c.1949 tandem breakers

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swei

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Can someone identify these and perhaps provide a little backstory on them? I'm particularly interested to learn if they are a thermal only design given some recent experience (not tripping on a direct short that vaporized about two inches of #12 THHN.) A Square-D employee referred to them as "Type MM" but I can't find much online using that name other than some relatively modern-looking miniature C-H breakers.


thanks in advance...
 

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ActionDave

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There is some old stuff that I have ripped out that has SQ D on the dead front and Westinghouse on the innards that look just like your pic. I can't remember the numbers on the breakers. I'll try and look on the shelf at the shop tomorrow.
 

jim dungar

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Square D XO type I believe. They are at least 60yrs. old, as I believe that sq.d went to qo type in the early 60's or even late 50's.

The OP clearly shows bolt-on style 1-pole MM breakers (M1 were 2-pole and M32 were 3-pole). These were likely built by Westinghouse and brand labeled by Square D and others. The last pricing, I have for a replacement MM breaker is dated 1970

You are correct that the XO family replaced some of the MM breakers and the QO family was introduced in the late 50's.
 

swei

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Thanks, guys.

Anyone know if they are a thermal-magnetic design, or some kind of thermal-only? Known issues with them?
 
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