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42VAC control voltage?

42VAC control voltage?

42VAC control voltage?

(OP)
I have just run across a tunnel boring machine made in England that is using 42VAC as a control voltage. I see that several control mfrs offer that as an optional (but never in stock) control voltage, but I am curious why someone would specify such an odd ball voltage. I'm doubtful that someone just pulled that number out of thin air, so I have to assume that there must be some sort of valid engineering justification or some problem that was solved by using 42VAC, I just can't think of one. Anyone know?  


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: 42VAC control voltage?

I've seen 48 vDC used in some industrial substations, and lots of 24 volt stuff and my UPS work on 72 volts, but 42?  That's a new one to me.

old field guy

RE: 42VAC control voltage?

At a guess I would say it could be the highest voltage which fits in to the category of safety extra low voltage (SELV), which is 50V maximum, under just about all likely extremes of voltage tolerance on the primary side of the control transformer. But I am probably wrong; it has been a day for being wrong!
  

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If we learn from our mistakes I'm getting a great education!
 

RE: 42VAC control voltage?

That sounds reasonable, Scotty. I was going to post a similar suggestion but you got there first.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 42VAC control voltage?

You are right there Scotty.

SELV actually always was 42 V maximum for so long time that my technical expertise once was challenged when I mentioned 42 V as the limit for safe voltage. Eyebrows up. "50 volts, you mean?" Of course I did, but since 42 is what you have and also is the nominal voltage, I had assimilated it as the standard.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 42VAC control voltage?

(OP)
I don't know. I've come to the conclusion that it's more likely just a a trick of the OEM to make sure replacement parts have to be ordered through him only... NOBODY has 42VAC coils in stock anywhere.


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