Hi Richard:
You commented that the major reason a surge suppressor
is not allowed from line to earth is because the surge
suppressor is prone to shorting.
Let us assume that the surge suppressor only fails when
a voltage is impressed across it.
For a voltage to appear across the surge suppressor, the
chassis of the product must be connected to ground. If
it is not connected to ground, then there is no current,
and there is no voltage across the suppressor.
So, a surge suppressor can only fail when the product is
truly grounded. If it is grounded, then the grounding
SHOULD provide the protection for which it is intended,
namely protection against electric shock in the event of
a failure of some sort from mains to ground!
When it fails, hopefully its impedance is sufficiently
low so as to cause the operation of an overcurrent device
(e.g, fuse or circuit-breaker). Hopefully, the operator
will notice that the circuit-breaker has tripped and the
unit is taken out of service for repair.
If the unit is not taken out of service, but moved to a
site where there is no ground, then the unit will indeed
be hazardous, with mains applied to the chassis through
the shorted suppressor. This is the sequence of events
that would lead to a shock hazard.
Evaluation of products for safety does not include moving
a unit (with a single fault) from a grounded state to an
ungrounded state.
Note that, in every product, we PRESUME failure of basic
insulation. Grounding is one scheme for mitigating the
failure of basic insulation. What is the logic -- or
engineering basis -- for permitting grounding as a mitigator
for failure of basic insulation but not for failure of a
surge suppressor?
I submit that not allowing a surge suppressor from line
to chassis because it is subject to shorting and causing
a shock is nonsense.
Nevertheless, the various certification houses indeed
have rules prohibiting surge suppressors from line to
chassis. Their justification is that the failure of the
surge suppressor WILL cause a shock hazard. The construction
is DEFINED as hazardous. No amount of engineering evaluation
or testing is going to change the certification house rules!
Best regards,
Rich
