Anytime you can design a system to electrically isolate the parts that you can touch from the insides, I'd recommend doing that. Some systems are already done this way, but some of them built to be inside something else don't bother, because the other casing it's supposed to be in would keep people safe from it, and it costs more to insulate and isolate everything, and complicates thermal management at least a little, too.
Often enough, you can't fully insulate an exterior casing of a PSU that's meant to go inside something else (which would've provided cooling or whatever), becuase of heat-shedding problems, since typically anythign that is electrically isolative is also thermally insulative to at least some degree, and/or blocks airflow necessary to convection-cool something important.
john61ct said:
The negative returns on ultimate load devices and battery banks are tied to the "Vehicle Common Reference" pseudo-ground, chassis frame, engine block etc.
I definitely don't recommend that.
That means the entire thing is electrified, and just waiting for a positive to rub thru the insulation and short to it. Or be pinched by it in a crash, and cause a short. (if the fuse or breaker doesn't pop, then a fire could result).
Iv'e seen this happen to various vehicles with 12v wiring, and in one case the battery exploded, ripping the hood off the car and flipping it up quite far in the air to land on another car in the parking lot, because someone trying to help jumpstart the car accidentally pushed the jumper cable clamp for positive against the car frame, where it sparked, welded, and was unremovable as the cables began to burn, and then the battery overheated, boiled, and bang. Since the battery terminal is where the jumper was clamped, the fuses on the system could do nothing to protect it because they were all downstream of that point.
I don't really want to think about the fire a large lithium pack could cause if something like that happened. Hopefully the design of the rest of the wiring harness would be able to prevent it...but it's safer not to use the frame for any electrical connections.
Also, it presents an electric shock hazard for anyone working on it. ON my CrazyBike2 (admittedly much more "science-project" than "engineering-award" quality :lol: ), I used the frame for ground, and ground was common between 12v lighting system and 52v traction system. I had a circuit breaker mounted on the frame just forward of the seat, that switched the positive of the traction system. Rainy day, and water conducted via dust/etc from the breaker connections to the frame, causing odd behavior and then tripping the breaker. Another time, I was holding the frame and moving the bike, and one hand was on a wiring harness that had an invisible insulation defect, and my sweaty hands conducted very well from that thru my body to the frame ground. Not much fun. If I had a higher voltage traction pack it's possible I wouldn't be posting this.
SB Cruiser trike early version had frame ground, and a positive 12v system wire direct from the battery pinched between a plastic case and the frame, shorting the battery (but not popping the breaker!) and setting fire to the wiring harness all the way back to the battery terminals. Thankfully I was able to stop and get the case open (which allowed the hot smoke to burst into flame in my face) and get the wires off the battery, before the battery itself could reach fire point.
I don't use frame grounds anymore, for anything.
If all electrical is kept completely isolated from all chassis, etc., it's safer.
I know cars are still built this way...and most of them don't catch fire from it...but it's still not as safe as not doing it, and some do still fail catastrophically from it. THe main reason it's still done is that using the frame means only half the wiring has to be done for the lighting system, since the other half is the frame. Cheaper in materials and labor. If the cost was teh same, I doubt they'd still do it.
