Re: Separately derived source grounding

Originally posted by websparky:

** Bennie,**

How close / far apart do these grounds need to be before there is high impedance?

It depends on what frequency you are talking about. Look at a 750 MCM cable; you would think there should be no problem, right? Wrong.

A 10-foot piece of 750 MCM exhibits 3.8 micro-henries of inductance. So at 60 hz you have close to 0 ohms, at 1 Mhz you would have 24 ohms of inductance, at 10 Mhz you have 240 ohms, at 100 Mhz you have 2.4Khz and so on. If you use a smaller gauge like 14 AWG there is not much difference in the high frequency range since a 10-foot 14 AWG exhibits about 4.3 micro-henries of inductance.

There are two types of ground design. One is the "power/safety" range (DC to about 1 Khz), which is the NEC arena. The other is the "performance range" (above 1 Khz), which is in the equipment designer arena.

The "single point ground" system Bennie is referring to is a popular method used by high-tech companies, but they are not in reference to earth, rather in reference to the power source which could be a battery plant or transformer. Establishing a “ground window” at the power source makes it work. Now if you are in a large facility you will have several power sources, hence several "ground windows" and several "single point isolated ground planes" systems. Now you have a design problem. How do keep impedance low between systems. Simple, you use multi-point grounding to create several loops.

There are good loops, and there are bad loops. For example in the data centers I design I use a raised floor signal grid to reference all the transformers on the technical floor space. I construct it out of # 4 AWG on 2 to 4 foot centers. The impedance between any two points on the grid is no more than 30 ohms at 45 Mhz or lower even if measured at opposite corners of 200 feet or more. It is not important if the grid is actually connected to earth or not, but it is to comply with NEC requirements. What makes it work is all the loops on 2-foot centers.

Now here is something to think about. A "single point ground" system has extremely high impedance at high frequencies. Go back to my example of a 10-foot piece of 750 MCM. Now imagine a 100-foot piece of # 12 AWG you use for an EGC or even a short section of a GEC. What is the impedance at a high frequency? It is so high you can consider it an open circuit.

Bennie think about that for a while and we will discuss multi-point grounding verses single point grounding and how the two can be used to compliment each other.