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Author Topic: brain storm an optimal human safety system for back line  (Read 85462 times)

John Roberts {JR}

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brain storm an optimal human safety system for back line
« on: December 12, 2014, 02:50:01 pm »

------clipped from earlier post-------

I have been wrestling with are Safety grounds good or bad conundrum where even a guitar player with a GFCI protected amp can get killed by hot ground external to his protected amp via the good amp safety ground (It actually happens).

I have looked into mA level fuses and apparently they made from fairy farts since they are insanely expensive (probably a medical equipment thing).

I figure I could stack a N-ch and P-ch MOSFET in series turned on to make a ground path with around 10 ohms on resistance, that I could open up electronically when I sense X mA of ground path current. The <$2 parts cost is much more reasonable than the crazy medical equipment fuse cost.  But the catch 22 is I need a safe isolated power supply, so to add a transformer or lump pushes the cost way up... BUT a tired 9V battery will still make around 7V which should be enough to turn on the two MOSFETs.

I would be crazy to sell these because somebody would probably sue me, but I am tempted to build one to see if it works.

-----end clip------

OK As I get older I get a little crazier but I am not beyond sussing this out.

#1 I found one US company making and selling the cheap in-line GFCI that sells for less than $30 in Lowes hardware chain. I haven't taken it apart yet (it uses an odd security screw holding the plastic clamshell package together).

#2 thinking about it some more I don't like the tired battery idea, 7V is marginal for turing on the MOSFETS hard and a PIA to keep fresh.
  A) one alternative is to incorporate this into a guitar pedal that already has a 9V supply. The incremental cost is now in the <$2 ballpark. A friend of mine makes and sells guitar pedals so I may bounce this idea pff him.
  B) If we build this into a smart GFCI power strip maybe include a PS and relay. Clearly an order of magnitude more parts cost but nothing says disconnected ground like a relay.

#3 What are the anticipated fault conditions we want to protect against that a stock GFCI outlet does not already cover?
   A) And this is the big one a properly ground bonded guitar amp played by a musician who encounters a hot ground microphone. The fix for this is opening up the ground connection "and" turning off power to the guitar amp, just in case.
   B) RPBG... perhaps a non contact sensor could detect if the ground is hot... This might also trip if a floating ground is measuring hot, perhaps a small shunt could prevent the false positive. Perhaps it is OK to not work unless it detects at least some impedance at the ground and no significant voltage.
   c) other???

Any thoughts.

JR
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Chris Hindle

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Re: brain storm an optimal human safety system for back line
« Reply #1 on: December 12, 2014, 03:38:49 pm »



#2 thinking about it some more I don't like the tired battery idea, 7V is marginal for turing on the MOSFETS hard and a PIA to keep fresh.
  A) one alternative is to incorporate this into a guitar pedal that already has a 9V supply. The incremental cost is now in the <$2 ballpark. A friend of mine makes and sells guitar pedals so I may bounce this idea pff him.
  B) If we build this into a smart GFCI power strip maybe include a PS and relay. Clearly an order of magnitude more parts cost but nothing says disconnected ground like a relay.

....Any thoughts.

JR
Build it into a Phantom powered DI ?? Still use the mic, but plug the lead through the "protected" DI. Many times I'll throw a DI on a guitar for backup if I am less than 100% confident his rig will make it through the night. Saved an embarased player more than once with this "trick"....
As a system owner/operator, I am more likely to "spread the protection" around than counting on (cheap-ass ::)) guitar players to protect themselves.
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #2 on: December 12, 2014, 05:05:27 pm »

Build it into a Phantom powered DI ?? Still use the mic, but plug the lead through the "protected" DI. Many times I'll throw a DI on a guitar for backup if I am less than 100% confident his rig will make it through the night. Saved an embarased player more than once with this "trick"....
As a system owner/operator, I am more likely to "spread the protection" around than counting on (cheap-ass ::)) guitar players to protect themselves.

My judgement is that it is easier to protect one guitar than tens of mic inputs, but this does argue for plastic mic handles and maybe even non-conductive mic screens.

I need to dig a little deeper into cost of low mA fuses. I guess a 10mA fuse which isn't as expensive as 2 mA or 5 mA fuses, in series with XLR pin 1 could still work with most phantom powered mics and probably not kill the average musician in case of an encounter with mains voltage. In theory a phantom input with both pin 2 and 3 shorted is more than 10 mA but that is not typical current draw. 10mA is higher than I like but not crazy, and better than nothing for PA guys who want to be proactive. .

JR

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Mike Sokol

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Re: brain storm an optimal human safety system for back line
« Reply #3 on: December 12, 2014, 05:44:02 pm »

JR,

Now I've not costed this idea out, nor even really looked at the math to know if this would work, so this is napkin engineering at it's finest. What if you put a DPST NO latching reed relay in series with the shield and hot of the guitar cable? These latching relays can be latched using a external permanent magnet, but unlatched with a relatively small current (DC, I think). Now, put a current transformer to monitor any 60 Hz AC current in the shield. In fact, run both the shield and signal conductor through the current transformer so it's only looking for current external to the guitar's signal. So would there be sufficient sensing voltage/current from the sensing transformer to directly trip and unlatch the relay, maybe with a diode to rectify it? Would this system be sensitive enough to trip at 10 mA fault current? If not, then something as simple as a 741 Op-Amp or other comparator IC to do the tripping, but that would need a battery or other power source which I'm not in love with. Now, once the latching relay trips, you could then push a button to move a permanent magnet close to the latching relay to reset it.

Again, don't beat on me if I've missed something by a few orders of magnitude or even thinking about this completely backwards, but I like the concept of this safety device being self powered, reset with a permanent magnet, opens up both the guitar shield and signal lines at the same time, and doesn't introduce any semiconductors into the guitar signal path to mess with their "sound".

Thoughts? Comments? Critiques?
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Robert Lofgren

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Re: brain storm an optimal human safety system for back line
« Reply #4 on: December 12, 2014, 06:01:41 pm »

   B) RPBG... perhaps a non contact sensor could detect if the ground is hot... This might also trip if a floating ground is measuring hot, perhaps a small shunt could prevent the false positive. Perhaps it is OK to not work unless it detects at least some impedance at the ground and no significant voltage.
   c) other???

Any thoughts.
I've been thinking of a smallish socket tester that includes an antenna. Connect some opamp/optocoupler to each pin on the mains socket. If the pin is hot then it will give us big noise on the opamp and we will be able to detect the level of each pin in the socket. A simple comparator should do the trick.

I think that the current transformer sounds like a great idea and I had similar thoughts as well.
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #5 on: December 12, 2014, 06:20:48 pm »

JR,

Now I've not costed this idea out, nor even really looked at the math to know if this would work, so this is napkin engineering at it's finest. What if you put a DPST NO latching reed relay in series with the shield and hot of the guitar cable? These latching relays can be latched using a external permanent magnet, but unlatched with a relatively small current (DC, I think).
This is definitely at the napkin stage so all ideas are game.

I am not familiar with the latching relay you can set with a magnet... sounds clever. My hypothetical relay was specified using a 10A ac power relay so if it was energized with mains voltage it would be sure to break the 120v circuit, the 200 V dc rating of your model might be marginal (I don't know whether DC is harder or easier than breaking AC, probably harder). I was thinking a normal non-latching relay for use with a power supply so when non-powered ground is normally open.   
Quote

Now, put a current transformer to monitor any 60 Hz AC current in the shield. In fact, run both the shield and signal conductor through the current transformer so it's only looking for current external to the guitar's signal. So would there be sufficient sensing voltage/current from the sensing transformer to directly trip and unlatch the relay, maybe with a diode to rectify it? Would this system be sensitive enough to trip at 10 mA fault current? If not, then something as simple as a 741 Op-Amp or other comparator IC to do the tripping, but that would need a battery or other power source which I'm not in love with. Now, once the latching relay trips, you could then push a button to move a permanent magnet close to the latching relay to reset it.
I need to look at these... with access to power and active sensing I can probably detect current with a modest DC series resistance and op amps.
Quote
Again, don't beat on me if I've missed something by a few orders of magnitude or even thinking about this completely backwards, but I like the concept of this safety device being self powered, reset with a permanent magnet, opens up both the guitar shield and signal lines at the same time, and doesn't introduce any semiconductors into the guitar signal path to mess with their "sound".
Yes the relay is harder to criticize.
Quote
Thoughts? Comments? Critiques?

I need to do some more homework.  The company I found who might be a candidate to make a special musician version also sells a patented LCDI (Leakage current detection and interruption)... I glanced at the patent (6,738,241,B1) and can't say that I get what it's good for, but these guys should know how to do what we want, as soon as we figure out what we want to do.

I do not see any smart detection of RPBG that I would like to build into a smart GFCI outlet strip... (open ground and mains if we detect voltage on the ground).

Their website does not sell direct but they make a bunch of GFCI strips and extension cords that look useful for back line as is. http://www.towermfg.com/gfci.htm

Keep the ideas coming....

In my ideal world we could make a power strip that can't be fooled, without breaking the bank.

JR
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Mike Sokol

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Re: brain storm an optimal human safety system for back line
« Reply #6 on: December 12, 2014, 06:49:41 pm »

I am not familiar with the latching relay you can set with a magnet... sounds clever. My hypothetical relay was specified using a 10A ac power relay so if it was energized with mains voltage it would be sure to break the 120v circuit, the 200 V dc rating of your model might be marginal (I don't know whether DC is harder or easier than breaking AC, probably harder). I was thinking a normal non-latching relay for use with a power supply so when non-powered ground is normally open. 

This latching reed relay is in the guitar signal line, not the AC power line. It would have to see 180 volts peak (in the USA / twice that in the UK) but essentially zero current since the megohm input on a guitar amp has perhaps micro-amps of actual audio signal. But it would have to absorb the fault current of a human (perhaps 100 mA) or even a guitar string on a hot mic (a few amperes maybe, until it trips open). I'm thinking of it as a self-power GFCI (Guitar-Fault-Circuit-Interrupter) which could be in a stomp box.

Yes, I know we're looking for something that doesn't require the musicians to add them into their signal chain, so perhaps a specialized backline AC power GFCI is the best solution. But I can't help but think that opening up the guitar signal cable is a good way to protect the guitarist from a "hot" mic as well as a "hot" chassis guitar amp. More to think about.   
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Frank DeWitt

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Re: brain storm an optimal human safety system for back line
« Reply #7 on: December 12, 2014, 08:40:31 pm »

But I can't help but think that opening up the guitar signal cable is a good way to protect the guitarist from a "hot" mic as well as a "hot" chassis guitar amp. More to think about.   

I can see the guitar forum thread now.  I would use the Sokol box but it colors my sound.

I know what you meen but if you use it with a 12AX7 input it is a very nice color.

No. If you use it at the input of your pedal board with a guitar with a active pickup you only get the slightest warming effect.  I actually like two of them to get the best sound.

« Last Edit: December 12, 2014, 08:42:33 pm by Frank DeWitt »
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #8 on: December 12, 2014, 08:59:12 pm »

This latching reed relay is in the guitar signal line, not the AC power line. It would have to see 180 volts peak (in the USA / twice that in the UK) but essentially zero current since the megohm input on a guitar amp has perhaps micro-amps of actual audio signal. But it would have to absorb the fault current of a human (perhaps 100 mA) or even a guitar string on a hot mic (a few amperes maybe, until it trips open). I'm thinking of it as a self-power GFCI (Guitar-Fault-Circuit-Interrupter) which could be in a stomp box.
I may be over engineering this but would like to do it once and be done (it's a small market) so ideally a solution for world voltages (i.e. 400V parts).

The voltage/current I am concerned about is hot ground mic to grounded guitar amp... which could be lots of amps metal to metal. If the relay gets stuck it isn't protecting anybody.
Quote


Yes, I know we're looking for something that doesn't require the musicians to add them into their signal chain, so perhaps a specialized backline AC power GFCI is the best solution. But I can't help but think that opening up the guitar signal cable is a good way to protect the guitarist from a "hot" mic as well as a "hot" chassis guitar amp. More to think about.   
As I already mentioned I will talk to my friend who owns a guitar pedal company to investigate building some protection inside.  Back when you were talking with that guitar maker who was adding fuses into his guitars (??I don't remember fuse value or company). I wonder if he is getting a decent price... I don't trust the quotes I've seen so far (too high).

I also thought about maybe putting a pair of 1/4" guitar jacks in the GFCI outlet strip with a relay for the muso to patch in series between his guitar and pedals or amp, but nobody want to add any extra cable length (capacitance).

JR
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #9 on: December 12, 2014, 09:05:36 pm »

I can see the guitar forum thread now.  I would use the Sokol box but it colors my sound.

I know what you meen but if you use it with a 12AX7 input it is a very nice color.

No. If you use it at the input of your pedal board with a guitar with a active pickup you only get the slightest warming effect.  I actually like two of them to get the best sound.
Indeed... while not exactly a guitar forum, there was a pretty extensive thread a few years ago in Geekslutz (more of a recording forum) discussing the preferred internal modification to the guitar. A small cap (.03uF or so) was inserted between the exposed metal parts of a guitar and the real ground that is still connected to the pickup and amp. 

So sonically this guitar tone should be unaffected, while hum pickup from proximate humans is well attenuated. 

JR
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