[sdiy] power line transient protection

[KHeck73 at aol.com]

In a message dated 4/10/2001 10:49:15 AM Eastern Daylight Time, 
jlarryh at iquest.net writes:



> I have worked in the power industry for many years and can tell you that I
> do not believe there is any 100% effective electronic solution to lightning
> protection.  However, solid state phase to ground arrestors are the most
> commonly applied on the high voltage system.
> 
> I can tell you the most common cause I see for lightning damage in homes is
> poor grounding practices.  I have seen several examples like the one below
> where a strike some distance (never that far though) away  has caused
> damage.  While it is possible for the transient to travel along the power
> line and into the home, it is unlikely.  The arrestors on the power line, at
> the step down transformer, and the step down transformer it self all serve
> to mitigate these.
> 
> While I am no lightning expect, I can offer this simple explanation.  During
> a lightning strike the current must obviously dissipate through the ground.
> The ground, not being a perfect conductor offers resistance to this current
> flow.  Therefore, voltage drop accumulates across the ground.  The voltage
> at the strike point is raised above normal ground voltage and it migrates
> back to normal the further you get away from the strike.  The difference
> between 2 voltages at difference points on the ground is called step
> potential in the power industry.  It can be thousands of volts per foot
> during a lightning strike.  Livestock can be killed by current flow induced
> through their bodies because of the difference in potential between their
> front and rear feet.
> 
> Anyhow, back to houses.  Many times, I have found that houses have multiple
> earth grounds.  This is not a good practice unless all the grounds are
> solidly bonded together with wire of significant size.  There should be only
> one ground, and everything should be grounded to that in a star
> configuration.  In the USA, it was common practice of the phone company at
> one time to ground their equipment on an incoming metal water pipe if that
> was closer than the power ground.  Cable TV did whatever they pleased.
> People installing home antennas, often installed arrestors on the signal
> line and put a ground rod at the base of the antenna.  The problem is that
> all of these ground attachments have the potential to be thousands of volts
> different than each other during a reasonable close lightning strike.  The
> problem in the house them becomes obvious as we start connecting things
> together.
> 
> So, I offer 2 pieces of advice for electronic equipment protection:
> 
> 1. check your grounding.  Have ONLY one ground rod, or bond them all
> together with # 2 or larger wire.  Of course, check local electrical code.
> 
> 2. Install a disconnect switch for your electronic gear.  I have a special
> circuit run to me gear from my mains that goes through a disconnect switch.
> It is only in the ON position when I am using it.  So, during storms, it is
> OFF, ad therefore, no transient damage can occur.  Of course, this will not
> protect the person who uses his gear during a storm.  :)  Essentially, this
> is no more than unplugging stuff.  But, much handier and therefore more
> likely to be actually done.
> 
> Larry Hendry
> 



As you said, different ground points have different potentials near a strike, 
and so the attached circuits can provide alternate quick paths for 
dissipation of current in the ground. In addition, there are currents induced 
in home wiring during a lightning strike that can help blow out your 
equipment.

The voltage and current waveforms within a lightning bolt look like a sharp 
attack, then decaying oscillation because it's a big capacitor discharge. The 
spikey waveform burst also emits powerful electromagnetism (EMF) into space 
with fundamental frequencies around 500 Hz to 20 Khz, and lots of harmonics 
up to many MHz. It lasts only a few milliseconds. This will induce currents 
in nearby objects and wires (especially if they are long or looped). 
Lightning generated EMF induced in your house wiring can make huge voltage/ 
current spikes at the wall sockets or on the phone line. Induced currents 
will try to find a common ground to complete a loop. Part of this circuit may 
be your gear, your computer, or even your body if you happen to be holding a 
telephone.

Best thing I know to do in storms, as has been said, is to physically 
disconnect the from the power 'antenna' feeding your home. This also removes 
paths to ground, one of which, by the way, is the neutral side of your AC 
line.

Lightning is one of nature's way of playing a big resonant circuit... and it 
is electrical! Music to Tesla's ears.

-Karl.


  
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