I see this as an absolute win. Be careful, folks. Just because it's DC doesn't mean it won't cause serious damage.
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Hm I have not heard about such an issue so far but I also don't have as many UPS as you.
I see you holding a red cable which could be +. When I switch UPS batteries, I do it the same way as it's recommended for car batteries to avoid sparks/arcs. Remove - (black) first, as it won't spark/arc. Then remove + (red) as it can't get a circuit closed any more, so also no spark/arc.
When plugging a car battery in, it's the other way around. + (red) goes in first and only then you connect - (black) to avoid spark/arc for both connectors again that way.
Either polarity will arc the same, the reason on cars for doing negative first is because you're supposed to attach it to the frame, so a spark doesn't set off any hydrogen in the battery and cause an explosion.
The frame is already grounded negatively. so if you remove red and happen to touch red post while leaning on metal car you make a circuit. so you pull negative first so even if you touch red it can't travel back to the battery
Additionally, if you where to drop a clamp or touch something by accident, it is most likely to contact negative, as that is the whole chassis. If you adhere to this order and drop the negative clamp, it will most likely just reconnect to negative. If you drop the positive clamp on the negative chassis nothing happens anymore, as you broke the circuit beforehand.
That's a good point as well.
Oh no. Half of this post is me making fun of myself. Mistakes were made. When you have a large array of batteries, it's important to wire them up correctly when replacing them. I definitely closed the circuit on some of the batteries when I shouldn't have.
I see how it could be confusing, and in the spirit of sending the correct message, I have added a clarifying edit.