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Ground lift

#43
rude_basstard said:
I have one of those. Doesn't do ****. I still get ground loops, and have to use an external DI. Stupid Ampeg not providing a ground lift on the built in DI on the SVT4PRO.
Click to expand...

It does exactly the same thing as a ground lift on any other DI - if you're still getting noise with one of those, it's because the DI sucks, or a cable/channel issue... That also doesn't surprise me, since IME Ampeg's DIs on their amps have always been pretty craptacular...


- georgestrings
 
#44
Bassamatic said:
There is no problem to lift the ground on the XLR - it is still grounded on the mixer side, In fact that is how audio connections are SUPPOSED to be made - grounded ONLY on one side to prevent ground loops! It's the way we wired many studio and PA racks. All cables grounded at one end only, and all the gear star grounded to a central point. Voila - no ground loops! Even my home stereo rack is wired that way.

Also - I am pretty sure that a GFCI has to be grounded to work - it senses current leakage to ground.
Click to expand...

A GFCI senses the current on the hot, and neutral, and when the difference exceeds 6 mA, it opens both conductors.

Here is what Leviton, (a manufacturer of GFCIs) says about this: [Invalid or Expired Link Removed]
 
#45
P Town said:
A GFCI senses the current on the hot, and neutral, and when the difference exceeds 6 mA, it opens both conductors.

Here is what Leviton, (a manufacturer of GFCIs) says about this: [Invalid or Expired Link Removed]
Click to expand...

Correct. It's a differential window detector. The circuit opens when the differential between the hot and neutral exceeds ~6mA, presumably this current is going somewhere it shouldn't.
 
#46
Are you guys certain it opens the neutral as well as the hot? My understanding is that they only open the hot, in which case they would be ineffective in protecting against a voltage differential between a mic and an instrument amp-the path to ground would still exist even after the GFCI tripped.
 
#47
I think the voltage differential from the faulty amp with no ground would be the one guarded against, and tripped out, although removing the input doesn't kill the amp dead until it has drained its capacitors, not so flash with ground still through the player!

I guess a hot mic would be less deadly with no convenient path to ground via the bass.
 
#48
Going from memory here, but IIRC EC&M magazine had an extensive article about GFCI's and I am just about positive that both the line side hot and neutral are disconnected under a trip condition.

I also remember something about there being a special type of GFCI for plug and cord type devices, which are the same as what's used in the construction industry for temp. power. Under these conditions, faulty line neutral conductors are more common and these GFCI's are required to open under loss of supply neutral conditions as well. Otherwise, the hot can be present and a fault current exist but the power to trip the shunt coil through the 120V source does not exist. I think there is a seperate UL listing type for this device.

The number of electrocution deaths has declined from ~600 per year in the late 1960's to about 300 today in spite of the massive increase in electrical devices and tools in use today. This is due to a number of reasons (other than but including GFCI's) such as solidly grounded chassis (3 prong plug), double-insulated construction on ungrouded equipment, battery powered tools, improved wiring practices including grounded receptacles, fixtures and appliances. Saving 300 lives a year times the 40 years is 12,000 lives saved... significant IMO.
 
#49
I just looked at a couple of different schematics for GFCI's and they both showed neutral as well as hot being disconnected.
Which suggests, does it not, that using one WITHOUT a ground lug may actually be SAFER?
If your amp chassis(and bass) goes hot and you touch a grounded object like a mic, click goes the GFCI. Safe.
But better is the fact that if you touch a HOT mic, not having the ground lug on your rig saves you from being zapped. A GFCI won't save you from that (unless it's on the PA). Only downside is you no longer have a way to drain noise from your bass. Seems like this would work. Wrong?
 
#51
dmusic148 said:
I just looked at a couple of different schematics for GFCI's and they both showed neutral as well as hot being disconnected.
Which suggests, does it not, that using one WITHOUT a ground lug may actually be SAFER?
If your amp chassis(and bass) goes hot and you touch a grounded object like a mic, click goes the GFCI. Safe.
But better is the fact that if you touch a HOT mic, not having the ground lug on your rig saves you from being zapped. A GFCI won't save you from that (unless it's on the PA). Only downside is you no longer have a way to drain noise from your bass. Seems like this would work. Wrong?
Click to expand...

I think you're right. Ground wires aren't that great protection for people from electrocution anyway: they are more a part of the "circuit protection" scheme. They do allow a breaker to see that the hot wire has shorted to the housing, but shorts to ground downstream of the load are still invisible to them, as that doesn't increase the load on the circuit. GFCI's are entirely for People Protection.
 
#53
Let me re-state that if you connect your amp to a GFCI, it will protect you from a ground fault in your amp ONLY, and NOT any other equipment you may come in contact with.

I have found quite a few old GFCI units that have been in service for a while to be defective, and recommend that you press the test, and reset button on any GFCI before you trust it. The safest practice is to do this each, and every time you are about to connect to the device.

For circuits where no grounding electrode conductor is provided, (meaning no ground wire), the test button on the GFCI, or any portable GFCI tester will not trip the unit. This is because the test button shunts current through a resistor to ground to perform the test. (While it is not practical, you could test an ungrounded GFCI by connecting a 22,000 Ohm resistor between the hot, side, and some other grounded circuit element. Not the neutral wire!)

GFCIs are prone to "nuisance tripping". I have found portable hair dryers, and vacuum cleaners to be common causes of this problem. The common feature these devices share are universal motors. The only cure I have found for nuisance tripping is to not use the equipment that causes the problem.

So to be completely safe from shock, or electrocution by a ground fault, all cord connected equipment you may come in contact with should be connected to a GFCI.

Especially if you are outdoors, or standing on a concrete floor.
 
#55
Downunderwonder said:
What say a paperclip drops in your amp, you hit a note and it wobbles the stage just the wrong way and 400v hits the chassis just as you step up to the mic? My fear is no GFIC will save you if the mains chassis ground has been lifted.
Click to expand...

I believe you are correct.

You will be doing the "KiloWatt Shuffle" on stage.

The audience will think its part of the act.

Look at it this way. Being a musician is still safer than farming.
 
#56
Downunderwonder said:
Question, if someone does get a heartstopping zap and falls clear of what zapped them, can CPR bring them back or is it game over?
Click to expand...

CPR MIGHT bring them back, or might not. What happens is that the 60Hz from the shock sends the victum's heart into fibrillation - rapid beating that accomplishes no blood pumping. CPR is just rhythmic external heart massage - making it pump blood due to the external compressions rather than from its own internal signal. CPR MIGHT keep the vicim alive until his or her heart stabilizes itself and returns to a normal rate, or until an EMT arrives to administer defibrillation. Defibrillation is just a massive DC shock that has a better chance of getting the heart to stop beating at the rapid rate and return to a normal rate. Defibrillation is not a 100% sure thing either, but its the best chance the victim has.
 
#57
Invalid Link Removed

Recommended read by Bob at QSC :)

"Generally, the most dramatic and cost-effective solution to system noise is to locate and eliminate the ground loops or other problems that allow noise to couple into signal paths in the first place. This approach solves the fundamental problem, which tampering with safety grounds does not."
 
#58
P Town said:
For circuits where no grounding electrode conductor is provided, (meaning no ground wire), the test button on the GFCI, or any portable GFCI tester will not trip the unit. This is because the test button shunts current through a resistor to ground to perform the test. (While it is not practical, you could test an ungrounded GFCI by connecting a 22,000 Ohm resistor between the hot, side, and some other grounded circuit element. Not the neutral wire!)
Click to expand...

The schematics I've seen place the test switch between hot and neutral, with a resistor. No ground required.
 
#60
Major Softie said:
I think you're right. Ground wires aren't that great protection for people from electrocution anyway: they are more a part of the "circuit protection" scheme. They do allow a breaker to see that the hot wire has shorted to the housing, but shorts to ground downstream of the load are still invisible to them, as that doesn't increase the load on the circuit. GFCI's are entirely for People Protection.
Click to expand...

Actually, solidly grounded chassis are important for electrocution protection because it holds the voltage rise under fault conditions to just a few volts relative to surrounding "grounds" (such as building steel, pipes, other grounded equipment). There is a UL test that looks at the effectivenes of this feature under fault conditions. If the chassis was ungrounded, and a fault to the chassis were to occur, the local ground (chassis, bass strings, bridge, stage metal, etc.) would rise up to line voltage. If a GFCI was present it would clear this ground fault as well, you would still likely feel the shock until the clearing.
 

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