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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: Mike Sokol on January 27, 2014, 01:06:27 pm

Title: Ground Continuity Current Testing
Post by: Mike Sokol on January 27, 2014, 01:06:27 pm
Here's a heads-up on a ground continuity current tester I've developed for the RV industry. It uses the 12-volt battery in the camper/trailer to send a 2-amp DC "fault" current though all the various bonding points and connectors. With a bit of reworking, it could probably be redesigned to work for the audio industry as well. The beauty of the 12-volt DC test is that it won't impose potentially shocking voltages on gear under test if the safety ground fails due to corrosion or loose connections. The bulb allows you to check for intermittent connections by flickering or dimming while you wiggle all the cables and connectors. The reason that a traditional ohmmeter sometimes fails on this test is that it only sends micro-amps of current for the test, while 2-amps is enough to created a significant voltage drop with any dodgy connections.

Take a look and tell me what you think...

Title: Re: Ground Continuity Current Testing
Post by: John Roberts {JR} on January 27, 2014, 03:09:33 pm
The Chinese battery logo bothers me...  :P

2a @ 12V can generate 24 watts of heat, while the light bulb in series will form a divider to prevent more than maybe half that to develop across some flaky wiring load. Even half that power concentrated into a small enough area could be a fire hazard. This test, especially if the lamp only lights dimly should NOT be left connected.

I have been thinking about probing for ground integrity with a HF signal to perhaps parse out a local bootleg ground from a valid long wired safety ground path, but this os low priority back burner stuff...  I still like the idea of coming up with idiot proof back line power strips and the like, that can detect all the common threats, to protect and serve the meat puppets.

JR
Title: Re: Ground Continuity Current Testing
Post by: Mike Sokol on January 27, 2014, 03:43:25 pm
The Chinese battery logo bothers me...  :P

2a @ 12V can generate 24 watts of heat, while the light bulb in series will form a divider to prevent more than maybe half that to develop across some flaky wiring load. Even half that power concentrated into a small enough area could be a fire hazard. This test, especially if the lamp only lights dimly should NOT be left connected. 

JR

The reason why I'm experimenting with this test is that new RVs from the factory are only high-pot tested, never ground continuity tested. And I've had dozens of emails from RV owners who were shocked from their RV and discovered their generator transfer switch or shore power connector was mis-wired at the factory or by a dealer. I think this test could really help sniff out the poor and intermittent connections. Here's the consumer version which plugs into a 12-volt lighter outlet in the RV. No Chinese brand battery visible..  :o
Title: Re: Ground Continuity Current Testing
Post by: John Roberts {JR} on January 27, 2014, 04:23:20 pm
I still repeat my concerns... If the lamp only lights dimly, DO NOT leave it connected, or look for smoke from whatever is dropping the rest of the voltage. Of course no light means no current and no fire hazard, only the mid-impedance situation. Something somewhere could get pretty hot.  :o

JR

Note: IC lamp impedance is not linear so won't be a very well behaved divider with anything other than another lamp. 
Title: Re: Ground Continuity Current Testing
Post by: Frank Koenig on January 27, 2014, 04:51:55 pm
An incandescent lamp can serve as a powerful piece of test equipment, at least in relation to its cost and complexity. I carry a 12 volt lamp furnished with clips in my vehicles. With a little creativity it can be used to troubleshoot most problems.

I first became aware of this as a child when we would go to an "electric" shop in downtown Palo Alto -- a sort of Norman Rockwell memory at this point. They would actually REPAIR home appliances like toasters and vacuum cleaners. The first test to which they would subject any new arrival was to plug it into a special receptacle on their test bench that was wired in series with a lamp in a porcelain lampholder on the adjacent wall. From that they could identify shorts, opens, faulty switches, and, I presume, a variety of ills from knowing the "bulb signature" of the appliance.

Sometimes you need a $20000 network analyzer, sometimes a $300 voltmeter, and often a bulb will do.

--Frank
Title: Re: Ground Continuity Current Testing
Post by: Jonathan Johnson on January 27, 2014, 05:02:33 pm
The Chinese battery logo bothers me...  :P

And failing to remove the shipping caps from the battery is going to cause a problem, too.  :)
Title: Re: Ground Continuity Current Testing
Post by: John Roberts {JR} on January 27, 2014, 05:20:07 pm
An incandescent lamp can serve as a powerful piece of test equipment, at least in relation to its cost and complexity. I carry a 12 volt lamp furnished with clips in my vehicles. With a little creativity it can be used to troubleshoot most problems.

I first became aware of this as a child when we would go to an "electric" shop in downtown Palo Alto -- a sort of Norman Rockwell memory at this point. They would actually REPAIR home appliances like toasters and vacuum cleaners. The first test to which they would subject any new arrival was to plug it into a special receptacle on their test bench that was wired in series with a lamp in a porcelain lampholder on the adjacent wall. From that they could identify shorts, opens, faulty switches, and, I presume, a variety of ills from knowing the "bulb signature" of the appliance.

Sometimes you need a $20000 network analyzer, sometimes a $300 voltmeter, and often a bulb will do.

--Frank
It is common industry practice to wire a 100W light bulb in series with power amp test stations. If there is a short circuit the lightbulb will light up and actually current limit at low single digit amps. If the lightbulb stays off, it is safe to apply full power.

JR

PS: I wonder what they will use when IC light bulbs are obsolete.
Title: Re: Ground Continuity Current Testing
Post by: Mike Sokol on January 27, 2014, 05:43:49 pm
It is common industry practice to wire a 100W light bulb in series with power amp test stations. If there is a short circuit the lightbulb will light up and actually current limit at low single digit amps. If the lightbulb stays off, it is safe to apply full power.

I started doing this on cars back in the 70's by clipping a 12-volt brake light bulb across the "dead" fuse to find a short in the wiring. When the bulb was dim, the radio was drawing a little power. When the build went bright, there was a short in the wiring. By flexing everything you can make the short come-and-go, which normally would be blowing the fuse, but which was current limited to 2 amps by the bulb. I built one of these gadgets last year for my kid's car when he kept blowing fuses. It's still a great idea that's cheap to build.

Quote
  PS: I wonder what they will use when IC light bulbs are obsolete.

I've actually thought about this. You just need a 20-watt resistor of the correct value (around 100 ohms) with an LED in parallel with it (don't forget the 1,000 ohm series resistor on the LED to keep from blowing it up). I'm going to build a ruggedized version of my tester for demonstrations, but the brake light bulb is easily understood by electricians and RV technicians.

I am taking JR's comments seriously. You can certainly make some heat with this thing, so it's definitely not a "walk-away" kinda test.   
Title: Re: Ground Continuity Current Testing
Post by: Mike Sokol on January 27, 2014, 06:06:12 pm
And failing to remove the shipping caps from the battery is going to cause a problem, too.  :)

Good golly, this is a tough room, isn't it?  ::)
Title: Re: Ground Continuity Current Testing
Post by: Stephen Swaffer on January 27, 2014, 08:46:51 pm
I think it would be valuable to be able to switch between a lamp and a fixed resistor with a dvm in parallel to allow you to quantify a reading.  Maybe that is over thinking its use, but I am thinking a .5 ohm resistance will only give 1 V drop and, especially in a bright setting, it may be difficult to notice the difference between a 12 V lamp at 12 V and at 11 V?  Especially without a side by side comparison.  That .5 ohms at a full 50 amps could translate into a 25 V drop.

But then I like working with numbers!
Title: Re: Ground Continuity Current Testing
Post by: Mike Sokol on January 27, 2014, 09:50:40 pm
I think it would be valuable to be able to switch between a lamp and a fixed resistor with a dvm in parallel to allow you to quantify a reading.  Maybe that is over thinking its use, but I am thinking a .5 ohm resistance will only give 1 V drop and, especially in a bright setting, it may be difficult to notice the difference between a 12 V lamp at 12 V and at 11 V?  Especially without a side by side comparison.  That .5 ohms at a full 50 amps could translate into a 25 V drop.

But then I like working with numbers!

Guys, don't over-think this. I'm not trying to get accurate resistance numbers, just testing an RV's shore power system to see if its ground fault path can carry enough current to successfully trip a 20-amp circuit breaker. To that end, I'm sure a bulb will be a good enough intermittent indicator, especially if you combine it with a voltmeter.

As I noted in my original post, this circuit is essentially one half of a Kelvin 4-wire Bridge with separate current and voltage paths used to measure a resistance by nulling it against a known calibrated resistance. Even with very long test wires, a Kelvin Bridge can easily measure down to fractions of an ohm resistance with ppm accuracy. That could be done with this half-bridge if we used a calibrated resistor instead of a bulb, but again we're only looking for under 1 ohm resistance to be code compliant. The general grounding failures in an RV are due to loose frame bolts, contact corrosion, and broken wires in dog-bone/pig-tail adapters used to connect to various campground pedestal outlets.

Once it gets warm outside I'm going to do some testing in various real-world scenarios and see how well it works.
Title: Re: Ground Continuity Current Testing
Post by: Frank DeWitt on January 27, 2014, 10:07:37 pm
I like it.  If you are concerned about the leave it connected start a fire issue then put a push button switch is series and make it a switch that requires a lot of pressure. 
Title: Re: Ground Continuity Current Testing
Post by: Lyle Williams on January 27, 2014, 10:20:54 pm
I like it.  If you are concerned about the leave it connected start a fire issue then put a push button switch is series and make it a switch that requires a lot of pressure.

If you don't want it to stay connected don't put a spring clip on the end!
Title: Re: Ground Continuity Current Testing
Post by: Stephen Swaffer on January 27, 2014, 10:58:49 pm
Or maybe a timer of some sort-that would let you keep your hands free to work connections/harnesses to find intermitents.

As to overthinking-I guess I was thinking more along the lines of general use.  I don't do much RV work, but I can think of a lot of places where testing with more than mA would be helpful-from trailer lights to grounding on AV equipment to general equipment troubleshooting.  I repaired a motor starter on a grain bin today (in -3 actual temp, - double digits windchill) where the fault would never had showed with an ohmmeter.  Common wire was touching frame-actually held by a screw that had arced in two-I noticed it arcing under load-a high current ohm test should have showed something most likely.  (And yes, it was using 240 volt 3 wire, with the same wire providing ground as well as neutral for control wiring-consider the location.)
Title: Re: Ground Continuity Current Testing
Post by: Mike Sokol on January 28, 2014, 08:06:03 am
If you don't want it to stay connected don't put a spring clip on the end!

The idea of the spring clip is to allow you to wiggle all the connections looking for an intermittent open. That's one of the biggest problems with RV wiring since there's so much vibration and salts from road travel. Often the hot-ground issues seem to come and go randomly, but I think a current load test will help find those intermittent open circuits.
Title: Re: Ground Continuity Current Testing
Post by: Lyle Williams on January 28, 2014, 02:50:42 pm
The idea of the spring clip is to allow you to wiggle all the connections looking for an intermittent open. That's one of the biggest problems with RV wiring since there's so much vibration and salts from road travel. Often the hot-ground issues seem to come and go randomly, but I think a current load test will help find those intermittent open circuits.

I don't mind the spring clip.  I just thought the simplest solution for those who were worried about a long term connection would be to just chop the thing off!  :-)

The Australian Standard for routine testing of portable appliances gives a few options for earth continuity testing.
  1. Just a VOM.
  2. 12v/200ma.
  3. 12v/10a.
  4. 12v and the greater of 1.5x appliance rating and 25a.
My read on this is higher currents are preferred, but they still want tests done by people who don't have access to high current testers.

I agree that there are some combinations of faults and poor wiring practices that could result in heat, smoke or even fire if exposed to 25a.  Isn't that the point of testing though?  Find the fault while you are looking for one (and can put out the fire etc) ...
Title: Re: Ground Continuity Current Testing
Post by: Mike Sokol on January 28, 2014, 04:13:30 pm
I agree that there are some combinations of faults and poor wiring practices that could result in heat, smoke or even fire if exposed to 25a.  Isn't that the point of testing though?  Find the fault while you are looking for one (and can put out the fire etc) ...

In the states we call that a "smoke test".  ;D
Title: Re: Ground Continuity Current Testing
Post by: John Roberts {JR} on January 28, 2014, 05:22:42 pm
You could sense for voltage at the low side of the light bulb. If it is low volts, that is OK, or if it high volts, that's OK (for not starting a fire)... Mid volts is the dangerous scenario.

A resistor feeding the base of a transistor from the bottom of the lamp lead, will turn in the transistor (driving a LED?), if the ground voltage is more than .5V or so. At 2A, .5V is generated across 250 mOhms. You could set this threshold higher if you think 1/4 ohm is a low enough impedance ground path. 

JR
Title: Re: Ground Continuity Current Testing
Post by: frank kayser on January 28, 2014, 06:08:27 pm
In the states we call that a "smoke test".  ;D

Heck, Mike.  That's what I call the point where I power up the sound system for the night!   ;D
Title: Re: Ground Continuity Current Testing
Post by: Mike Sokol on January 28, 2014, 08:10:42 pm
Heck, Mike.  That's what I call the point where I power up the sound system for the night!   ;D

Indeed...  8)

Be careful not to release the magic smoke as that's what makes the electronics work. ;)  Of course, you can get a Lucas Smoke recharging kit if you really need it. http://www3.telus.net/bc_triumph_registry/smoke.htm (http://www3.telus.net/bc_triumph_registry/smoke.htm)

Title: Re: Ground Continuity Current Testing
Post by: frank kayser on January 28, 2014, 09:03:10 pm
Indeed...  8)

Be careful not to release the magic smoke as that's what makes the electronics work. ;)  Of course, you can get a Lucas Smoke recharging kit if you really need it.

Had me laughing out loud with that one.  Praise be, Lucas, Prince of Darkness (and the reason the British drink warm beer)
Title: Re: Ground Continuity Current Testing
Post by: Chris Hindle on January 29, 2014, 08:25:21 am
Indeed...  8)

Be careful not to release the magic smoke as that's what makes the electronics work. ;)  Of course, you can get a Lucas Smoke recharging kit if you really need it. http://www3.telus.net/bc_triumph_registry/smoke.htm (http://www3.telus.net/bc_triumph_registry/smoke.htm)
Un-fxxxxn-believable.
That's priceless.
In my early days of auto mechanics, I learned to loath MG and Triumph electrical systems.
(Even in the mid 80's they were still known an "Lucas Electric". God knows what they are calling themselves today)
Very old mechanics joke..... Why do Englishmen like warm beer ?  They have Lucas refrigerators.
Chris
Title: Re: Ground Continuity Current Testing
Post by: Lyle Williams on January 29, 2014, 04:49:02 pm
On day one of a new university job, a friend (smart guy, with a PhD) was sent down to stores for "a long stand".  You know the way this gag is meant to play out.

Anyway, instead he goes to the pub for most of the day and comes back to the office late in the afternoon saying "they searched for hours, but couldn't find a long stand".  The guys who sent him on the fool's errand knew they had been played, but really couldn't complain.  :-)