The secondary of a transformer supplies a 208Y/120-volt system that has no direct electrical connection to the conductors of the supply system. How must this derived system be bonded and grounded?
- AThe neutral point should be left unbonded and connected back to the main service equipment, where the main bonding jumper already sets the ground reference for the premises
One bonding jumper at the service cannot serve a system that has no electrical connection to the supply conductors. Fault current on the secondary must return to the secondary windings, so an unbonded neutral may never draw enough current to open the device.
- BA bonding jumper should be installed at every downstream panelboard so that fault current has several parallel return paths and the impedance of any single path cannot delay the operation of the overcurrent device
Multiple jumpers put the grounded conductor and the equipment grounding system in parallel, so ordinary neutral current returns on conduit and ground conductors. That is objectionable current, and it produces stray voltage rather than faster clearing.
- A system bonding jumper connects the derived grounded conductor to the equipment grounding conductors at one point, and a grounding electrode conductor runs from that same system to a suitable electrode
- DThe equipment grounding conductor brought from the primary side serves as the complete grounding means for the secondary, because it already ties the transformer enclosure back to the service electrode
The primary equipment grounding conductor bonds the enclosure, which is necessary but not sufficient. It never establishes the neutral-to-ground reference the derived system needs, and it leaves secondary faults with no defined return to the secondary windings.
Why C is correct
A separately derived system needs its own neutral-to-ground connection so that a line-to-ground fault on the secondary has a low-impedance path back to the derived source. Article 250 requires that system bonding jumper at exactly one location, plus a grounding electrode conductor from that same location to a suitable electrode.
What this question is testing
This tests recognition of a separately derived system and the three pieces that make it work: a single system bonding jumper, a grounding electrode conductor, and the supply-side bonding jumper carrying fault current back to the source.
On the job
Buildings with a stray-voltage or noisy-ground complaint frequently turn out to have a transformer bonded in two places, typically once at the transformer and again in the downstream panel where an installer left the factory bonding screw in place. Pull the covers, find every neutral-to-ground connection, and leave exactly one. Also confirm the grounding electrode conductor is sized from the largest derived ungrounded conductor and lands on a qualifying electrode such as building steel or a metal water pipe near the transformer.
Memory technique
One system, one bonding jumper, one electrode connection.
Exam tip
If the question says no direct electrical connection between primary and secondary conductors, it is a separately derived system, which brings the whole Article 250 derived-system package.
Where to look it up
Article 250 Part II, the section on grounding separately derived alternating-current systems, and the sizing table for the grounding electrode conductor of a derived system.