Earth pit instead of earting cables

[AusLee]

Hi,

I am working on a football stadium with 4 lighting towers. Each tower has a 120kVA power panel at the bottow with incoming directly from the MDB.

So along with the feeder cables, i have 4 isolated earthing conductors running from the MDB earthing bar to the earthing bar of each of the 4 power panels.

Making the sums, this is like 1Km of 150mm2 cables. I want to save that money to afford other things in the project.

For this i want to run only the normal feeder cables from the MDB to the remote power panels and make an earth pit near each of these panels and connect the earth bar to it.

The question is am i violating any code? Do i need ground fault protection in the remote panels or i can count on the magnetic trip of the breakers in the MDB?

One solution proposed to me is to use an armoured cable in the already installed ducts and use the sheath as earthing conductor, which works also because cost of armoured cable is < cost of 4C cable + 1/2C conductor. I find that second solution more attractive since armoured means also higher insulation = higher permissible current.

Regards.

Define Expensive: 17,000 USD for one 4P HT 3200 PACB.

 

[busbar]

 
It seems like it would depend on which regional "code" applies.

From a practical standpoint, in low-voltage distribution in ANSI regions, using “earth return” to clear upstream overcurrent devices is considered ineffective and almost always avoided. It is somewhat more apparent when the term ‘bonding’ is used in place of ‘grounding’ to describe interconnection between electrical enclosures.

A primary reason for local (per-tower) ground electrodes on taller metallic structures is effective lightning protection, so both bonding and local electrodes have their place for the described installation.

 

[alehman]

The proposed arrangement is very dangerous and violates the U.S. National Electrical Code. The reason is that you can't depend on the earth as a return path for fault current to have low enough resistance to trip the circuit breaker. You *must* run a properly sized grounding conductor from the source.

It is perfectly acceptable (and probably advisable) to supplement that with a ground rod or other electrode at each tower to reduce step potential.

 

[mc5w]

When doing amp checks on 120 volt 60 Hertz fluorescent lighting using a true RMS ammeter built for use with VFDs and so forth, I have found that when 1 circuit at a time is energized about 90% or the hot lead current flow back through the neutral an 10% comes back through the equipment grounds.

In an electric discharge lighting application the equipment ground is a current carrying conductor and you do need a redundant grounding conductor in case a conduit joint breaks or otherwise open circuit or high resistance. I have had to open up enough walls and reassemble an old conduit joint enough times that redundant grounding conductors are needed in all occupancies and not just in hospitals and explosionproof areas.

Mike Cole, mc5w@earthlink.net

 

[rbulsara]

I agree with alehman...

 

[mc5w]

Come to think of it, you MUST have at least 1 ground rod at each lighting pole to provide a path for LIGHTNING. If the lightning tries to flow back through the equipment ground there will be enough inductive and resistive voltage drop to toast ALL the conductors in the underground circuit.

The first ground rod at each pole needs to be as close to the pole as practicable. If the soil conditions will allow it the ground rod should be at the center bottom of the foundation and sticking at least 8 feet below the foundation. Then, run a rather hefty ground wire up through the foundation and bondit to all reinforcing bars and then run the ground wire up into the base of the pole and connect to the pole. The ground wire does need to be insulated from 2 inches below to 2 inches above where it emerges from the concrete so that the basic environment of the concrete will not corrode the wire. The rest of the wire should be bare to increase contact with the concrete to lower ground resistance. Also, all reinforcing bars need to be in contact with each other and the ground wire. Any gaps between rebars will cause the concrete to exploded there if lightning should stike.

If soil conditions are not good enough then the first ground rod should be about 3 feet ( 1 meter ) from the foundation or the outside of the foundation should be a copper plate electrode.

Mike Cole, mc5w@earthlink.net