Joe, I agree with your assessment. To summarize L-G, N-G modes are not necessary at the service entrance, and earth ground has no real function in the operation of a TVSS device. Earth is just a reference point and is used for safety. Down stream from the entrance N-G and L-G modes become useful because a lot of sensitive electronic equipment has components like caps, mov’s, and RFI filters installed in these modes for FCC requirements. Again, the earth ground has no function. However, the EGC does become a factor b/c if we were to have a surge current flowing in the neutral circuit and/or EGC, this would cause an excessive voltage drop between N-G, therefore a SPD between the 2 would limit the potential difference to a safe operating limit felt across the equipment terminals
Here is a passage of what I have written in the Emerald Book:
“Low-voltage end-user type surge protective devices are often described as transient suppressors, but their operation is really a diversion of the surge current through a low impedance path preventing the rise of high voltages across the load terminals. For large surge currents, this diversion is best accomplished in two stages. The first diversion should be performed at the entrance to the building, typically by conventional surge arrestors rated for this duty (a class “C” device). Then, any residual voltage resulting from the action of the arrestor can be dealt with by a second protective device at the power panel, or at the terminals of a connected load (a class “B” or “A” device) . Note that class “A” devices are sometimes called point of use devices.”
SCOTT, I looked at your drawings and correct me if I am wrong. Your devices appear to be class “A” devices aka Point of Use devices. I agree the LC network would dampen any transient on the load side, and recognize the value of the N-G and L-G modes. These would great devices on the service side except you would not need any of the N-G and L-G modes. But the problem is they would have to be service rated equipment and very expensive to pass service current ratings of 100 + amps. In theory I know they are possible, but I have not run across any in my adventures. In fact your devices look a lot like the ones used in Triplett Surge Reference Equalizers for Point Of Use devices. A very fine product indeed.
[This message has been edited by dereckbc (edited 01-18-2006).]
