Presumed RF Interference

Wed Mar 8 20:29:16 UTC 2006

Ian Mason wrote:
> 
> 
> On 6 Mar 2006, at 15:06, ww at styx.org wrote:
> 
>>
>> On Mon, Mar 06, 2006 at 09:49:39AM -0500, Steven M. Bellovin wrote:
>>
>>>
>>> On Mon, 06 Mar 2006 21:17:17 +1100
>>> Matthew Sullivan <matthew at sorbs.net> wrote:
>>>
>>>> (In the
>>>> UK where I served my apprenticeship, we were required to provide  earth
>>>> bonding to the copper plumbing system, additional bonding at every
>>>> exposed fitting - this caused a few issues when plumbers first  
>>>> starting
>>>> using PVC pipes)...
>>>
>>>
>>> The US National Electrical Code (which has no national force of law;
>>> it's a model code voluntarily adopted by many jurisdictions) now bars
>>> grounding to pipes except within (as I recall) six feet of where the
>>> pipe enters the building, for precisely that reason.
>>
>>
>> The use in modern times of teflon tape at joints in copper
>> piping makes them unuseable for earth grounds even near the entry
>> point to the building. A long (e.g. 2-3 meters) copper stake must be
>> driven for a proper earth ground, or else a large copper mesh mat if
>> the ground is rocky -- unless you are certain that the copper piping
>> that you want to use extends a significant distance underground and
>> unbroken.
>>
> 
> The purpose here is not to use the piping *as* a ground, but to  ensure 
> that the piping *is* at ground potential. Otherwise, if an  electrical 
> failure causes the pipe to reach a dangerous potential  then so does the 
> water in it, then so do the hands you're washing in  that water. Thus if 
> there's an electrical discontinuity in the piping  it is even more 
> important to earth bond any conductive piping/taps  etc. that are on the 
> non-earth side of that discontinuity. The same  applies too to gas 
> piping except here the principal risk is static,  sparks and the 
> subsequent explosion.
> 
> 
> 

I think it is also important to note that NEC 250.52(B) prohibits gas 
piping as a grounding electrode(1990 or so). The gas pipe ceased as a 
grounding electrode due to the dielectric fitting at the meter. The gas 
company did not want a bond around the meter because it defeated the 
isolation fitting.

The presence of gas is not relevant, IIRC.

In the old days, it was a big no no (at least according to the hourly 
wage fellows who actually do the work) to hook the gas line "as" ground 
other than any incidental grounding which ocurs in a gas furnace as an 
example.

Good place for resources is http://www.mikeholt.com in the forums. 
Decent community of knowledgeable folk there.

Good luck, and "no" do not use your body/fingers/arms/etc to connect 
various pieces of equipment to see if a voltage exists:-) That's best 
left to close friends who stand near electric fences.

I had problems in the mid 1990's in an older home where the galvanized 
water supply pipe was the primary ground. Over time, corrosion of the 
pipe reduced conductivity, and lightening storms toasted a few expensive 
items (e.g. ISDN gear, sun workstation, etc) before finally driving a 
few grounding bars into the soil in the basement.

Cheers,
andy