Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding
Meter and test every show
Kemper Watson:
Provided for a show yesterday in North Atlanta.. Stage and generator are provided by the same guy every year and I noticed he had brand new spider boxes and cabling. Just out of curiosity I metered the outlets. All was fine. Then I plugged in the polarity checker. There was a hot-neutral swap somewhere. He took apart the BRAND NEW California connectors and found not only a miswired connector but a messy job as well. He spent an hour taking apart and reconnecting his cables.. Meter and check every show..
Mike Sokol:
--- Quote from: Kemper Watson on October 13, 2013, 11:50:46 AM ---Meter and check every show..
--- End quote ---
I agree 100%. It only takes a few minutes to meter the power, and that can save you thousands of dollars in electrical damage. For instance, I did a seminar a few years back in a gymnasium at a big church, and was told to plug in my PA and video system anywhere. Well, I found a brand new receptacle on the back of the stage which was close to my rack, so I simply plugged in. My rack had a Furman power distro feeding most of the gear, but I had recently added an extra wireless mic receiver and didn't have enough outlets on the Furman distro, so I foolishly put a power strip on the floor BEFORE the rack and plugged in the extra RF receiver. When I plugged into the outlet, I noticed two things. The Sennheiser RF receiver display got VERY BRIGHT for a few seconds and then went out. And the voltage monitor bar on the Furman distro was pegged to the right. That prevented me from flipping on the power to the rest of the rack. I then metered the stage outlet and found what appeared to be a standard 15-amp 120-volt Edison receptacle had 240-volts on it. When I complained to the church maintenance guy, he said I must have plugged into the "special outlet" he had rewired to 240-volts for the floor buffer. Yikes!!! Of course it's a code violation to wire any 120-volt outlet to 240-volts, and certainly crazy not to mark it as 240-volts. But there it was, and it blew up a $1,000 receiver in seconds. After that, I began checking voltage and grounding at all new sites I work, especially when there's a new outlet in and old building. That's a danger sign especially in a church since a lot of volunteers and maintenance guys do these outlet "upgrades" and often don't understand the electrical code or how to measure voltage.
Frank Koenig:
Metering power before plugging in is like checking the fuel quantity on an airplane before departure. Just do it, even if you "know" it's OK.
I have an observation on using a regular volt meter, as opposed to a non-contact "AC voltage checker", for verifying that grounds are not hot. I do not suggest that anyone do this as a matter of course, as it's a little non-standard and potentially hazardous, and put it out here for the amusement of folks who have more of an experimental mind set when it comes to electricity.
Get up on your rubber soles and grab one probe of the meter. You have now become a capacitively-coupled ground reference to the universe (or at least your immediate vicinity). The other probe can now be used to check that equipment grounding conductors and grounded circuit conductors are grounded, and that hots are hot. If your meter is healthy you're connecting yourself to the line through a 10 M Ohm resister, so you won't die, even if you're grounded. (Hell, you won't even trip the GFI.) The reason for getting up on your soles is in case the meter is internally shorted or you plugged the leads into the current hole by mistake.
Actually, you could make up a special lead with a, say, 10 M Ohm, high-voltage resistor in series. With triple redundancy (meter, external resistor, and dry rubber soles) this would be pretty safe.
Like I said, only by qualified personnel, laboratory use only, etc.
--Frank
Mike Sokol:
--- Quote from: Frank Koenig on October 14, 2013, 10:37:48 AM ---
Get up on your rubber soles and grab one probe of the meter. You have now become a capacitively-coupled ground reference to the universe (or at least your immediate vicinity). The other probe can now be used to check that equipment grounding conductors and grounded circuit conductors are grounded, and that hots are hot. If your meter is healthy you're connecting yourself to the line through a 10 M Ohm resister, so you won't die, even if you're grounded. (Hell, you won't even trip the GFI.) The reason for getting up on your soles is in case the meter is internally shorted or you plugged the leads into the current hole by mistake.
--Frank
--- End quote ---
While this does work, and we used to do this with a neon bulb back in the "old days", I think it's way too dangerous for the casual user (as you note in your post). Just spend the $20 and get a NCVT such as a Klein NCVT-1 or Fluke VoltAlert. I'm also an advocate for using a NCVT on a newly wired stage as a quick hot-ground check on backline guitar amps and such. Simply walk around the powered-up stage with your NCVT and touch the microphones and guitar/stage amps. If it lights up and beeps, that's the warning sign that something is not grounded properly. I've had my microphones blamed for shocking the guitar players, when in fact he had an old amp he just bought at a pawn shop without a ground pin on the plug, and he was getting shocked from his amp. But because he felt the shock on his lips, the mic (and my sound system) was blamed for the shock.
Many years ago I had the fun ??? of doing sound for Chumbawumba on the roof of a garage in Wash DC. This was an unlicensed event that supposed to get them arrested as a publicity stunt. Getting more fun, isn't it. Well there was a contractor generator down on the ground feeding our power distro up on the roof, and while metering the power I measured 90 volts between the "ground" of the PA system on the roof, and the "grounded" safety rail around the roof. Of course, we had built a stage to get the band up to the level of the safety rail, and I was worried that a guitar player getting a shock between his guitar and the rail might take a plunge off the roof to the street some 40 feet below. A little too much publicity for my taste, so I did some troubleshooting and found that the generator down on the ground didn't have a ground rod. Running out of time before the show, I found a piece of rebar, drove it into the dirt next to the generator, and used a pair of vise grip pliers to bond it to the generator frame with a short run of heavy wire. Yes, I taped the vice grip handle to make sure it didn't shake loose, and it properly eliminated the hot ground on the stage up above. But without metering the power distro, I could have been involved in something very bad with potential litigation. I ALWAYS test outside stages with generators for proper grounding. Too many things to go wrong otherwise.
John Roberts {JR}:
--- Quote from: Frank Koenig on October 14, 2013, 10:37:48 AM ---Metering power before plugging in is like checking the fuel quantity on an airplane before departure. Just do it, even if you "know" it's OK.
I have an observation on using a regular volt meter, as opposed to a non-contact "AC voltage checker", for verifying that grounds are not hot. I do not suggest that anyone do this as a matter of course, as it's a little non-standard and potentially hazardous, and put it out here for the amusement of folks who have more of an experimental mind set when it comes to electricity.
Get up on your rubber soles and grab one probe of the meter. You have now become a capacitively-coupled ground reference to the universe (or at least your immediate vicinity). The other probe can now be used to check that equipment grounding conductors and grounded circuit conductors are grounded, and that hots are hot. If your meter is healthy you're connecting yourself to the line through a 10 M Ohm resister, so you won't die, even if you're grounded. (Hell, you won't even trip the GFI.) The reason for getting up on your soles is in case the meter is internally shorted or you plugged the leads into the current hole by mistake.
Actually, you could make up a special lead with a, say, 10 M Ohm, high-voltage resistor in series. With triple redundancy (meter, external resistor, and dry rubber soles) this would be pretty safe.
Like I said, only by qualified personnel, laboratory use only, etc.
--Frank
--- End quote ---
This needs a strong "don't try this yourself' disclaimer.
I have done this myself and it kind of works, but on a public forum how can we be sure that the users will remember to only use voltage measurement ranges that should safely limit current to modest levels. There are too many ways to use a VOM that are not current limited to low mA levels.
So do not do this,,,
Frank knows his way around test equipment and electricity, but another data point from last time I did some research into who actually gets electrocuted. A disproportionate number of deaths are engineers. technicians, and electrical professionals who should know better. Sometimes we get a little blasé about the risks.
JR
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