Friday: Understanding System Bonding Jumpers in Transformers
Demystifying separately derived systems and why establishing a neutral-to-ground bond handles catastrophic faults.
When you install an isolation transformer to drop 480V down to a 208/120V panel, you are creating what the electrical code calls a Separately Derived System. The energy coming out of the secondary winding has absolutely no physical connection to the primary winding.
This creates a serious safety challenge. If a hot phase touches the metal frame of a tool on the new 120V system, the fault current desperately wants to return to its source—the transformer secondary. But if there is no established path back to the neutral point of that secondary winding, the fault current has nowhere to go. The breaker will not trip. The metal frame will stay energized at 120V waiting for a worker to touch it.
This is why the System Bonding Jumper is arguably the single most important piece of copper in the installation.
By running a properly sized jumper from the newly created neutral secondary terminal (X0) directly to the transformer enclosure ground, you construct a low-impedance highway. If a fault occurs anywhere downstream, the massive surge of current blasts along the equipment grounding wire, jumps across the system bonding jumper back to the X0 source, and instantly trips the overcurrent breaker, clearing the hazard.