You bought a robot "capable of handling a 400lb load". That means it can do that at maximum reach, reliably, repeatably, and without overload.
It also means that in the location you have it the robot can easily punch a hole in the wall, ceiling, or floor. We had special test cells for our robots. There were holes everywhere because, while slinging a 400lb weight, something like the gripper failed and the weight flew into the wall.
The Unimate had an emergency stop button that immediately shut it down. We required floor markings and no-one was ever allowed in the boundary while the robot had power.
We prototyped spot welding on car lines (the things you see in all the car videos are a task we pioneered). The robot chased a car frame mounted on an 18-foot 2-foot thick I-beam (roughly 8000lbs) carrying a 70lb weld gun. One day the signal wires from all of the position encoders failed and the robot "went to its all-0 position" up and to the right. It moved there in less than 1 second. During that time it PICKED UP the I-beam, tore up the car frame, and trashed the weld gun. Fortunately things broke (not the robot, it was fine) , before the I-beam moved far.
I will tell you from experience that if a robot can reliably, repeatedly handle 400lbs at full reach you don't want to be in the same room unless you are in "unreachable areas" like in areas it cannot access. A robot capable of handling such loads will turn you into yogurt without slowing down.
I assume that a robot capable of handling 400lb must have a pretty beefy power supply. You can probably get an intuitive sense of how much damage a robot can do to you by imagining what would happen if all of the power output was directed directly at your body (in the form of heat/electrical/kinetic). A tiny 5v motor might on a breadboard might be enough warm your hand up or fling a small pebble toward you, but a motor that requires one of those bigger 400v 3-phase 60A supplies can probably pull enough energy to melt iron or generate the same kinetic energy as some military ordnances.
