[Elecraft] Antistatic mat without electrical ground

Tue May 13 13:29:09 EDT 2008

Don, and others

In older houses in the UK the neutral is bonded to a real ground at the fuse
box, where the ground is usually a long copper rod just a few feet away.
There is rarely in my experience more than a volt or two on the neutral.

More modern houses use PME (protective multiple earthing) where the neutral
is bonded not at the house but locally at the final step-down transformer-
the argument being that this forms a higher quality ground I believe.  The
real danger with PME is that the neutral may well then rise above 0 volts,
but in normal circumstances, because plumbing inside the house is bonded to
'ground', no potential (!) exists for electrocution.  Until, that is, a ham
decides to ground his equipment 'properly' outside.  Then large amounts of
current can potentially flow!

There was an excellent piece on this in Radcom many years ago by Peter
Chadwick G3RZP I think.  Ian, GM3SEK will no doubt remember and also know
far more than me about the issue!

--
73 Stephen G4SJP

KX-1, K2, and K3 ordered Christmas Eve!

On 12/05/2008 23:13, "Don Wilhelm" <w3fpr at embarqmail.com> wrote:

> David,
> 
> Very good advice in general.
> In this particular case, the original poster was in the US which is much
> more relaxed about withholding mains power from older residences that do
> not conform to the newer electrical codes (grandfather clauses, etc.).
> In addition, here in the US, one side of the 120 volt receptacles is
> connected to neutral - and the neutral is connected to earth ground at
> the mains service entry point.  I believe in the UK, both sides of the
> 240 volt service are isolated from ground which can create more exposure.
> 
> In this specific case, there has to be an existing ground rod at the
> electrical mains entry, and rather than driving an additional ground
> rod, it would be safe practice to add a direct wire from that mains
> ground to the location where the anti-static mat and wrist strap are
> located - Warning - if an additional ground rod is driven, it must be
> connected to the service entry ground by a #6 or larger wire to comply
> with NEC code.  The actual ground rod may be buried under the surface of
> the earth since many building inspectors and power companies insist on
> it to keep the ground rod from being removed by the unaware homeowner
> who finds an 'ugly obstacle' which interferes with his lawn mowing
> efforts.  When I built my house, the building inspector strongly
> suggested that I sink my ground rod under the surface, but I explained
> my reasons for not doing so, and he backed off.  I like to check and
> tighten my ground rod clamps once a year.
> 
> 73,
> Don W3FPR
> 
> 
> 
> David Woolley (E.L) wrote:
>> Apologies for breaking threading.
>> 
>> Paul G4LNA wrote:
>> 
>>> Just run a wire to a rod outside, it doesn't matter how long the
>>> wire is, as you say it's not an RF ground. I wouldn't use the gas
>>> pipe for safety reasons.
>> 
>> This advice is dangerous and doesn't even really achieve what one
>> needs to achieve.
>> 
>> It's dangerous because, if there is a fault which connects live to any
>> exposed metalwork, you have introduced a low resistance path to earth
>> into an environment where the the fault was much less dangerous
>> because only high resistance return paths existed.
>> 
>> What you are really trying to do make sure that all, even partially,
>> conducting surfaces are at the same voltage, including you and the
>> soldering iron tip.  Normally you can rely on internal surfaces being
>> insulated or connected to mains earth, because that is what modern
>> electrical codes require, so it is normally appropriate to connect the
>> mat to mains earth.
>> 
>> As far as I know, in the UK, you would have electricity supply refused
>> if you had two wire mains sockets (shaver sockets are specially
>> isolated), except for environments which are specifically designed to
>> be earth free, in which case you would not be allowed to have anything
>> connected to earth.
>> 
>> If you only have two wire sockets, or, more generally, if you do not
>> know how the mains earth is arranged, you really need to consult a
>> competent electrician before doing anything that involves earthing at
>> DC to mains frequencies.  You should treat any metal connected to true
>> earth, and inside, as though it was at mains voltage, and you should
>> treat any wires going to the outside, as being at mains voltage, when
>> outside.
>> 
>> My own guess is that you either need to completely rewire to include
>> an earth wire and properly earth all relevant appliances and bond
>> other conductive surfaces to that earth, or to establish a work area
>> that is a body's length from any electrical outlet, or equipment,
>> provide conductive flooring, bond all surfaces electrically and use a
>> battery powered soldering iron.  (Your electrical codes may be more
>> specific about the separation distance.)  I think there are variations
>> on this that provide some mains power, but I wouldn't want to suggest
>> anything without detailed knowledge of local electrical codes.
>> 
>> In general, if you put in earth rods, they must be connected to the
>> mains earth or completely isolated from the electrical system, by
>> sufficient distance that it is impossible to touch anything that can
>> come into contact to one of them at the same time as anything that can
>> come into contact with the other.
>> 
>> 
>> WY5Q wrote:
>>> 
>>> My house does not have grounded electrical outlets. What is the best
>>> way to protect against static during assembly of the K3 kit? Ground
>>> the mat to a ground rod outside? Or to a metal gas pipe (for
>>> grounding me, not as RF ground!)? There are no water pipes accessible
>>> my build location.
>>>     
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