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Old 09-19-13, 07:12 AM   #41
stevehull
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Originally Posted by NeilBlanchard View Post
You can be shocked with any electrical system. Why are solar PV panels any different?
Here is something that I can address with knowledge and first hand experience.

AC (50 or 60 Hz) is FAR more dangerous to the human body that the same voltage with DC (up to a point).

DC shock causes thermal burns, but it must break down the insulation value of the skin. Sweat does this (salty conductive fluid), but you can easily grab both terminals of a 12 V auto battery and get no shock. Somewhere in excess of 200-300 V DC, the current leak through the skin becomes clearly noticeably and you get local heating and spot burns (third degree). Clearly depends on the current. Just a few amps will not do much.

DC potential higher than the above will cause flash burns and significant thermal injury. Very, very ugly . . .

By contrast, AC current is a killer, both at lower voltages and at far, far lower currents. AC current passes through the skin far more easily than DC and only mA of AC current will cause skeletal muscles to either violently contract or spasm. In the case of the heart, a lethal electrical condition called ventricular fibrillation (VF) occurs. Without immediate defibrillation, brain death occurs as the heart is not pumping. Not ugly, just a dead firefighter with no external signs of any burn or shock.

Two scenarios - both grid tied; one with a central inverter and another using microinverters. One has a large DC buss on the roof that is energized (during the day) regardless of the AC main power being off or on. This DC potential is very unlikely to cause a firefighter any trouble.

The second situation (microinverters) presents large potential AC with large current loads on the roof. Spraying water here can cause shock (if wires are melting exposing current), but removal of the AC main shuts them down (as mentioned again and again).

Spraying water on a hot AC circuit is something firefighters deal with ALL the time. They kill the power to the structure and they go at it. Once power is off to a PV system, the microinverters are off, the central inverter is off and you are left with DC. If microinverters, then the individual panel DC is low (not much more than car battery) or if panels in series, then up to a couple hundred V of DC.

But either is not especially dangerous.

Non grid tied gets problematic as there is no AC main to turn off the system. If a central inverter, then you could be chopping, spraying water on a 120 or 240 V AC main within the structure with high currents or have a less dangerous large DC voltage (and current) on the roof to contend with. The firefighter doesn't know.

That is why I feel a central roof mounted highly visible disconnect adjacent to the panel system is important so that all (regardless of training on DC, AC, grid tied or battery back up) PV installations can be fought equally - and quickly.

I don't want a firefighter to be calling to the central station talking back and forth to his (or her) chief, trying to describe what they have (or don't have) on the roof if they see a PV system.

What shocked me was the disinformation that firefighters are getting (such as PV panels are always on, they are energized for 7-10 days even with no sunlight, etc).

As has been mentioned, firefighters are professionals, but they (and we) are best protected if they can deal ONLY with the fire and can quickly remove any electrical current issues from consideration. That is why I like the roof mounted, code enforced disconnect switch (even if it is redundant with AC main being turned off).

Is this a large expense? No.

Steve

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Old 09-19-13, 09:42 AM   #42
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Steve,
Thanks for the info.
One thing, I know when I installed my intertie system that the electrical inspector was adamant about a shut off that is labeled and easily accessible from the ground, not on the roof and also adamant about metal conduit to the box from the modules.

My off grid system is on poles and there is a combiner box on each pole with a breaker that can be turned off.

Rob
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Old 09-19-13, 10:29 AM   #43
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Steve,
Thanks for the info.
One thing, I know when I installed my intertie system that the electrical inspector was adamant about a shut off that is labeled and easily accessible from the ground, not on the roof and also adamant about metal conduit to the box from the modules.
Rob
Both superb ideas.

Steve

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