How do I determine correct ground

[Martin0679]

Hi:

I am not an electrician and I have a very basic knowledge about electrical systems. I need some advice. Here is my situation.

I have an office in an old house in Bogota SA. The wiring is old and contains no grounding system. I had a local electrical contractor install an exterior electrical system in all of the offices. We added a ground to this new system by driving a long copper rod into the earth and then through the panel and to the new system. We use this new system exclusively for our work stations (computer, lighting and occasionally a 1500 watt heater.).

We have grown to around 15 work stations and now another contractor is telling me we need to add a second ground. I have asked why, but I get a non technical answer. Additionally it is very common for contractors to be misleading here in South America

Can anyone please give me some guidance?
Thanks
Martin

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[ November 26, 2005, 02:36 AM: Message edited by: charlie b ]

 

[john m. caloggero]

Re: How do I determine correct ground

Having presented NEC seminars in South America for the past 20 yrs. Grounding of electrical systems at the service is something that is neglected and ignored. It would usually take me two days of teaching to get them to understand the necessity of proper grounding, especially when it comes to computer systems.
Where a second electrical system is installed, it is absolutely necessary to make sure that all systems are bonded together, including communications and CATV systems. This will minimize voltage differential between the systems during lightning storms or high voltage cross overs.
They do not like you to use the premises metal water pipe system for grounding because of the stray currents causing damage to the pipes. I had a tough time convincing them of the benifits. Most of the stray currents are due to using general purpose enclosures outdoors.

 

[Martin0679]

Re: How do I determine correct ground

Hello John:

Thanks for the input and I think you can understand my frustration. I am still confused. Should I just attach the grounding system to the water system or do I need a second ground?

Thanks
Martin

 

[iwire]

Re: How do I determine correct ground

Originally posted by Martin0679:
I am still confused. Should I just attach the grounding system to the water system or do I need a second ground?

If you are under the NEC and you have a metal water piping system in your house it has to bonded (connected) to the electrical system grounding conductors. 250.104(A)

If that metal water line continues underground in direct contact with the earth for 3.0 m (10 ft) or more then it must also be considered a grounding electrode and must be connected to the electrical system grounding conductors. 250.50

For your purposes you can think of those two issues as one in the same.

Now when a structures only grounding electrode is a metal water pipe in direct contact with the earth for 3.0 m (10 ft) or more then it must also be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(7). 250.53(D)(2)

The easiest way to comply with that is by driving a ground rod 8' long into the earth.

However a single electrode consisting of a rod that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7). 250.56


Also this additional rod shall not be less than 1.8 m (6 ft) apart. 250.56


So in short you need two ground rods spaced no less that 6' (unless you can prove one provides less than 25 ohms of resistance) and you must also use the metal water line if it is direct contact with the earth for 10'.


All of those items are bonded together into the grounding electrode system and connected to the buildings electrical system at the location of the buildings service (main) disconnect switch. 250.24(A)(1)

By the way, that is my opinion, some will say you are all set with one rod.

For the cost of a rod I just automatically put in two.

[ November 26, 2005, 08:16 AM: Message edited by: iwire ]