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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 => Topic started by: Kemper Watson on October 13, 2013, 11:50:46 am

Title: Meter and test every show
Post by: Kemper Watson on October 13, 2013, 11:50:46 am
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..
Title: Re: Meter and test every show
Post by: Mike Sokol on October 14, 2013, 08:47:52 am
Meter and check every show..

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. 
Title: Re: Meter and test every show
Post by: 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
Title: Re: Meter and test every show
Post by: Mike Sokol on October 14, 2013, 11:03:26 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

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.
Title: Re: Meter and test every show
Post by: John Roberts {JR} on October 14, 2013, 11:43:52 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
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 blasť about the risks.

JR 
Title: Re: Meter and test every show
Post by: Mike Sokol on October 14, 2013, 12:00:27 pm
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 blasť about the risks.

JR

Yup... DO NOT DO THIS!!! It's way too dangerous and an unnecessary risk.

We "professionals" do get too casual around live voltage. When I was a young electrical engineer working for a major glass manufacturer, there was an EE in another plant (basically doing the same job as me) who was discussing a problem with management about how an electrician had received a shock from a live 11,000 volt panel because he hadn't tested his gloves properly. The electrician lived, but the engineer didn't because as part of his demonstration to the plant manager he poked the live 11,000 volt bus with a pencil. Of course, graphite is conductive and it killed the engineer on the spot. I got really serious about live electrical panels after that incident.

That's also why I don't let anybody talk to me or distract me in any way when I'm working in a live panel out of necessity. I hate doing that, but sometimes it's the only way to get the job done. However, I then take extra safety precautions including having somebody with a cell phone close by who can call 911 and start CPR if something goes wrong. And I always use my PPE (Personal Protection Equipment). Safety glasses have saved my eyesight on more than one occasion.

There are OLD electricians and BOLD electricians, but very few OLD and BOLD electricians.
Title: Re: Meter and test every show
Post by: Tim McCulloch on October 14, 2013, 12:07:01 pm
Mike, how about a little detour (new topic) on arc flash and PPE?
Title: Re: Meter and test every show
Post by: Frank Koenig on October 14, 2013, 02:11:28 pm
A disproportionate number of deaths are engineers. technicians, and electrical professionals who should know better.

Wise words. I wasn't trying to cause trouble, just generate a little discussion and thinking. And I'm going to get  one of those NCVTs for my kit. -F
Title: Re: Meter and test every show
Post by: Jamin Lynch on October 14, 2013, 03:51:42 pm
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..

HHMMM? Did a licensed electrician change out the wiring or just the guy who brought out the generator? You're gonna burn the place down.
Title: Re: Meter and test every show
Post by: Kemper Watson on October 14, 2013, 04:21:07 pm
HHMMM? Did a licensed electrician change out the wiring or just the guy who brought out the generator? You're gonna burn the place down.

It was the owner of the generator, not "some guy that brought the genny" He went through all the BRAND NEW cables he bought and straightened  the problem out.

Edit: He bought the cables from a new distributor somewhere on line. I had never had a problem with his distro's ever, in 6-7 years of working with him..Stand up guy. He will never NOT check brand new cables again..
Title: Re: Meter and test every show
Post by: Jared Bartimus on October 14, 2013, 04:21:40 pm
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

I don't know about other digital meters but mine (extech ex330) shows 10-20V if you just touch one probe to 120V, the other can be completely disconnected.  The meter has a non contact voltage detector built in but it isn't sensitive enough to pick up which is hot.  I wouldn't want to rely on it 100% but it is safer than using your body as a ground.
Title: Re: Meter and test every show
Post by: Kemper Watson on October 14, 2013, 04:28:45 pm
It was the owner of the generator, not "some guy that brought the genny" He went through all the BRAND NEW cables he bought and straightened  the problem out.

Edit: He bought the cables from a new distributor somewhere on line. I had never had a problem with his distro's ever, in 6-7 years of working with him ..Stand up guy. He will never NOT check brand new cables again..

I don't know why this posted like this.. Delete if necessary
Title: Re: Meter and test every show
Post by: John Roberts {JR} on October 14, 2013, 04:35:17 pm
I don't know about other digital meters but mine (extech ex330) shows 10-20V if you just touch one probe to 120V, the other can be completely disconnected.  The meter has a non contact voltage detector built in but it isn't sensitive enough to pick up which is hot.  I wouldn't want to rely on it 100% but it is safer than using your body as a ground.
Again not really encouraging off-sheet use of test equipment, my cheap rat-shack digital VOM measures 10-15VAC on hot, and around 2.5VAC on neutral, with only one lead touching anything.

Just to be safe remove the other meter lead completely when ignoring our earlier advice to not do this.  ;D

JR

Title: Re: Meter and test every show
Post by: Mike Sokol on October 14, 2013, 05:29:12 pm
Mike, how about a little detour (new topic) on arc flash and PPE?
Good idea. I'm hanging out in Austin Texas teaching seminars for a few days (and supposed to be grading mid-term exams) so it will probably be next week before I have some free time.

Certainly as we work with ever higher amperage distro prower, an arc flash can be even more dangerous than being shocked by 120-volts. The reason is, you can probably have your heart restarted if you're "just" shocked, but if you're up close to an arc flash without your PPE (Personal Protection Equipment), you will probably require skin grafts and rehabilitation if you even survive the explosion of superheated copper plasma coming at you faster than the speed of sound.

That's why you shouldn't work in live electrical panels if you can avoid it. And going into a hot panel "bare back" without proper training and protective gear is an invitation to becoming a very bad looking corpse. I have lots of pictures I'll share next week on a new thread.   
Title: Re: Meter and test every show
Post by: Mac Kerr on October 14, 2013, 05:48:03 pm
Good idea. I'm hanging out in Austin Texas teaching seminars for a few days (and supposed to be grading mid-term exams) so it will probably be next week before I have some free time.

I guess you're not going to any music festivals. ACL looks pretty wet.

Mac
Title: Re: Meter and test every show
Post by: Mike Sokol on October 14, 2013, 06:35:37 pm
I guess you're not going to any music festivals. ACL looks pretty wet.

Mac
Austin had 12 inches of rain here in the last 48 hours. Yikes!!!
Title: Re: Meter and test every show
Post by: Ryan C. Davis on October 14, 2013, 10:57:55 pm
Austin had 12 inches of rain here in the last 48 hours. Yikes!!!

Yep, massive amounts of rain. BUT according to Bob L, new england can just "soak up" 12" of rain. Too bad ACL isn't in Boston.
Title: Re: Meter and test every show
Post by: Jared Bartimus on October 14, 2013, 11:25:59 pm
Again not really encouraging off-sheet use of test equipment, my cheap rat-shack digital VOM measures 10-15VAC on hot, and around 2.5VAC on neutral, with only one lead touching anything.

Just to be safe remove the other meter lead completely when ignoring our earlier advice to not do this.  ;D

JR

I would never recommend doing it as a primary method but if you don't have a non contact meter (get one) and only attach one lead it shouldn't add any other danger.  It is usually something I just notice when checking voltages normally.  If when I make the first connection with a probe it doesn't spike correctly I get suspicious.
Title: Re: Meter and test every show
Post by: Jonathan Johnson on October 15, 2013, 11:49:31 am
It is usually something I just notice when checking voltages normally.  If when I make the first connection with a probe it doesn't spike correctly I get suspicious.

Ah, yes. It's important to be familiar with your equipment and its normal behavior. It's always a good idea to test your test equipment against known conditions before testing unknown conditions -- every time you pull it out of the toolbox.

If you didn't successfully test your meter against a working source of known voltage, don't trust it when it says 0V. A broken lead can kill you.
Title: Re: Meter and test every show
Post by: John Roberts {JR} on October 15, 2013, 12:39:36 pm
While this is somewhat related, we need to careful even when metering an outlet for voltage. If the VOM is in some other range than AC voltage, it is possible that the meter with one lead plugged into hot could pass lethal current to the other lead.

Good practice would be to plug into the ground or neutral first, and not touch metal on any loose lead ever.

JR

PS: I am not aware of anyone getting hurt this way, but it could happen so is worth mentioning.
Title: Re: Meter and test every show
Post by: TJ (Tom) Cornish on October 15, 2013, 12:59:06 pm
While this is somewhat related, we need to careful even when metering an outlet for voltage. If the VOM is in some other range than AC voltage, it is possible that the meter with one lead plugged into hot could pass lethal current to the other lead.

Good practice would be to plug into the ground or neutral first, and not touch metal on any loose lead ever.

JR

PS: I am not aware of anyone getting hurt this way, but it could happen so is worth mentioning.
And even more so if the leads are in the current shunt terminals.
Title: Re: Meter and test every show
Post by: brian maddox on October 15, 2013, 04:29:42 pm
Just wanted to take a moment to reinforce the central premise of this thread.  Meter.  Always.  Every time. Hot to Neutral.  Hot to Ground.  Neutral to Ground.

Everybody that has been doing this for any length of time has encountered something truly stupid regarding power.  The worst i ever experienced was 60V between neutral and ground on an outdoor tie-in with feeder that disappeared into the distance and went who knows where.  Took me over an hour to get the 'house electrician' to track down the issue and fix it.

Location?  None other than the White House Rose Garden.  Even the electrician for the freakin' PRESIDENT can get this stuff wrong.....
Title: Re: Meter and test every show
Post by: Mike Sokol on October 15, 2013, 05:17:59 pm
PS: I am not aware of anyone getting hurt this way, but it could happen so is worth mentioning.

Now matter how much I try to idiot proof something, there always seems to be better idiots coming along.
Title: Re: Meter and test every show
Post by: John Roberts {JR} on October 15, 2013, 06:50:06 pm
Now matter how much I try to idiot proof something, there always seems to be better idiots coming along.
  I used to design products for peavey customers.  :o

I believe there is a natural law, that no matter what you do to idiot-proof a design or procedure, the idiots will adapt and evolve to overcome said prophylaxis.

JR
Title: Re: Meter and test every show
Post by: Frank Koenig on October 15, 2013, 08:20:12 pm
Mike, how about a little detour (new topic) on arc flash and PPE?

And if we're requesting topics, how about a discussion of the interrupting current rating of circuit breakers? 10 kA and 22 kA are common ratings for small beakers. When is it OK to use one or the other, or neither?
-F
Title: Re: Meter and test every show
Post by: Mike Sokol on October 15, 2013, 09:04:59 pm
And if we're requesting topics, how about a discussion of the interrupting current rating of circuit breakers? 10 kA and 22 kA are common ratings for small beakers. When is it OK to use one or the other, or neither?
-F

How about I create a sticky topic just for suggested new threads? That way if someone  watching the forum sees something in the suggested topic area they want to elaborate on, they can start the new thread and we can run with it. And no, JR can't start a thread about "better idiots" as much as that would be relevant. Or is that a good idea? 
Title: Re: Meter and test every show
Post by: Jared Bartimus on October 16, 2013, 11:47:21 am
Everybody that has been doing this for any length of time has encountered something truly stupid regarding power.  The worst i ever experienced was 60V between neutral and ground on an outdoor tie-in with feeder that disappeared into the distance and went who knows where.  Took me over an hour to get the 'house electrician' to track down the issue and fix it.

It has been a while since I researched it but I believe that is either due to unbonded neutral/ground or completely disconnected ground somewhere upstream combined with some devices leaking 60V to ground.  Definitely don't mess with the panels yourself (although it might be worth it to walk along your feeder cable and make sure nothing is messed up) but it might save a little time to mention the possibility to some of the less competent electricians out there.
Title: Re: Meter and test every show
Post by: Mike Sokol on October 16, 2013, 07:58:42 pm
  I used to design products for peavey customers.  :o

I believe there is a natural law, that no matter what you do to idiot-proof a design or procedure, the idiots will adapt and evolve to overcome said prophylaxis.

JR

So it's an evolutionary thing, like cockroaches getting resistant to DDT? I suspected as much...