Thread: Bonding the vibration damper

For years I have used this statement in my reports whenever I come across a air handler that has a vibration damper installed... "The vibration damper at the metal duct for the furnace should also be considered when evaluating the bonding system. There is no visible bonding jumper at this location." I have had several electricians not know what I was referring to... I'm throwing this out (and anticipating the slaughter ) as to whether I need to reevaluate being picky or ignorant.

Thanks in advance.

Cheers,


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I will try to get this started for you. The bottom of the damper is secured to equipment that should be connected to an EGC supplied in the circuit wiring. Do you consider the top half likely to be energized?

I have never seen ductwork bonded in a residential setting, vibration damper or not. In a commercial application I have seen it but it was many years ago. Honestly I have looked and I can not find a requirement or recommendation for ductwork bonding.

Reis, if you have cite or source would you post it? You have my curiosity going on this now.

Originally Posted by Reis Pearson

For years I have used this statement in my reports whenever I come across a air handler that has a vibration damper installed... "The vibration damper at the metal duct for the furnace should also be considered when evaluating the bonding system. There is no visible bonding jumper at this location." I have had several electricians not know what I was referring to... I'm throwing this out (and anticipating the slaughter ) as to whether I need to reevaluate being picky or ignorant.

Thanks in advance.

Cheers,

There is a requirement that all conductive materials that are likely to become energized be connected together and to the supply system grounded equipment to provide an effective ground path. There is no specific mention of the bonding of metal air duct that I can find with the exception of a fine print note in 250.104(B) NEC 2002 and up through 2011.
Fine print notes are usually considered commentary though, and are not normally enforced.

In a perfect world all things conductive should be bonded, so there is no difference in potential - - - anywhere. That is obviously impractical, ( conductive stairs, window frames, fireplace enclosures, conductive components connected to non-conductive supply systems like faucet sets on a PEX and PVC water system, metallic vent piping on a plastic housed bath fan, etc. ). So the question becomes where do you draw the line ? I'm thinkin you're going too far beyond SOPs.

Considering all the sharp edges that may be present with duct work and the close proximity of non metallic cables in many residences, I would place duct work in the "likely to become energized" catagory. Probably more of an issue than it appears to be in kitchens with metallic ducted under counter vents near sinks and various appliances.

In my 17 years inspecting, I have never seen metal ductwork bonded in single famiy homes nor has that ever been a topic of conversation, that I have been involved in, in the inspector circles here in the ATL.

I quite often see a ground wire connected to the air handler. It looks like a plain old ground wire from the power supply cable but I didn't see anything connected to the metal duct on the other side of the damper. A few time out of curiosity, not practice, I checked resistance across the damper and found 0 to very low Ohms. Perhaps the damper material has metallic threads in it?

I recall hearing a few stories regarding electrocution from ducts, though I have no records other than this one from a 1998 NIOSH report summarizing workplace deaths from electricity:

"94-17 A HVAC contractor and his employee were electrocuted while installing aluminum straps to anchor ductwork to floor joists in a crawlspace. The victim contacted an energized strap, then was grabbed by the contractor. Electrical energy flowed through both men to ground."

Douglas Hansen
Code Check- Help With Building Codes

I agree that it should be bonded to ground, and I would expect to see a bonding conductor either back to ground or across the flex vibration isolation section.

(bold and underlining are mine)
- 250.4 General Requirements for Grounding and Bonding.
- - The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
- - - (A) Grounded Systems.
- - - - (1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
FPN: An important consideration for limiting the imposed voltage is the routing of bonding and grounding conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided.
- - - - (2) Grounding of Electrical Equipment. Normally non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.
- - - - (3) Bonding of Electrical Equipment. Normally non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
- - - - (4) Bonding of Electrically Conductive Materials and Other Equipment. Normally non–current-carrying electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
- - - - (5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

I have yet to see bonding on duct work ever, and I have been inspecting for over 20 years.

Recently had geo thermal installed and the return and supply have vibration sleeves and they are not bonded fwiw.

Originally Posted by Reis Pearson

[SIZE=2]I'm throwing this out (and anticipating the slaughter ) as to whether I need to reevaluate being picky or ignorant.

I'll take your question literally and say that there is no reason to bond the isolation damper. As for a bonding jumper around the damper - never seen it.