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For two varistors connected between both sides of Mains (L & N) and Protective Earthing (PE) through the same GDT, is it acceptable that each varistor withstands a voltage based on half of equipment input voltage rating?

More specifically you asked:  For a product with input voltage 100-240 Vac submitted for UL / CSA / IEC 62368-1 evaluation, does Annex G.8.1 allow using two MOVs between L and N with a midpoint connection to PE through a GDT, if each varistor’s Maximum Continuous Voltage (MCOV) rating (e.g., 215 Vac) passes 1.25 times the half of rated voltage ( 240 / 2 * 1.25 =  150 Vac) of equipment, so therefore the sum of  the two varistors (e.g., 215 Vac + 215 Vac = 430 Vac) in series has a total MCOV of greater than the minimum required voltage (240 Vac * 1.25 = 300 Vac) per the upper voltage of the Rated Voltage Range of the equipment? 




The construction under discussion consists of two varistors connected between Line and Neutral with a  GDT connected midpoint between the two varistors and Protective Earthing. When the GDT is activated, this results in a varistor and GDT in series between L and PE (MOVA+GDT) and a varistor and a GDT in series between N and PE (MOVB+GDT), and, when the GDT is not activated, two varistors in series between L and N (MOVA+MOVB).


In response, per sub-clause 5.5.7, where an SPD is used between the Mains and Protective Earthing, it shall consist of a varistor and a GDT connected in series and the varistor shall comply with Clause G.8 in Annex G.  Per G.8.1, the MCOV of a varistor shall be at least 1.25 times the Rated Voltage (or the upper voltage of the Rated Voltage Range) of the equipment.


With above scenario, the MCOV rating of each varistor of the two is required to be at least 1.25 times the upper voltage of the Rated Voltage Range of the equipment.  For the example given, the MCOV rating of each varistor should be at least 300 Vac (i.e., calculated by 240 Vac * 1.25).  Therefore, a varistor rated 215 Vac (i.e., less than 300 Vac) is deemed not sufficient per the presently published requirements in Annex G.8.1.


(Note, the currently published requirements are based primarily on consideration of the overvoltage implications of the L or N to PE effects and not the L to N effects.  Any proposal (with technical rationale) to adjust this would need to be subjected to the IEC committee expert review process and should be proposed to IEC TC108.)


As this general forum is not intended to analyze and provide guidance on specific designs, we suggest that you contact the UL office you work with if you have a specific design or construction that you wish to discuss.

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