Rich, as always, you present good arguments. Nevertheless, others may not be swayed by them. Consider the "Y" capacitor. It too is connected from line to earth, but it is specially designed to heal itself in case of a failure of the dielectric. It does not fail in a shorted condition. Should not the same safety criteria be placed on other components placed from line to earth? If would seem silly to have a less reliable (safety wise) part in parallel with a "Y" capacitor.
---------- From: Rich Nute To: WOODS, RICHARD Cc: [email protected] Subject: Re: Surge Suppressors List-Post: [email protected] Date: Wednesday, May 14, 1997 3:04PM 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
