You're right, normally the supply-side bonding jumper (SSBJ) in a service disconnect should bond the metal enclosure to the neutral. The image you're referring to likely depicts a temporary power skid, and the grounding and bonding configuration can be complex depending on its specific application and design.Here are the conditions under which you might find a seemingly "isolated" neutral bar connected to the SSBJ on a temporary power skid, even though it primarily bonds the enclosure

1. Separately Derived System (SDS) - Most Likely Scenario
Definition

The temp power skid is acting as a separately derived system (SDS), meaning it's generating its own power, or receiving power from a source with isolation between the input and output windings. Think of a generator, or a transformer designed to isolate.

Why it matters

NEC Article 250 has specific rules for grounding and bonding SDS.

Configuration



Transformer or Generator

The skid likely contains a transformer or generator. The

secondary of the transformer (or the generator output) is the source of the SDS. SSBJ Location

The SSBJ would be installed

within the skid at the location of the first disconnecting means for the derived system. This is where the neutral is bonded to the enclosure. Dedicated Neutral Bar

The insulated neutral bar would be connected to the SSBJ within the skid. This ensures that the neutral is properly grounded to the enclosure, establishing the necessary low-impedance ground fault path.

Purpose

This configuration ensures that in the event of a ground fault on the load side of the skid, sufficient current will flow through the grounding path to trip the overcurrent protective device. It also limits the voltage to ground.

Important Consideration

The incoming supply (from the utility or another source) may have a grounded neutral, but that ground is not directly connected to the neutral on the load side of the SDS (inside the skid). The only grounding of the neutral happens via the SSBJ at the SDS.

2. Specific Engineering Design for Ground Fault Protection

In very rare cases, a design engineer might specify a different grounding configuration for specialized applications, such as to: Limit circulating ground currents. Address sensitive electronic equipment. This would be highly unusual and require thorough documentation and justification.

Do not assume this is the case without specific instructions and engineering diagrams.3. Mistake or Improper Wiring



Possible, but should be ruled out. Incorrect wiring is always a possibility. If you suspect this, immediately shut down the system and have it inspected by a qualified electrician.

Key Considerations & Troubleshooting Inspect the Skid Thoroughly

Look for documentation, wiring diagrams, and the location of the transformer or generator (if present).

Identify the Source of Power

Where is the power coming from into the skid? Is it a direct connection to the utility, or is it from a transformer/generator?

Verify the Bonding

Ensure that the metal enclosure of the skid is properly bonded to ground via the SSBJ at the point of the SDS.

Check for Proper Grounding Electrode System

The skid (as an SDS) may need its own grounding electrode system (ground rod, etc.) depending on the NEC requirements and local codes.

NEC 250 Compliance

Ensure the entire system complies with NEC Article 250 (Grounding and Bonding).*

Qualified Electrician



Always consult with a qualified electrician familiar with SDS and temporary power systems. Grounding and bonding are critical for safety. Incorrect configurations can be extremely dangerous.

In summary, the presence of an insulated neutral bar connected to the SSBJ is most likely due to the temp power skid acting as a Separately Derived System (SDS). However, it is crucial to verify the specific configuration, the source of power, and to consult with a qualified electrician to ensure that the system is grounded and bonded correctly and safely, per NEC Article 250. Never assume anything. Verify the connections.

Warning: Working with electrical systems can be dangerous. Only qualified personnel should perform inspections, testing, and modifications. If you are not qualified, seek professional assistance. Flag for review