Separately Derived System
A separately derived system (SDS) is an electrical power system whose conductors have no direct electrical connection, including the neutral or grounded conductor, to conductors of another power source or utility service, except through grounding and bonding connections. This concept is fundamental for designing grounding, bonding, overcurrent protection, and transfer switching for transformers, generators, and UPS systems in industrial settings.
On a shop floor, SDS applies to equipment like dry-type transformers (e.g., 480V to 208Y/120V), standby generators with switched neutrals, and UPS systems with isolation transformers. These sources create independent grounded neutrals requiring a system bonding jumper and grounding electrode conductor at the source or first disconnect per NEC 250.30 or CEC Rule 10-212. Proper installation ensures fault clearing, reduces touch voltages, and affects arc flash labels. Maintenance teams must identify SDS for lockout/tagout, as secondary circuits can remain energized when primary is de-energized. Misclassification leads to parallel neutral paths, nuisance tripping, and code violations.
When is a transformer considered a separately derived system?
A two-winding transformer is an SDS when its secondary is isolated from the primary, with no direct conductor connection. The primary is supplied only by ungrounded conductors, and the secondary creates a new grounded neutral not solidly tied to the primary neutral. This applies to standard 480→208Y/120V distribution transformers, requiring their own neutral bonding and grounding electrode connections per NEC 250.30 or CEC Rule 10-212.
How does neutral switching in transfer switches define SDS for generators?
If the transfer switch switches the neutral along with phases (e.g., 3-pole with switched neutral), the generator output is electrically isolated from the service when in generator mode, making it an SDS. If the neutral remains solidly connected between generator and service, the generator shares the same neutral and is not an SDS. For SDS generators, NEC 250.30 and CEC Rule 10-212 require a system bonding jumper and GEC at the generator or first disconnect.
How are grounding electrode conductor and system bonding jumper located for SDS in Canada and the US?
For SDS, both NEC and CEC require the system bonding jumper to connect the SDS grounded conductor to the equipment grounding system at one of three locations: the source (transformer or generator terminal box), the first switch or disconnecting means, or a common tie point where multiple SDS terminate. The grounding electrode conductor must terminate at the same location as the bonding jumper, ensuring a coherent bonding scheme. Improper location, such as bonding at a remote downstream panel, is a common code violation.