Merlion Audio

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Theater Speakers

Are my home theater speakers a fire hazard?

A friend of my husband's installed our inexpensive home theater system as a favor. He has the wires for the speakers (mounted around the room) coming up out of the vents in the floor. Granted, the vents are in a good spot for speaker placement. But is this a fire hazard?

Under most circumstances the risk of fire due to a short in loudspeaker cables is virtually non-existent.

If your loudspeaker cables use the typical PVC insulation found on most lower-cost or generic A/V cabling then you do run a somewhat higher risk of developing a problem. Prolonged or repeated exposure to elevated temperature and airflow rates can, over time, result in degradation of the PVC insulation in large part due to plasticizer migration and volatility. This can lead to increased brittleness and crazing in the exposed portion of the insulation, which could eventually result in failure of the insulation.

Under these circumstances the most likely result is that the protection circuit that protects the output stage of the audio amplifier would kick-in, or worse, the output stage of the amplifier could self-destruct; depending on how well the unit was designed and built. There are very few (well-designed) audio amplifiers that would pose a fire hazard when the speaker outputs are shorted.

You have a couple of fairly simple choices to effectively mitigate the threat of potential problems: (1) replace the loudspeaker cabling with either a jacketed loudspeaker cabling or use polyolefin (i.e., polypropylene) insulated cables; or (2) install 1/2" to 1" diameter plenum rated or riser-rated halogen-free (aka low smoke zero halogen or LSZH) nonmetallic corrugated flexible conduit (aka “innerduct,”) over the (speaker) cabling wherever the cabling passes through the air ducting. Electrical and datacom/telecom suppliers, better hardware stores and A/V dealers/installers should have innerduct in stock.

One last item - you can reduce the amount of self-heating that occurs with higher cable resistances by choosing loudspeaker cabling that has a larger wire gauge. Speaker cabling is generally available in stock wire gauge sizes ranging from 4 AWG up to 16 AWG and higher; the lower the number the larger the gauge of wire. As a general rule of thumb you should always use the lowest wire gauge you can afford and never less than a single run of 14 AWG cable. Though rarely a significant issue, self-heating varies in direct proportion to the cable’s resistance, (which increases as the wire gauge decreases, i.e., smaller overall conductor cross-sectional area and/or the cable length increases,) the impedance of the loudspeaker and the power output of the audio amplifier.

Carlon Product Brochures Index

http://www.carlon.com/Brochures.html

Hal-Free Riser-Gard® Nonmetallic Corrugated Flexible Conduit

http://www.carlon.com/Master%20Catalog/Flexible_Raceway_Brochure.pdf

Belden Low Smoke Zero Halogen Speaker Wire and Cable

http://www.belden.com/search/index.cfm?q=halogen+AND+speaker

Understanding In-wall Speaker, Video and Audio Cable Ratings

http://www.bluejeanscable.com/articles/inwallrating.htm

Cabling What You Don't Know Can Kill You

http://www.wireville.com/news/Cabling%20Can%20Kill%20You.html

Cabling Safety in Multi-Story Structures

http://www.wireville.com/news/Cabling%20Safety%20in%20Multi-Story%20Structures.pdf

Wire Management and Life Safety Articles

http://www.hhrobertson.com/ls_docs.cfm

Guide to Low-Voltage and Limited-Energy Systems

http://www.mikeholt.com/documents/lowvoltage/pdf/LowVoltBook.pdf

 

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