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From: jtchew@csa2.lbl.gov (JOSEPH T CHEW)
Newsgroups: sci.electronics,sci.geo.meteorology,misc.consumers.house
Subject: Re: using the bathroom during lightning storm
Summary: A ground by any other name may have impedance not the same
Message-ID: <13568@dog.ee.lbl.gov>
Date: 24 May 91 23:17:22 GMT
References: <1991May23.223109.19908@cbfsb.att.com> <1524@cvbnetPrime.COM> <1991May24.210108.9982@solbourne.com>
Reply-To: jtchew@csa2.lbl.gov
Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA
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X-Local-Date: Fri, 24 May 91 15:33:53 PDT

>Correct me if I'm wrong here, but isn't the metal plumbing in your
>house grounded?  

In theory, yes.  The cold-water system is often regarded as one of
the best grounds in the house.  

However, gotta watch the broad statements. The drain/waste/vent  
plumbing, which includes the waste stack below the toilet and the vent 
pipe above it, is trending towards ABS, a rugged plastic, in many areas.  
Also, metal D/W/V pipes are often joined by rubber connectors.

The cold water supply might also include plastic (PVC) links, so the 
only grounding there, at some points, would be by virtue of the water's 
conductivity.  

Various systems are often mixed by do-it-yourselfers or even contractors
without caring about electrical matters.  Significantly, my cold and
hot water pipes and gas pipe were bonded together (at the water heater)
at the behest of the electrical inspector (who regarded the D/W/V pipes
as a don't-care), not of the plumbing inspector.

So here's a scenario: the best lightning target on a roof might well 
be a piece of metal D/W/V pipe going down to a toilet, but thereafter
grounded only by a spark + water path through the toilet and into the
cold-water system.  And/or other good grounds nearby.  A weenie roast,
so to speak, seems improbable but plausible.

Remember that at a few tens of kiloamperes, fractional-ohm differences
get really significant.

--Joe
"Just another personal opinion from the People's Republic of Berkeley"