On Sat, 4 Jul 2009 23:40:18 -0700, John E.
wrote:

I have been asked to offer an opinion in a sensitive situation.

A machinist moved his shop across town and required some rewiring (3-phase
outlets, conduit, etc.) in order to locate some machines where he wanted
them.

He hires a guy who's not a pro (and later discovers is not insured) but has
done shop wiring before and had a good attitude and track record. The guy
does good work. No complaints about the quality of his work.

Owner throws the switch, all works fine.

The story continues 4 weeks later when the very expensive CNC fries its
controller PCB to the tune of $4000.

Turns out the voltage in the shop was upward of 245 and the taps in the CNC's
power supply were set for 220.

What is the legal and moral responsibility of each party?

What will not be helpful are replies about the character or intelligence of
either of the players or their actions.

The nominal utility power as supplied is supposed to be 240V. 5
volts over is a touch hot, but not out of the bounds of normal
tolerances - turn everything on in the neighborhood some hot August
afternoon with the AC units cranked, and tell me what the voltage
reads then...

If they were seeing 250V - 255V or more, then I'd call the Utility
and get the transformer taps knocked down a notch.

If the power supply on the CNC computer had changeable taps, and the
last guy that touched it didn't have any reason to look to see what it
was set for, IMHO it's nobody's fault. Especially if the shop they
moved from and the one they moved to had the same nominal operating
voltage, and they knew it - I'm not going to open 50 machines looking
for the unexpected when I'm charging by the hour unless I have a good
reason to... Just "Git Er Done" and go home.

If he had a reason to look inside and saw it was on the 220V tap he
should have moved it to the 240V - or told the owner - it's good
practice to follow but there's no responsibility to look involved.

And I wouldn't expect 255V on the 220V tap to kill it. Now if it
was set for 208V input and you fed it off the 'High Leg' from an Open
Delta service that's hovering around 280V to ground, THEN I'd expect
fireworks. Open Delta High Leg voltages can bounce around and go even
higher, then something flashes over...

That would be the /one/ time I'd call it against the Handyman,
putting the high leg on the control circuit would be a big goof. You
are supposed to put the regular 240V legs on the A and C phases coming
in, and the 'High Leg' Orange lead to B phase and NOT the controls.

The average power supply is supposed to feed +5V, +12V & -12V etc.
to the computer board, and have Crowbar protection so that's all that
gets through. If the supply blows up and lets line voltage through to
fry the controller board (even if you put an over-voltage on the
input) that's a badly built power supply.

Otherwise, it's entirely possible that it just reached End Of Life
and decided to go out in a spectacular manner, and the move had
nothing to do with it. The timer that makes things blow up three days
out of warranty finally went off.

Unless you want to spend a lot of money on Electronic Forensics to
analyze the power supply failure, "The world may never know..."

-- Bruce --

PS - Have to trim off alt-r.c.m to make this go, 4 crosspost limit.