I think this discussion has bogged down into too much
detail.
Overcurrent protection is required to provide protection
against overheating and fire (and sometimes shock) when
a fault occurs.
For plug-and-socket connected products, a mains fuse is
used to provide protection against LOAD faults, i.e.,
faults that cause overcurrent from line to neutral.
A single fuse (in either the line or the neutral) provides
protection against load faults.
For grounded installations and grounded (Class I) products,
a fuse in the line (live) conductor is also used to provide
protection against GROUND faults, i.e., faults between the
line conductors and protective (grounding) conductors.
A single fuse in the line (live) conductor provides
protection against both load (line-to-neutral) faults and
ground (line-to-ground) faults.
The discussion has considered the need for a fuse in each
side of the mains in a plug-and-socket connected product.
If the plug and socket are polarized (i.e., line connected
to line and neutral connected to neutral through the
socket), then a single fuse in the line conductor provides
protection for both load and ground faults.
Polarized plugs and sockets include: UK, North America,
Australia/NZ.
If the plug and socket are not polarized, then a single
fuse (in the product) provides protection for load faults
only. However, the installation overcurrent device (which,
by definition is in the live side) provides protection
against ground faults.
An example of a non-polarized plug is the Euro standard
plug. It is reversible at the socket. (The French plug
and socket, while not reversible, is nevertheless non-polar
because neither the socket nor the plug need be wired in an
arrangement to maintain polarization.)
So, when connected to a non-polar system, the product fuse
provides protection against load faults, and the
installation "fuse" provides protection against ground
faults.
The reason we put a fuse in the product is that the product,
under load fault conditions, may overheat at a current less
than the rating of the installation "fuse."
We MAY want to put a fuse in a product to also provide
protection against ground faults. If the construction of
the product was such that a ground fault might cause
overheating at a current less than the rating of the
installation "fuse," then it would be appropriate for the
product to provide a fuse for such protection.
This would be a highly unusual product construction.
Normally, there is basic insulation between the line and
the ground. We presume that failure of basic insulation
is very low impedance such that the installation "fuse"
will operate. However, it is conceivable that the
construction may allow for a high-impedance fault that
would cause overheating at a current less than that of
the installation "fuse" protection.
If such a condition existed in the product, and if the
product was expected to be connected to a non-polar system,
then two fuses would be required (to assure a fuse is always
in the live conductor to protect against a ground fault).
An issue of much less importance to my mind is the
protection of a serviceman.
If a Class I product with a single fuse is connected to
a non-polar system, and the plug is such that the fuse is
in the neutral side, and the fuse operates (due to a load
fault), then the mains circuit remains live.
If a Class I product with two fuses is connected to either
a polar or a non-polar system, and the fuse in the neutral
operates (due to a load fault), then the mains circuit
remains live.
If a Class II product is connected via a non-polar plug,
and the plug is such that the fuse is in the neutral side,
and the fuse operates (due to a load fault), then the mains
circuit remains live.
Our safety standards are not concerned with the first or
third situation, but require serviceman warnings for the
second situation.
Go figure.
Best regards,
Rich
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