DC-rated switches [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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Eric,

Wiring switch contact in series helps extinguish the arc when the 
contacts break. Two gaps in series would have half the voltage across 
each one, except the action is never perfectly symmetrical, so one will 
be weaker, and will extinguish much sooner during break.

When a switch is wired to break both sides of a circuit, each pole of 
the switch is in series with the circuit, just at different places 
(which makes no difference from the point of view of current because it 
is the same everywhere through the circuit). So, it takes advantage of 
the arc suppression of series-contacts.

The switch is rated for V and A under assumption that the contacts WILL 
all be used by wiring them in series (one way or the other). So, if you 
break only one side, you are REQUIRED to wire the poles in series or 
you are violating the ratings (the design intention) of the switch! At 
worst, it will sustain an arc and the contacts will melt, etc.

An aside -- this applies to pressure switches used in water systems 
too! It comes up when a 115V pump is us used and one side is neutral, 
or a DC pump is used and one side is neg-ground (not to be broken).

I hope this is clear.

Windy



>
>> I recently visited a grid-tie PV system where a siemens Heavy Duty 
>> Switch
>> (Type VBII) was used for the array disconnect. In a negative grounded
>> system do these switches need to have the positive conductor pass 
>> through
>> two (or more) poles of the switch in DC applications? I can't find
>> anything on the Seimens site to indicate yea or nay.
>>
>>
>> Eric Smiley,  MASc
>> Project Leader - Technology Centre
>> British Columbia Institute of Technology 

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