"AC ground lift plug"

[jim.i]

I have been told this device is a good thing to have in my gig bag, if the amp being used does not have a ground lift option. Can someone explain what "ground lift" does, and is it a good idea to have this device?

[Ken Baker]

I have been told this device is a good thing to have in my gig bag, if the amp being used does not have a ground lift option. Can someone explain what "ground lift" does, and is it a good idea to have this device?

Is whoever told you this trying to kill you? I ask because this is probably what they're talking about; a 3 prong adapter. What's kind of scary is these are the result from a Google image search for "AC ground lift plug".



Ken's Terminology for the purposes of this discussion:

Audio ground refers to the ground connection that runs to the various signal sources: instruments, mics, keyboards, etc, as well as the internal ground connections of preamps & such that these source devices are connected to. Audio ground is typically used as an electrical reference for the rest of the circuitry.

Mains ground refers to the earth-based ground connection that goes along with AC power (mains) connections. The internal power supply and chassis of our audio gear is connected to this ground. That round plug on power cords is a mains ground connection, as are most cold water pipes. In the event of a power issue, such as an internal short, mains power is shunted to earth ground via a low-resistance connection and is isolated from the rest of the equipment.

Occasionally we run into odd grounding issues when there are multiple audio devices that are all connected to a common audio ground. For example, your bass amp's audio ground and a PA system's audio ground might not play well together. You could get zapped or you could hear odd sounding noises. So we "lift" or disconnect the audio ground of one of the offending systems. This will many times fix the odd sounds and zappings. Most pro audio gear has a ground lift switch on the back, and this is what it's for. If lifting ground makes the noise worse then there might be mains-related ground issues, and that MUST be fixed for safety purposes.

Lifting mains ground removes a critical safety feature from the connected equipment. Using one of these 3 prong adapters disconnects the power circuit from earth ground. If a power circuit fault developed inside the equipment, such as an amp or PA, people could get hurt or killed. We generally want to avoid that.

Ken...

[jim.i]

Glad i asked...

[Ken Baker]

Let's expand just a little more. Start by looking at that 3 prong adapter again:



These are NOT designed to be a ground lift, but a ground connect. Years ago, many/most electrical receptacles (outlets) were two prong. Hot & neutral, no ground. Then appliances and other goods starting coming available that required a ground connection. Thus was born the 3 prong plug we use today for many devices. The problem was that few places had the matching receptacles, and codes didn't require that existing circuits be ripped out and replaced unless there was a remodel to go along. The workaround is the 3 prong adapter. You plug the 3 prong device into its female side and plug its male side into your 2 prong receptacle. What you're supposed to do, and few people actually did, is unscrew the receptacle's coverplate screw, plug in the adapter with its green ground connection over the screw hole, then put the coverplate screw back in. So long as the box in the wall is grounded (most were), you're safe. This is what the adapter is for, though it's use has been bastardized over the years. This "ground lift" is absolutely scary.

Ken...

[jim.i]

So Ken, using this 3-2 adapter, 1)if the ground connection prong is not screwed in via the cover plate screw or 2)the electrical outlet box itself is not grounded, then this adapter has zero grounding effect and a potentially dangerous connection is made. Right?

If one is using an amp without a "ground lift" button/switch, one just has to live with any hum that may occur: solution, if there is one, is trying to minimize pieces of gear plugged into same outlets. Right?

So if one plays at a venue that only has old-style 2-slot wall AC jacks, is there a best safest way to plug in an amp?

[Ken Baker]

So Ken, using this 3-2 adapter, 1)if the ground connection prong is not screwed in via the cover plate screw or 2)the electrical outlet box itself is not grounded, then this adapter has zero grounding effect and a potentially dangerous connection is made. Right?

Correct.

If one is using an amp without a "ground lift" button/switch, one just has to live with any hum that may occur: solution, if there is one, is trying to minimize pieces of gear plugged into same outlets. Right?

Outlet choice may or may not help. The better solution would be minimize noise sources or proximity to them.

So if one plays at a venue that only has old-style 2-slot wall AC jacks, is there a best safest way to plug in an amp?

Not really, unless you run an extension cord to the pharmacy next door.

Ken...

[dukeplaysbass]

Sorry, I'm late to the party here, but wouldn't the other issue be that with an ungrounded amp, you risk being shocked by mics and/or making contact with other instruments?

[Ken Baker]

Sorry, I'm late to the party here, but wouldn't the other issue be that with an ungrounded amp, you risk being shocked by mics and/or making contact with other instruments?

Yup! Absolutely.

When the chassis ground (what I refer to as audio ground above) of the various amps aren't close in level, little zaps can happen. They might be felt as a tingle on the mic or fingers on the strings. it shouldn't happen, but it does - probably more so with older (tube) equipment. Add a bad or failed mains ground and someone might get knocked on their ass. It might not be too bad when all the gear is solid state with their relatively low voltages. But a tube amp with a wiring fault and a bad mains ground might throw hundreds of volts onto someone.

Ken...

[Ken Baker]

Here's another reason for good electrical practices.

Watch the vocalist in the white t-shirt.



Know your venues, folks!

Ken...

[deltafred]

I have been told this device is a good thing to have in my gig bag,

Absolutely not.

A far better thing to carry is one of the plug in mains testers that check that the mains outlet is wired correctly and that the earth/ground connection is good.

Many years ago I was rehearsing and as I went up to a mic to sing I touched my lips on it and got a full 240VAC mains shock (I was holding a grounded bass). It was like being kicked in the teeth by a donkey, one second I was stood at the mic next I was about 5 feet back. The ground connection had broken in the wall socket and was touching the live. Since that day I have been ultra cautious and check the band power distribution and cords on a very regular basis.

We have a mains checker permanently plugged in to a multi-way outlet and unless it shows everything is good I refuse to plug in. Fortunately in the UK the mains supply in venues is pretty good now, it hasn't always been that way though!