Phantom Power Water and Snakes

[Brad Weber]

I am a little surprised how far from the original question regarding a stage box located "near a decent amount of water" the discussion appears to have gone. On one hand, you cannot be too safe but on the other, show me an outdoor festival that does not have the potential of a stage box or snake being near water during use.

Most of the safety issues being discussed are not caused by the audio system but rather by electrical systems that cause the audio system devices and/or cabling to become the safety ground path, often due to improper wiring or safety grounding. If any part of an audio or AV system is near, over, in, etc. water then a safe and properly designed and wired electrical and safety ground distribution system is that much more important.

[DTV-Engineer]

Originally Posted by Brad Weber

...show me an outdoor festival that does not have the potential of a stage box or snake being near water during use.

"Shall we go on sinning so that grace may increase?" (Romans 6:1)

Originally Posted by Brad Weber

Most of the safety issues being discussed are not caused by the audio system but rather by electrical systems that cause the audio system devices and/or cabling to become the safety ground path, often due to improper wiring or safety grounding.

This is not a safe assumption. There are many audio-visual devices which do not have a grounding power connection: CD players come immediately to mind. It takes very little moisture to cause significant leakage current to the chassis, and typically it will float up to around 60 volts -- the midpoint between the hot and neutral connections.

Blessings,

-- Jeff

[avtech57]

Just on the off chance someone is still reading this forum..how about the equipment ? which would die first.. you or it? Not trying to sound funny but having delt with electronics and home appliances repairs for 35 yrs the short across the 48 volt connection caused by excess moisture would surely wipe out the mixer fairly fast

[DTV-Engineer]

Originally Posted by avtech57

Just on the off chance someone is still reading this forum..how about the equipment ? which would die first.. you or it? Not trying to sound funny but having delt with electronics and home appliances repairs for 35 yrs the short across the 48 volt connection caused by excess moisture would surely wipe out the mixer fairly fast

No, not in this instance. Mixer preamps are explicitly designed to withstand the 48 volts on the input (which makes sense since the mixer is the source of this voltage in the first place); the +48 volt phantom supply is applied to the balanced pair through current limiting resistors so that the mixer will shrug off even a direct short to ground. This is a basic design requirement, since it's not uncommon to find a mic with the transformer center tap connected to ground.

-- Jeff

[tedanderson]

Originally Posted by avtech57

the short across the 48 volt connection caused by excess moisture would surely wipe out the mixer fairly fast

Not necessarily. Low voltage power supplies typically can remain in a shorted state for extended periods of time without being damaged. If there is any resistance between the short, the circuit of the power supply can remain charged indefinitely.

[DTV-Engineer]

Originally Posted by tedanderson

Not necessarily. Low voltage power supplies typically can remain in a shorted state for extended periods of time without being damaged. If there is any resistance between the short, the circuit of the power supply can remain charged indefinitely.

This only applies to linear power supplies with foldback current limiting, or to some switching supplies that do auto-shutdown. Assuming that a supply will withstand a shorted output is not good practice, and can lead to a damged supply at best, or a fire at worst.

-- Jeff

[tedanderson]

Assuming that a supply will withstand a shorted output is not good practice, and can lead to a damged supply at best, or a fire at worst.

Actually it is a good practice to make this assumption. The previous poster suggested that a short will quickly kill the device and subsequently prevent someone from being electricuted. But even when it is shorted, (while under water) it is safer to assume that the circuit is still charged and is still dangerous.

[avtech57]

Think best thing is ..DON'T GO NEAR THE WATER!!

[DTV-Engineer]

Originally Posted by tedanderson

Actually it is a good practice to make this assumption. The previous poster suggested that a short will quickly kill the device and subsequently prevent someone from being electricuted. But even when it is shorted, (while under water) it is safer to assume that the circuit is still charged and is still dangerous.

Okay... I obviously need to be clearer, because the implications of your answer are incorrect to the point of being explicitly dangerous.

1) From a engineering design point of view, assuming that a voltage source will withstand a short is bad practice. If your application requires this functionality, it is essential to ensure that the supply does in fact have foldback limiting or some other appropriate scheme to prevent catastrophic failure.

2) From an operational point of view, assumpting that equipment will withstand abnormal conditions in the absence of specific knowledge is also bad practice, and can easily lead to equipment damage or worse.

3) From a safety point of view, discussion of moisture creating a short is irrelevant; even salt water, which is considerably more conductive than fresh water, still comes nowhere close to creating a near-short condition that would make power supply shutdown likely, except for extremely high surface areas and high ion concentrations -- which is more akin to conditions designed for electroplating, not the sort of environment a mic connector is ever likely to encounter. A supply with foldback limiting will simply continue to source its limiting current but at a reduced voltage, according to the effective resistance it sees, and will not shut off. Some switching supplies will automatically attempt to restart in an overcurrent situation, resulting in continued brief pulses at the rated current through the load. Depending on internal power supply shutdown simply is simply not safe, even if you know exactly how the supply is designed.

But setting all of this aside, the question we are most recently addressing concerns the effect of the phantom supply being terminated in moisture, and whether it was a greater threat to the mixer or to personnel. Since the mixer is obviously designed to withstand its own phantom supply even if the input is indeed shorted, the only remaining issue would be safety.

The best advice remains: keep electrical systems away from water.

-- Jeff

[EmilBarnabas]

More importantly, be sure your generator is properly grounded. This is usually done with a wire connected to the frame of the generator and a copper rod driven six feet into the ground. Local electrical codes may vary on the length of the rod, wire size, etc.

Also, make sure the electrical outlets on the generator are GFCI protected. If you use a remote power distribution panel, make sure those electrical outlets are GFCI protected.

A properly grounded and GFCI protected power system should eliminate most of your electrical/water concerns.

Can you keep the box end of the snake in a plastic box? That should keep it from getting wet and keep people from accidentally touching it.

[AVtechie]

I'd just like to point out that that the GFIC is a "last ditch" safety backup. You should never depend upon it working to provide safety. As DTV-Engineer stated: "keep electrical systems away from water." I would modify his #1 comment to: "From a safety point of view, assuming that a voltage source won't withstand a short is bad practice."

You should always have multiple layers of protection, such that any one failure does not result in an unsafe system. And you must always make sure that all of the layers of protection are working properly all of the time. Water is especially bad because I could certainly envision a case where water in the wrong place could bypass multiple safeties at once.

On a side note, old phantom power systems were often +48V power supplies with a current limiting resistor in series. The resistor was sized to keep the current near the nominal specs, even if shorted to ground. It was intended to be bullet-proof (read "roadie") so the electrical guys wouldn't have to replace fuses multiple times per day.

[technerd]

Just make sure you ground all AV gear to code by a PRO. Balance signal feeds correctly. Throw away 49 cent ground lifts. That will get someone killed.
I find many do-it-yourself church member installs have floating grounds everywhere. When I get calls for bad hums in systems, I have read as high as 60 volts on baptistry gooseneck mics when I put a meter on it to a copper pipe. When churches add another building to the existing one I have found that the back of the church where the front of house board is and the stage area are on separate feeders coming in from the power company. With 2 separate ground rods outside. That is a hazard for the media. Amp racks hidden in the back of stages are forever wired on different phases from the front of house gear. You can loose some members that way!