PSNet,
Unreliability of earthing/grounding
In the PSNet discussion on touch current, a comment was made regarding
earthing/grounding as it applies to cord connected equipment using the most
common North American NEMA-15 or EURO 10A plugs types (called Pluggable Type
A connections in the 60950 family of standards).
The common use of 3-wire earthed/grounded plugs and sockets is a fairly
recent addition to the wiring codes. This requirement was generally invoked
around 1960 in North America, West Germany & Austria. From this it is easy
to understand that this means that yet today there are many
unearthed/ungrounded 2-wire installations.
There is not a lot of worldwide data available, but in the US
Underwriters
Labs has developed a tracking scheme to estimate the relative proportion of
earthed/grounded to unearthed/ungrounded installations, especially relating
to domestic dwellings. From their information, around the turn of the
Millennium about ½ of the homes are still 2-wire installations. Added to
this would be a smaller percentage of businesses (since they are more likely
to get a full electrical remodeling than homes).
Data is not as readily available for other developed countries or
regions,
but something similar seems reasonable.
In the EU, Holland still does not install 3-wire, earthed sockets in all
locations (although any installation could be easily earthed locally because
of the soil conditions). Norway, on the other hand, cannot easily provide a
local earth since the country is built upon a granite rock so earthing is
only guaranteed when a specific wire is carried thru the electrical system
for that purpose - (which often leads to the derogatory comment that the
Nordic countries are dragging their feet on some changes proposed to product
standards).
Anecdotal evidence for the developing countries reflects the fact that
earthing is not reliable because of the inconsistencies of installation
practices.
Because of these conditions, the earthing/grounding is not reliable and
the
loss of earth/ground under these conditions is the normal case, not a fault.
All of this is compounded by the tremendous expansion of the use of many
products that were, in the past considered commercial, in the individual
home office or small office developed by converting homes to business use.
This not only includes PC’s and fax-machines but will also include
workstations, copiers, servers, telephone switches and ATM’s (for delivering
not only cash, but phone cards, phones and other hi-value items) which are
installed rather ubiquitously. In the US, any shop that carries electrical
supplies also carries a 3-wire to 2-wire adapter – commonly known as a
cheater.
The prudent manufacturer will know that the design of the equipment
should
take this into account – even though this may not be properly accounted for
in the safety standard that is normally used.
br, Pete
Peter E Perkins, PE
Principal Product Safety Consultant
Tigard, ORe 97281-3427
503/452-1201 fone/fax
[email protected]
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