Faraday Cage

[ayakimov]

At our college, a researcher in his electrophysiology lab wants an isolated dedicated ground, separate from the building ground system. He wants to "connect his equipment to this isolated ground system, thereby preventing ground loops and minimizing the electrical/magnetic fields that result from ground loops". Is his resoning correct, and is this allowed per the NEC? Thanks in advance. Andrew

 

[weh3]

If I understand the isolated ground correctly, it is no different from the regular ground. It is a separate electrode but still connected to the regular one at the service entrance.

William

 

[weh3]

Which is to say, there would be no benefit as he contemplates from using an isolated ground. If he is talking about a completely separate grounding electrode for his equipment, that is not allowed.

William

 

[itsmoked]

weh3 I believe his whole intension is to get away from the "house" ground.

To wit: Isn't this a case where the power in the cage is separately derived (via isolation transformer). Can a separate ground not be used in this case?

 

[weh3]

Yes. I think the whole idea is spitting in the ocean, though. There are enough stray fields from power lines and wall circuits to make those due to a ground loop negligible. A properly implemented grounding system would not have ground loops anyway.

The ground rod for the isolated ground is driven into the same planet as the service ground.

William

 

[wareagle]

An isolated ground is the "green wire that is connected to the main panel and grounding electrode. That green wire may connected to a plug and pass thru several panels and equipment with out being connected to any thing til it is connected to the ground at the main switch.
You may not install a 2nd grounding electrode with out bonding it to the main electrode at the main switch. This the ultimate sin with reguards to safety. All of the electrical wiring to power the 120 volt equipment will carry an equipment grounding conductor(green wire) that is connected to the main switch.
There is an isolation transformer made by ULTRAGUARD and by many others that will allow a minimum of noise and transients to pass from primary to secondary.
I suggest you investigate getting one of these devices to eliminate you concerns.

 

[weh3]

Are you saying the electrode for a separately derived system should be bonded to the service electrode?

William

 

[itsmoked]

Thanks wareagle.

weh3 is correct in the same planet aspect.

ayakimov: I would expect you have;

Faraday cage.

You have an isolation transformer.

The transformer essentially is at the wall interface.

Only the power leads exiting the transformer enters the cage.

The ground that came with the power feeding the transformer grounds(connects)to the transformer case.

This same ground connects to the cage.

If you want more *ground* add stick a rod/system in the nearby ground and also connect it to the cage.

Remember to use a shielded transformer or it will conduct all sorts of stuff, besides power, into the cage.

I used to listen to a transistor radio in my school's Faraday cage. ????

 

[wareagle]

weh3
If you are asking me that question the answer is yes. The typical installation is to bring primary conductors with and equipment grounding conductor(EGC) which is connected to the grounding electrode conductor at the main switch.
The EGC is terminated on the frame of the transformer and the neutral bonding jumper is connected to the frame of the transformer. Usually the neutral is grounded to the building steel of some other approved electrode. See 250.30 Grounding Separately Derived Alternating Current Systems.
250.b0 Grounding Electrode Systems states that all of the electrodes listed in 250.52 A1..A6 must be bonded if they exixt.

 

[peebee]

Check out Army Publication TM 5-690, "Grounding and Bonding in Command, Control, Communications, Computer, Intelligence, Surveillance, and Reconnaissance Facilities".

It is one of the most thorough and up to date reviews of grounding, including signal reference grounding, that I have ever seen. It generally appears to comply with NEC requirements -- which is NOT the case for many many other "respectable" sources of information on grounding.

Not only will it tell you about bonding your SRG/Faraday cage to ground (single point to power system ground), it will tell you how big the grid should be (grid spacing = 1/8 of wavelength of highest frequency of interest).

It's available for free at

http://www.army.mil/usapa/eng/