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CajunWon

join:2005-12-30
Cary, NC

2 edits
reply to Zach1

Re: Surge Protection Residential Whole House

CATV, after the CATV STB. The old(ish) antenna is my lightning rod & only connected to ground.

The Coax is grounded to the rod, as is the copper water lines & electrical ground wires.


EricNepean

join:2009-11-30
Nepean, ON

In a lightning surge, most of the voltage drop is due to the wire inductance, not resistance, and the current pulse doesn't last very long.

A standard model for a lightning surge is a a current surge that rises from 0 to 10kA in 8us, then drops to 5kA 20 us after start (Ref 2 below); the whole event is over in under 50us. (According to Ref 3 below, this probably under represents a lightning surge somewhat, but at least it provides a reference for some kind of estimate)

A rough calculation shows the the current rise is 1.25kA/us, and for 1/8"dia wire the inductance is about 530nH/ft. The resulting voltage drop is 660V/ft. The inductance doesn't get much smaller unless you use really BIG conductors like water pipes.

So one key thing is to make the leads to your surge arrestors as SHORT as possible (don't worry about size), and in the case of multiple surge arrestors (CATV,AC, phone) to connect them to a common location.

My power entry & panel is at one end of the house, water pipes enter at the other corner; there is a 75 foot "ground" cable between the two, in accordance with Canadian electrical code. bah, humbug. I'd be much better off with ground rods installed near the power panel.

I considered grounding my NAS directly to the water pipes at house entry, and then I realized that in event of a lightning strike, the lightning surge would run from the power entry at the panel via the ground wire to the water pipe entrance. The house AC power lines could rise to 75 ft * 660V/ft = 50kV relative to that water pipe at that point. ZZZZAP Instead I grounded the NAS in the regular way, to the AC wiring ground, this way a similar current surge may travel down the hot, neutral and ground.

references:
1) Inductance calculator (1/8" wire, 400 inches) »www.consultrsr.com/resources/eis/induct5.htm
2) IEC 61000-4-5 section 6.1.1 »www.sanki-e.com/uploadimg/conten···6817.pdf
3) "How to Protect Your House and Its Contents from Lightning"
IEEE Guide for Surge Protection of Equipment Connected to AC Power and Communication Circuits, section 2.3.2