Learn English – What do you call a number that is a power of 2

As another American EE, I would use "mains" only in certain contexts...

After the hurricane, the hospital powered life support equipment from diesel generators for 36 hours, then switched back to the mains.

"Grid" would also be acceptable.

If you're referring to 120VAC (RMS) without caring about source, the accepted U.S. industry-wide term is 'line voltage'.

I think "double-checked" or "cross-checked" would be more accurate than "redundant" here. If the intention was that the system was still usable even if one of the computers fails altogether, that would be a redundant system, but (I assume) it isn't; the cross-checking that both operators agree on the numbers is a necessary part of the system if we want to detect errors.

"cross-checked" is perhaps more precise; the point is to have two independent sources to check against each other, whereas "double-checked" could be a single source being looked at twice. (Though ultimately there's one source, the actual ball with the number.)

Compare cross-check and double-check.

(The next stage would be to have three operators, and to accept results if two out of three agree. Then the third computer is redundant for error detection, but not for error correction.)