Hi Ken:
> What determines whether equipment gets a green wire or not? TVs, toasters,
> handheld hair dryers and just about anything for home use get two wire power
> cords. Computers and other ITE get three wire cords. Is the distinction
> commercial vs. residential (class A vs. Class B)? It doesn't seem purely
> safety related, since a metal toaster would appear to be more prone to
> dangerous electrical faults than a doubly-isolated all plastic handheld hair
> dryer. What is the rule here?
There is no rule.
Two-wire (double-insulated) products are equally safe
as three-wire (grounded) products. Regardless
whether metal-enclosed or plastic-enclosed.
The safety difference is that a double-insulated
product is independent of the building installation,
while the grounded product is dependent on the
building installation (i.e, the building wiring
must have grounded outlets throughout). In modern
buildings, all outlets are grounded, so the safety
difference is moot.
Whether the product is double-insulated or grounded
is a matter of the "inertia" of the industry or of
the manufacturer.
Home appliances, including TVs, existed before the
advent of grounding. Hence they were 2-wire, and
they remain 2-wire. Inertia.
Based on a straw poll of my fellow employees, about
one-third live in houses with 2-wire outlets. So,
it is prudent to build double-insulated products for
residential use.
On the other hand, PCs were derived from large,
commercial computers that had 3-wire cords. So, the
inertia was grounded products. Most PCs and computer
peripherals are grounded.
Portable electric tools originally were 2-wire, metal
encased. Then, with the advent of grounding, they
became 3-wire, metal encased. But, grounding was not
reliable in construction sites, and many were injured
by electric shock. So, the portable tools industry
changed to 2-wire, plastic-encased tools.
Another factor is EMC compliance. For higher current
products with input rectifiers and energy storage
capacitors, EMC filtering is easier with a ground wire.
Toasters, hair dryers and similar heating appliances
need no EMC filtering.
>From my point of view, the choice of double-insulated
versus grounding is one of customer convenience. A
2-wire product can be used anywhere; a grounded product
can be used only with a grounded outlet.
There is a minor safety issue with 2-wire electronic
products with EMC filters. Under some circumstances,
the leakage current, although well under the allowable
limits, can be detected by the body, and can be rather
annoying. (Under the correct circumstances, the body
can detect leakage currents -- through the skin -- in
the 40 microampere range; I've done it!)
Best regards,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
AiO Division Tel : +1 858 655 3329
16399 West Bernardo Drive FAX : +1 858 655 4979
San Diego, California 92127 e-mail: [email protected]
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