inverter overload (was transformer inrush) [RE-wrenches]

Windy Dankoff, Dankoff Solar windy at dankoffsolar.com
Sun Jan 11 10:13:20 PST 2004


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Hi Kurt.

Yes, the contacts are just like any DPST switch. But, I am not 
suggesting to PARALLEL the two sets of contacts. I am suggesting to 
SERIES them.

Wear or damage on the contacts occurs from arcing during the moments 
of make and break, especially break. Because parallel contacts do not 
make and break at the exact same time, there is little or no 
advantage gained by connecting them in parallel. During break, an arc 
may be sustained too long across whichever contacts have the smaller 
gap.

Wiring contacts in series quickly suppresses the arc. Again, one set 
always breaks first, but when the other set breaks, the voltage 
divides across the two gaps. Voltage at the contacts determines how 
far the arc can jump. The arc cannot sustain the jump across both 
gaps in series. So, the arc isn't prevented, but it is extinguished 
before it has time to melt the contacts.

BTW, look at the way the switch is constructed. Each of the two poles 
has 2 sets of contacts in series, for arc suppression. When you 
series the 2 poles, you actually have 4 contacts in series!

Physical reality:

Under-rated contacts carrying DC tend to form a crater on one contact 
and a pointed deposit (stalactite?) on the other. Eventually the 
pointed deposit may grow long enough to maintain contact when the 
mechanism opens. My experience is that if you file off the deposit, 
it tends to go maybe twice as long before it happens again. Of 
course, it's better to prevent this occurrence, but good to know you 
can buy time with a file or some sand paper.

Under-rated contacts carrying AC have symmetrical wear and (I think) 
tend to spatter away and leave the contacts open -- but this is 
unpredictable. They can weld together on occasion.

If you suspect that pressure switches are being overstressed in your 
applications, I suggest you inspect them. If they are holding up OK, 
you can make your own call as to whether to file them, leave them 
as-is, or install a larger switch or relay.

Finally, it's always wise to have a contingency in place if a switch 
fails -- like a pressure relief valve in a pump system, plumbed to a 
safe drainage spot.

I've learned all this from observation and experience. Unlike solid 
state electronics, here is a case where you can see things happen. I 
suggest watching contacts as they work under load, and inspect them 
over time, to get the best feel for what's happening.

Finally, in case of any doubt, it's much cheaper to replace a 
pressure switch with a bigger one than to clean up the mess that may 
occur after contacts weld together.

Windy





>Windy wrote:
>
>>As long as you use all four terminals on the pressure switch, the
>>contacts are all in series -- whether it switches both sides of the
>>power (2 hots), or one side, with a series-link between both the sets
>>of contacts.
>
>Hey Windy,
>
>Not sure I am following you here, maybe you are stating it from a code
>perspective.  It may not meet code, but I believe I am paralleling the
>two sets of contacts and I use two jumpers.  Aren't the contacts on this
>switch just like a 2pst relay, or for that matter any 2pst switch.  Some
>(Allan) might argue that they don't make and break at exactly the same
>moment, and that could be more to the issue.  I'll consider a relay deal
>in the future, but at this moment I'm not feeling a need to replace a
>bunch of pressure switches.
>
>Kurt Nelson

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