sundance-communications.com/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems Intercom Systems

Like the man said, the polarity convention in telephony is that ground is positive. Therefore, battery ("hot") is negative. Therefore anytime battery is referenced in text or on an SD we use the negative symbol in front of the number.

There are a few specialized places in telephony where the D-C supply is center-tapped, so that one side is higher than ground, and the other side is lower. In those cases, you will see "-v" and "+v".

I see. Makes sense. Thank you for clarifying!

the only Intercom that I ever used that required an A lead was the 401 A manual intercom. The later 401 B did not need one, nor did any of the dial intercoms (207, 347 etc) that I worked on. As Arthur said, the packaged intercom systems worked very well. You just plugged in a 25 pair cable, cross connected power in and T,R,L and signal leads out.

Very simple, very effective and essentially bulletproof.

Sam

I think I -kind of- know the answer to this... but why are telephone systems positive ground? Since we are on the subject, it would be an interesting thing to know why. Something similar to the cathodic systems used on ships, isn't it? A corrosion thing?

It's a question I always asked and while I think I got a straight answer once, I'll be damned if I can remember it.

What I heard most often was " electrons flow from the negative to the positive."

At this point in my dotage, I just accept it.

I will also say that I did a buildout for a cell site once and they called for +24 vdc. When I asked why, all I got was blank looks.

Sam