Modem lightning protection

The "standard" pulse for lightning simulation is the 8/20 us (rise/fall time) pulse, so with a 10 kA pulse, the rise time is about

1 kA/us. A conductor has an inductance of about 1 uH/m, at that dI/dt, the voltage difference is 1 kV/m along the conductor. For that reason the cable length from the spark gap ground side to the common grounding point should be well below 1 m.

I couldn't get it by searching on NIST's own site, but Google gave me a link of

which works for me.

Matt Roberds

:-) I like my modem, and don't want the hassle and downtime to replace it.

Note that this likely worst case is 30 kA to the house - 10kA on each hot service wire, and another 10 kA on the neutral. That would be 30 kA to the earthing system.

Nice calculation. I would state it as distance from the service panel, where arc-over may occur, to the common bonding point. And from a service panel protector to the common bonding point.

The currents from phone and cable entry protectors are likely lower than

10 kA, but the same applies.

There are service panel protectors that include wire-through protectors for cable and phone that minimize the distance to the common bonding point. Using them after the usual phone/cable entry protector is a way of dealing with entry protectors that are distant from the power service.

The problems of keeping a reasonable voltage between power and signal conductors at the service point is one reason you might want a plug-in protector on sensitive electronics with both power and signal connections.

Similar numbers from the IEEE guide:

-- page 23 The lead length from a service panel protector to the busses is critical. With a 10kA surge and 6" leads, the additional voltage drop is 810V (added to the clamping voltage of the protector}. There would be

2 leads involved - 810V for 12".

-- page 33 A 30 ft distant cable entry protector with a 3 kA surge to earth system produces a 10 kV voltage drop - about 1 kV per meter, but the risetime is 3 us (not sure why they used that risetime - I believe your 8/20 pulse is the standard).

And for larger wire sizes - "Doubling the wire diameter, e.g., going from a #14 to a #8 AWG size, only decreases the inductance by ~15%".

The voltage drop numbers illustrate why the "ground" at the building will be far above "absolute" earth potential - the wire from the common bonding point to the earthing electrodes will significantly long.

And then there is the resistance from the electrode to earth. Ground rods are slightly better than nothing. An acceptable rod for the NEC has a resistance-to-earth of 25 ohms or less. More likely 2 rods are installed and the resistance does not matter. And the voltage drop in the earth away from the rod is something like 70% in the first 3 ft.

"Ground rings" are another acceptable electrode in the NEC. The wire (#2 or larger) for the ring will have the same voltage drop problems, and should have the same kind of voltage drop away from the ring as a ground rod, but the ring is obviously longer. I believe this is a decent electrode.

For new construction in the US, a "concrete encased electrode" (usually called a Ufer ground) is often required. It is a decent electrode.

Thanks to George and mroberds for finding and correcting the NIST link error.

When using single phase spark gaps, there are two (US) or three (the rest of the world) ground wires to the grounding bar, each carrying 10 kA. If two or three phase spark gaps are used with only a single grounding terminal, you would indeed need to carry 30 kA.

One other question is the possible spark gap between neutral (N) and ground (PE), some wiring conventions need it, other conventions connect N and PE together to PEN at exactly this grounding bar.

They seem to be using higher rise times than I used, with 1 kA/us the

150 mm wire would have a voltage drop of 150 V.

The resistance is not the problem, the inductance is. Using multiple wires in parallel at some distance from each other, the inductance (and hence voltage drop) will fall to 1/2 or 1/3 for two resp. three wires in parallel.

It is no problem if you have full control of all installations e.g. building a big factory from scratch in the middle of the jungle, so you can design the electric system, grounding system, lightning rods, serial and ethernet connection etc. and have competent installers. The system works fine even during daily tropical thunderstorms.

However, in the real worlds, installations have been added several times during the last 50 to 200 years, during which wiring standards and economical systems (communist/capitalist) may have changed several times. There is very hard to get any reliable up to date drawings.

Even if the drawings calls for separate Neutral (N), Ground (PE), Technical Earth (TE) and signal grounds, a "bright" local installer notices that these are all grounds, so lets just daisy chain these together and connect to the Neutral and save a lot of copper wire :-).

Um, we were talking about a HOME.

If you're still getting stuff zapped, FIX IT. It's not rocket surgery. Particularly in your, red herring, big factory.

SO FIX IT! Good grief!