DC system - Neg to ground

[JAYWERNER]

Can someone tell me why we take the negative side of a 12V battery and bond to metal structures such as a trailer or large boat? Why not run a separate positive (switched or un-switched) and negative wire to each light/load to complete the circuit back to the source and not rely on the metal structures to complete the circuit? Is there a safety issue with not bonding the negative to the metal structures? What if it's higher voltage say 100 volt DC or higher? Would it make a difference if the positive was bonded to the steel rather than the negative side? Is there an issue doing with bonding the positive side to the metal structures with current flow of special electronic equipment, ie diodes, etc?

Thanks in advance -

 

[iwire]

At 6, 12 or 24 VDC there is no shock hazard and the frame / structure is a very low resistance reliable path for current when done correctly. No sense spending time and money running a conductor where a safe one already exists.

Now on the other hand as the voltage gets higher there becomes a shock hazard and most likely in those cases we would still bond one line of the supply (just like we do with transformers and generators) yet run a separate circuit conductor for carrying circuit current leaving the frame / structure free from current flow.

 

[meternerd]

Not all DC systems are grounded/bonded. Voltage is not really a determining factor. You'll find that almost all (if not ALL) DC control systems in electrical substations are ungrounded. Ditto for military emergency supplies on watercraft (such as submarines) or other emergency sources. The reason is that an unintentional ground on either leg will not result in a short circuit. A short would require both legs to be grounded. Most ungrounded systems have a ground fault detection system, similar to an ungrounded AC system, such as ungrounded Delta. Polarity of the grounded leg would not matter. Some older cars had the positive side grounded. Grounding one side was done to reduce the number of conductors needed, thus reducing cost.