When Horizontal Bus is NOT HCB

Horizontal Does Not Automatically Mean HCB

Just because a piece of bus runs horizontally (left-to-right) as in Figure 1 does not mean you should select HCB.

HCB is very specific.

It applies when:

• The conductors are horizontal and
• The ends of the conductors are pointed toward the worker

A classic example is stabs in a motor control center (MCC) of a pad mounted transformer as shown in Figure 2. In that case, the horizontal conductors terminate in a way that effectively “aims” the arc plasma outward toward the worker.

That geometry matters — a lot.

Why Geometry Is So Important

In an arc flash event, the arc plasma expands away from the source. The electrode orientation influences how that energy is directed.

With HCB, because the conductors are horizontal and their ends are facing outward, the arc tends to be projected more directly out of the enclosure opening. The result:

• More aggressive energy ejection
• Higher incident energy at the working distance
• Often more conservative (higher) PPE requirements

This is very different from:

• A bus running horizontally across the back of a switchboard
• A typical left-to-right horizontal bus stack

In those cases, you might actually be dealing with VCB, depending on how the arc would develop and where the conductors are oriented relative to the worker.

Practical Takeaway

Don’t select HCB just because you see horizontal bus.

Ask yourself:

1. Are the conductors horizontal?
2. Are the ends of those conductors pointed toward the worker?
3. Would the arc plasma be projected outward in that direction?

If the answer to #2 is no, it’s probably not HCB.

Always base the electrode configuration on arc behavior and physical geometry, not just the direction the bus happens to run.

That small modeling choice can significantly change calculated incident energy — and ultimately the PPE selection.