X, no problem, lets move on. Think about this for a while. Let’s go back to your assumption that a low impedance earth ground is a factor, before I do that, some good reference material is IEEE STD 1100-1992 (aka The Emerald Book POWER GROUNDING SENSITIVE ELECTRONIC EQUIPMENT). I am co-authoring the 2006 release and have some insight. Also NFPA -780 is excellent material.
Let’s assume we go to the trouble of installing a ring ground with chemical rods and get that elusive 5-ohm or less ground. Now we bond our service with a lot of overkill using a 10-fott long piece of 750 MCM ground electrode conductor. Most people would think this is a killer system no lightning could damage. Would you agree?
Now let’s apply some basic physics and electrical principals. Lightning and surges are high frequency events so inductance and impedance comes in to play. So let’s assume the event is lightning and it hits the weather head, the worst case scenario. Due to the very fast current rise time of lightning the impedance of that 10-foot long 750 KCM cable becomes a few kilo-ohms, for our example let’s say 2000 ohms. 2000 ohms is in series with our killer 5 ohm ground electrode system would be 2000 + 5 = 2005 ohms at the N-G bond point. Do you see that logic?
If so, I now pose the question to you. What difference does it make if the ground electrode system impedance is 5-ohms or say a two rod 50-ohm system? If it were a 50-ohm system the N-G bond point would be 2000 + 50 = 2050. Is there any significant difference between 2005 and 2050 ohms? Now for the reality bites section; The 5 or 50-ohm ground electrode measurement is measured at DC or power frequencies which have nothing to do with High Frequency (HF). At HF the impedance would be many magnitudes higher depending on the frequency of interest.
Now for a final thought or two. If lightning were to hit the weather head as we used in our example, the voltage at the N-G bond would be several thousand volts above local earth ground. Let’s just say 10,000 for argument sake. Although the reference ground point (the N-G bond) goes to 10,000 volts, so does our Line and neutral voltage. The job of the TVSS is to clamp the voltages between terminal points to a survivable level. Since N-G is bonded by a bolted connection that mode and L-G mode is covered at the service entrance only. All we have to do is clamp is the L-L and L-N modes that will be felt by the terminal equipment. If we clamp the L-L to say 400 volts, and L-N to 330 volts we should be OK. Yes the voltage would rise to 10,000 on all circuit conductors with respect to local earth ground but not between the EGC-N-L that the equipment is seeing.
Do not going away thinking a low impedance ground is not important, because it can be when we are talking about lightning protection systems. We will save that topic for another time.
Hope that helps.
Dereck
