection is shared between many garages, and I really don’t know what the circuitry is. There may exist some potential on this wire due to some bad contact and I have no control over it.

The separate grounding pin provides real connection to the Earth so should be the perfect for the safety. But first I have to make the it, and second - the grounding resistance may increase with time due to corrosion etc.

The proper grounding is the key safety measure for the HV and I would like to do it the proper way. So what are your opinions on this?

BTW - all machines at my garage are grounded by the N wire, but they use only 380V supply.
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Frank Sanns
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Re: Grounding - neutral wire or grounding pin

Post by Frank Sanns »

The neutral wire is really back to the center tap of the transformer. While it is grounded, it is a long path pack to the pole and the transformer. This path offers resistance and inductance that can cause local voltage build up during a voltage spike. Grounds are best when they are closest to the source and conducted with straps or bars. A water pipe is probably nearby and will offer a better choice than counting on a neutral.
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Richard Hull
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Re: Grounding - neutral wire or grounding pin

Post by Richard Hull »

A driven rod is always ideal. Hooking any number of rods to any number of neutral points in adjacent buildings junction or breaker/fuse boxes just lowers the impedance of the entire system, regardless. There are never too many driven ground rods. Any ground currents between them would be minimal. I have written about grounding here in the past. In my Tesla coiling days this was a must have....A good ground.

I once again post images of the outside grounding that I maintain for my lab. the round PVC system uses a couple of driven rods in a pit fill with about 1 foot of sand and regular loadings of rock salt which rain soaks into the ground.

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Re: Grounding - neutral wire or grounding pin

Post by Bob Reite »

Never use the neutral for an equipment ground. It's only for getting the current from the load back. Use the green or bare ground wire.
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Maciek Szymanski
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Re: Grounding - neutral wire or grounding pin

Post by Maciek Szymanski »

Thank you for responses. The grounding system is of TN-C type so there is no separate protective ground wire and no metal water pipes. The lawn adjacent to my garage is not my property - it's a public ground, so I can't do any big earthworks there, but driving a single grounding pin next to the wall of the garage complex isn't an issue.
Then I've again two options either the TT grounding with the grounding bar connected only to my grounding pin or TN-C-S with the grounding bar connected to both the grounding pin and the neutral wire. Personally I'm opting for the TN-C-S: the pin will reduce the grounding impedance but for other hand if there is any problem with the grounding pin or its connection there will be still the path to the good grounding of the local transformer.
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Dennis P Brown
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Re: Grounding - neutral wire or grounding pin

Post by Dennis P Brown »

I have a well and ground to the metal pipe near the pump; this connects me to the water table - of course, one needs a well. This method can still be done for a municipal water system. Just add a section of copper pipe to your water pipe (pressurized side) in the house. This shouldn't be a very difficult job for someone of your skill level.

One could drive a long metal spike into the ground to reach the water table if that isn't too far from the surface.
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Rich Feldman
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Re: Grounding - neutral wire or grounding pin

Post by Rich Feldman »

There are two safety reasons why the grounding pin of every receptacle needs an intentional metallic path to the transformer secondary winding,
and a locally driven earth ground rod or water pipe connection is insufficient. In NEC land, for new work; details depend on authority having jurisdiction.
Dennis's water well pipe, and municipal water mains, as primary earth connections are outside of my experience.

Consider an equipment fault that creates a milli-ohms short between "hot" conductor and the metal case.
1. Grounding path should keep the case voltage safely low. A 10 ohm or even 1 ohm salted earth connection can't do that. (note 1)
2. Grounding path should draw at least hundreds of amps, so the circuit breaker trips immediately. (note 1)

Note 1. When earth contact resistance matters, consider contacts at four places as well as the earth between them:
* Driven ground rod at the utility pole, to which transformer secondary is connected.
* Driven ground rod at service entrance to main building, to which all neutral (groundED) and groundING conductors are connected (in NEC land).
* Local ground rod or pipe at a shop or outbuilding, if present (not allowed to be primary grounding connection in NEC land, AFAIK).
* Contact by a shockable person, e.g. bare feet on concrete slab, or hand on a water faucet which may have metal path to earth but not metal path to transformer winding.

When my house was repiped a few years ago, the new copper stuff got a heavy wire connection to the driven ground rod at electricity service entrance.
That's to protect against pipe plant being energized, not to make the pipes a permitted path for electrical grounding elsewhere. Don't want to shock any plumbers or water users, if pipe ever needs repair or gets locally replaced with plastic.

Electrical codes have evolved to recognize increased use of plastic in water mains, in water connections from meters to buildings, and inside buildings.
Please consider my points above, when there's a milli-ohm fault from live wire to a grounded thing, and an ohm of earth resistance back to neutral side of transformer secondary.
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Dennis P Brown
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Re: Grounding - neutral wire or grounding pin

Post by Dennis P Brown »

Agree with Rich - with lethal systems - following code is always the first and absoultly best choice. Other methods may or may not be valid. His last point in his list is vital and would be of great concern if one does use a water pipe (of course, as I mentioned, the metal pipe used MUST be filled with water that is directly coupled to the main water system so it is a valid ground - for a home well this is certainly the case; not necessarily so for other systems. The main (power company's) x-former ground is always a first and best choice.
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