Hi Bill,
I hope all is well. "
Manual Override" can mean a lot of things, as you know and have done a great job exploring. For us, we typically interpret this as providing a means of "emergency/maintenance control" for motorized equipment which also features a moderate amount of complexity which may cause problems down the
line. Typically our "
manual override" is a back up, push
button style control
system mounted on the
face of the Motor Control Cabinet (though not always). Sometimes these are entirely redundant and use separate fuses,
etc. Today, we provide these as standard, even when they are not specified (so long as the TC does not object), so as to provide a means of running a show should some non-safety critical
system (such as a positioning
encoder or touch
screen panel) become damaged or fail at a bad time. Its a fact that no matter how well built electronics are, eventually they will stop working. We dont want to
strand a TD or
stage hand right before a show, but we also do this for
safety reasons which I'll explain below. Most of us (Protech,
Clancy, Secoa, Texas Scenic,
etc) can provide these at little or no additional cost.
In a staggering number of our inspections of existing facilities (both of our own manufacture and others), operators have been trained to and routinely operate motorized damaged equipment by manually depressing the line-contactor inside of the MCC. Not only this dangerous to the operator due to
ARC FLASH/High
voltage shock without proper
PPE, but by depressing the
line contactor the machine is bypassing ALL
safety equipment. This includes mechanical limit switches and emergency stops. As this is not the way they these switches are designed to be operated, contactors can even permanently weld themselves in place while engaged! The only way to stop this is to hit the main
disconnect either inside the panel or locally on the
winch. While I've never seen THAT happen, I've seen plenty of damaged caused by two-blocking hoists via the
contactor (as the limit
switch is not operating), unbeknownst to the operator. This is a potentially fatal situation and machinery should NEVER be run this way.
Some consultants that we have worked with object to this practice because they want NOTHING to work if
anything is wrong with the
system. I understand that perspective, and we will comply if that is what they think is best for their clients, but in my experience, technicians will find a way to make the show go on - and that includes the behavior described above. Its just a fact of Show Business. Sometimes we provide a "middleground" solution such as a secondary key which must be inserted and held during the move to "authorize" it.
Some special equipment (performer hoists in particular) are built with backup systems that are designed to ignore ALL
feedback/
safety equipment and will drive the
hoist so long as it has
power. An example would be a fire
blocking access to the
manual release system, with non responsive controls and a performer in the air - they need to get down NOW. Another I can think of was a set of automated acoustical doors (4-5k lb concrete walls) which needed to travel UP, without
power, during a fire. For that we built a
counterweight assist
hoist and a hydraulic damping
system, similar to a
fire curtain, only in reverse. Pretty cool.
Heck, even popping the fan
cover and operating the motor fan by
hand will operate the
hoist "manually"... though you'll probably want to bring a lunch and some podcasts or books on tape as it can take several hundred (or sometimes thousands) of turns to equal one revolution of the output shaft.
Please
send me a PM or give me a
call if there's anything I can do to help. Sounds like a nice project!
Tanner
Protech