IG - Clean power ??

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jkim780

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I have looked archives and I know this topic has been discussed so many times in the past, but electronics or IT, things of that nature are not really my strong suite, so you will have to forgive me for my ignorance.

when you say isolated ground, you all mentioning clean power, common mode electrical noise reduction, etc. But what do you mean by "clean power" (other than harmonic distortion - it seems this is different from harmonics)? what is this "noise" you talking about? how does this noise be generated? why this "noise" is problematic? how does it effect the electrical circuits?

I guess what I don't understand is that why, all of sudden(at least for me), this ground has this much importance other than fault current path? When I hear the "ground", all I can think of is 1) to limit the voltages cuased by the lighting and such, 2) to stabilize the voltage under the normal conditions, 3) to provide effective ground-fault current path.

Thank you for your answer in advance.
 
Re: IG - Clean power ??

jkim780, Noise manifests from rfi and emi that eminate from sources such as unfiltered or unshielded electrical equipment. A car passing by with a faulty ignition or an old refrigerator wired upstream on the same 120V line can be 'noise' sources to sensitive electronic equipment. (a Pop or static in the audio output of a stereo is a good example)

Clean ground is usually referring to isolating a power receptacle outlet to a dedicated 120Vac branch circuit wired and grounded directly back to the main disconnect panel. This is an attempt to eliminate other 'noisy' electrical equipment from being plugged into the same 'clean' line.

The line can also be twisted in a spiral to reduce cross-coupling interference and/or by isolating the cable from other parallel power source cable runs.

rbj, Seattle
 
Re: IG - Clean power ??

rbj, Thanks for your replay. Now I know what is the noise and where it cames from. I guess what I still don't understand is

1) What does the ground do to your sensitive equipment other than I mentioned above?

2) How does the IG give you a "clean power", if your voltage source is already corrupted?

3) Why do we care about the ground, not the L-N, if the ground gives you only fault current path?

I guess what I am still confusing is between the neutral and the ground. Is the ground same as the neutral in electronic equipment?

[ January 19, 2005, 09:49 AM: Message edited by: jkim780 ]
 
Re: IG - Clean power ??

Jkim780 I will try to help out.

Q1. What does the ground do to your sensitive equipment other than I mentioned above?

A1. Provides a signal reference point in some equipment that have I/O ports like PC?s, printers, A/V, etc. some of the interconnecting media uses a cable with a signal ground like RS-232 and coax

Q2. How does the IG give you a "clean power", if your voltage source is already corrupted?

A2. Good question it does not. The only purpose of IG is a possible means of preventing common mode noise from entering the IG. It cannot eliminate or reduce noise, not possible, so if you put garbage in you get garbage out. Common mode noise is developed from the multi-point ground system. Small currents will flow in multi-point ground systems that develop potential differences along the length of the circuit. The receiver circuits in data bit detectors and analog receivers pick up these small potential differences (common mode noise). This common mode noise can possible corrupt data bits, or be amplified in analog circuits.

The theory behind IGR is a single point ground will prevent common mode currents from flowing. This will only work if the IG is originated at the N-G bond point and the IG is kept completely isolated as it passes through enclosures and raceways. However the NEC does not require the IG to originate at the N-G bond point. It can originate anywhere down stream in a branch panel. The other problem is the IG becomes corrupted when a signal cable using a ground reference like RS-232 and coax from a different branch circuit connects interconnecting equipment. See A1.

Q3. Why do we care about the ground, not the L-N, if the ground gives you only fault current path?

A3. We do care about L-N, also L-G and N-G. There are two types of noise ?normal mode noise? (L-N, or L-L) and ?common mode noise? (L-G and N-G). All sensitive electronic equipment has to meet FCC regulation to prevent harmful RFI. To do this they install filters from L-G and N-G. These filters inject noise currents into the ground circuits. Sensitive electronic equipment also uses MOV?s installed between L-N, L-G, and N-G which contribute to more noise injection from leakage and normal operation.

IGR are practically useless for the reasons already given. In order to be effective the equipment has to be isolated from any incidental contact from ground like concrete and building steel. It cannot be interconnected by any grounded signal cable (like RS-232 or coax) supplied from a different branch circuit or different service from interconnected buildings.

The only way to clean up common mode noise is from a separately derived system like an isolation transformer or dual conversion UPS. A isolation transformer can offer up to 140 dB of common mode noise rejection ratio.

Now for the good news. IGR is not really needed in today?s electronic systems. The common mode noise problems have been engineered out by using optical or balanced transmission media like e-net. For those system like A/V that still use ground as a reference common signal isolation transformers can be used to eliminate noise.

OK, I am finished now.

[ January 19, 2005, 11:14 AM: Message edited by: dereckbc ]
 
Re: IG - Clean power ??

Thanks Dereck, I don't think I can even come close to your thorough explanation.


jkim780,
I guess what I am still confusing is between the neutral and the ground. Is the ground same as the neutral in electronic equipment?
Click to expand...
Yes, as is for any other ac powered equipment. I am referring to the neutral bus as the circuit return connection of 'grounded' conductors. I did not mention the equipment 'grounding' conductor that is associated with ground fault current and shielding paths.

rbj, Seattle
 
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