Bonding heating duct

[porterh]

We have a large medical office building in the construction stage. There many VAV boxes in the ceiling. Each VAV box has a canvas vibration connector separating two pieces of the metal duct that connects to the VAV box. The isolated piece of duct is hung from the building steel. The electrical contractor says that a bonding jumper is not required citing 250.104(B) FPN.

Section 250.104(B) is titled "Other Metal Piping". The FPN talks about "metal ducts".

Can bonding across the isolation canvas connector be enforced?

Thanks

Porterh

 

[barbeer]

If the isolated sections of ductwork are "likely to become energized" which I would say not, then bonding would be required. I would think depending on construction (steel trusses, etc...) the ductwork may be bonded from the supporting structure and hangers.

 

[lpelectric]

We have a large medical office building in the construction stage. There many VAV boxes in the ceiling. Each VAV box has a canvas vibration connector separating two pieces of the metal duct that connects to the VAV box. The isolated piece of duct is hung from the building steel. The electrical contractor says that a bonding jumper is not required citing 250.104(B) FPN.

Section 250.104(B) is titled "Other Metal Piping". The FPN talks about "metal ducts".

Can bonding across the isolation canvas connector be enforced?

Thanks

Porterh

The FPN is not enforceable. Any electrical equipment, i.e., valves, motorized dampers, etc, would have an EGC that would serve to bond the metal. An isolated section of duct work would probably not be considered "likely to be energized".

Having said that, I should relate a legal case in which I gave professional testimony after an electrocution of a boiler technician a few years ago.
A metal hot air heating system using ducting for conveyance of heated air had been abandoned in place and parts of the ducting removed leaving the floor plenum and grate intact and thru the floor adjacent to the newly installed hot water boiler.
An electrician had shoddily pulled a NM cable thru a small hole in the floor exposing the phase conductor thru the NM jacket and laying up against the metal ducting. This ducting was electrically isolated from any ground-fault return path and remained energized for an undetermined length of time. When the heater technician stood on the floor grate and touched the piping of the newly installed plumbing system, a circuit was made between the equipment grounding conductor of the circulator motor thru the technicians body to the energized floor grate. This was sufficient to electrocute him.
He was a young man in his thirties with two children and a wife.
The term "likely to be energized" is grey at best, but it is always wise to err on the side of good judgement. :smile:

 

[bphgravity]

I agree that it is a gray area that requires a judgment call based on the specific conditions of the installation in question. An all-inclusive NO or YES is not reasonable.

 

[ryan_618]

I made a proposal (that I knew would fail) to require duct bonding, just to get a statement from CMP 5. Here is the proposal, substantiation and panel comment:

(D) Other Metal. Where installed in or attached to a building or structure, metal objects that are likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 250.122, using the rating of the circuit that is likely to energize the piping system(s). The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.
Substantiation: It is evident that code understands the fact that bonding isolated metal is an enhancement in safety. This can be verified by referring to the fine print notes of 250.104(B) and 250.116. Because there are, in fact, documented deaths that have occurred because of the lack of bonding of such things as nonstructural metal wall framing and ducting systems, this concept should be a requirement, and not just a suggestion in the form of a fine print note. This proposal uses the existing text of 250.104(B) in an effort to provide consistent code language between the two subsections.
A companion proposal to delete the FPN to 250.116 will be submitted for
correlating purposes.
Panel Meeting Action: Reject
Panel Statement: The terms ?other metal? and ?metal objects? are vague and unenforceable as used in this context.
Number Eligible to Vote: 15
Ballot Results: Affirmative: 15

 

[don_resqcapt19]

Ryan,
Then they need to delete 250.4(A)(4). It is just as vague and unenforceable. Also in my opinion this section would require that the duct be bonded if it is likely to become energized. Likely is a tough criteria to meet and very subjective.
Don