Cheater Plugs OK with GFCI?

[PeteD]

Hi Guys:

From what I know, a GFCI works in a similar way to a circuit breaker on a grounded outlet. It senses a voltage imbalance (short) as small as few milliamps between the hot and neutral and trips in 1/30 of a second. Does this afford the same protection as a grounded outlet? I have temporarily inserted one on the plug of my BFD (which is plugged into a GFCI) and it did eliminate my hum. The link below provides some details.

http://electronics.howstuffworks.com/question117.htm

Additional interesting info that makes sense:
"For many years, audio equipment was manufactured with two wire plugs as
well, up to the early 90's, in some cases. The manner of construction of
most of this equipment is identical to equipment specified for three wire
safety ground operation. Internally, there is no difference at all in the
safety considerations of the transformers for these devices. Since the early
90's, all this two-wire equipment has had polarized plugs. There is a reason
for this. One is that it is desirable that the on-off switch must break the
hot side of the circuit. The other is to take advantage of the fact that one
of the two prongs on the power cord is actually at ground potential. When a
transformer is wound, one terminus of the winding is at the iron core. The
other is on the outside. When the inner winding termination is connected to
neutral, this provides protection against a transformer short. Even if the
inside of winding shorts to the frame, which is extremely unlikely, it will
not elevate the potential of the amplifier chassis above neutral, which is
actually at ground potential." (http://www.gatago.com/rec/audio/opinion/3731827.html)

Finally, look at your lamps, etc that are still two prong. I don't think there is any more of a chance of audio equipment shorting than these other household items. Either way, you still get 120V.

I do not want to take risks so feel free to tell me if I am missing something.

Thanks,
Pete

[brucek]

It senses a voltage imbalance (short) as small as few milliamps between the hot and neutral and trips in 1/30 of a second. Does this afford the same protection as a grounded outlet?

No........Safer - yes....

You are using a GFI in an ungrounded condition. It is safer than a normal receptacle used in an ungrounded (cheated) condition.

When a normal outlet is wired properly, and a three wire device is plugged into it, the metal case of the device is now attached directly to your house safety ground system. If a short occurs internally in the device and the case becomes live, then the breaker for that circuit will immediately trip indicating the fault. If you have the third safety wire cheated or removed and this happens, the live metal case will likely electocute you before the breaker will trip. The 120 volts will travel through you if you touch a ground. That's obviously bad news.

What happens when you use an ungrounded GFI? Well, when a fault occurs in the device, the case again becomes live. There is no "difference" in the loads at the GFI so it doesn't trip yet. When you touch the live case, the hot current now exceeds the neutral current and the GFI trips. See the small problem you still have. You don't get electrocuted, but you get a shock. How bad a shock is determined by the speed of the GFI.

You're safer - not safe.........better than simply using the cheater

Finally, look at your lamps, etc that are still two prong. I don't think there is any more of a chance of audio equipment shorting than these other household items.

Nonsense....

brucek

[PeteD]

I appreciate your conservatism on this issue. However, my last receiver (a Sony from the early 90s that I replaced last year) was 2-prong.

I disagree with you regarding the lamp, considering how cheap they are and the fact that the lamp cord is often threaded through a metal shaft with no grommets to protect the wiring from getting cut. Even blow dryers are pretty cheaply made compared to most audio equipment, in my opinion.

It seems that if we rely on GFCI outlet to protect us while blowdrying in the bathroom, we should be OK with the BFD, especially since people use a blowdryer everyday and I will probably touch my BFD once every couple of months.

That said, I understand that the safest recommendation is appropriate on your part.

Somewhat related question: I have a pro-sound amp with a ground lift switch. Same danger as a cheater plug (I assume)?

[brucek]

Any device that is sold with a two prong polarized plug is double insulated to prevent shock. A three prong device does not enjoy this double insulation, but rather has the advantage of a grounded outer case that connects to the safety cold conductor of a three prong plug. (the latter being the condition that is over-ridden with a cheater). A hair dryer is a double insulated device. The GFI plugs in bathrooms are not to protect against failed hairdryers. They're provided for accidents such as dropping the hair dryer in the bathtub......

Audio equipment is particularly susceptible to a 'hot case' condition since it employs multi small capacitive components connected directly from hot to case for RF bypass. In general, it's the probability that the DC ground will become hot from a possible failure of any number of components that enhances the odds of a live case. This is because the DC ground in electronic equipment is connected directly to the AC ground except for those that are double insulated or enjoy a "ground lift" capability.

I have a pro-sound amp with a ground lift switch. Same danger as a cheater plug (I assume)?

Absolutely not. The outer case of equipment with a ground lift is always connected directly to safety ground. The ground lift simply inserts a small resistance between DC ground and AC ground. This changes the potential between AC and DC ground and inhibits ground loop current, but still allows a breaker to blow in the event of a short. Seems simple, but the entire DC ground has to be isolated from AC ground before this feature can be offered. It's quite an expense for the manufacturer to accomplish this....

brucek

[PeteD]

"A hair dryer is a double insulated device. The GFI plugs in bathrooms are not to protect against failed hairdryers. They're provided for accidents such as dropping the hair dryer in the bathtub......"

I realize that. I was just thinking if it affords safety in that situation (small shock), hopefully it will do they same for audio equipment...

"Audio equipment is particularly susceptible to a 'hot case' condition since it employs multi small capacitive components connected directly from hot to case for RF bypass. In general, it's the probability that the DC ground will become hot from a possible failure of any number of components that enhances the odds of a live case. This is because the DC ground in electronic equipment is connected directly to the AC ground except for those that are double insulated or enjoy a "ground lift" capability."

Makes sense. I suppose the stored electricity in the capacitors are gonna get you even with a GFCI, if they short to the case.

"Absolutely not. The outer case of equipment with a ground lift is always connected directly to safety ground. The ground lift simply inserts a small resistance between DC ground and AC ground. This changes the potential between AC and DC ground and inhibits ground loop current, but still allows a breaker to blow in the event of a short. Seems simple, but the entire DC ground has to be isolated from AC ground before this feature can be offered. It's quite an expense for the manufacturer to accomplish this...."

Very informative...Too bad more manufacturers do not incorporate this, but if it is expensive that is the reality. I suppose if I read the "new BFD proposal" thread again, the ground lift switch would be included in that proposal...

Thanks for the information, it reminds me of my mandatory EE class in college (I went CE). Circuits never really interested me until I owned a home and got into home theater...

[PeteD]

brucek wrote:

The outer case of equipment with a ground lift is always connected directly to safety ground.

Given this situation, can I ground my BFD case to my amp case and avoid ground loop, while keeping a safe ground?

[brucek]

Given this situation, can I ground my BFD case to my amp case and avoid ground loop, while keeping a safe ground?

Yes, and there is some justification to that scenario (star grounding), but often more than not that doesn't work to eliminate the ground loop. If the small voltage differential that is causing the ground loop originated at the amplifiers safety conductor, then it would work.

Realize that the outer case of your BFD is already connected to the amplifier case and every other case in your system by means of the interconnects and as long as they're kept connected, the BFD is connected to safety. Relying on connection to other equipment is considered bad form and somewhat dangerous though (unsuspecting person could disconnect the interconnects).

You've covered yourself fairly well though by using a GFI and also by having the BFD connected to grounded components.

brucek

[Wayne A. Pflughaupt]

I have a pro-sound amp with a ground lift switch. Same danger as a cheater plug (I assume)?

I believe what it does is lift the ground of the balanced audio connection (i.e., pin 1 of an XLR). That could be the same thing that brucek described – I don’t know for sure...

Regards,
Wayne

[tdamocles]

PeteD wrote:

Hi Guys:

From what I know, a GFCI works in a similar way to a circuit breaker on a grounded outlet. It senses a voltage imbalance (short) as small as few milliamps between the hot and neutral and trips in 1/30 of a second. Does this afford the same protection as a grounded outlet? I have temporarily inserted one on the plug of my BFD (which is plugged into a GFCI) and it did eliminate my hum. The link below provides some details.

http://electronics.howstuffworks.com/question117.htm

Additional interesting info that makes sense:
"For many years, audio equipment was manufactured with two wire plugs as
well, up to the early 90's, in some cases. The manner of construction of
most of this equipment is identical to equipment specified for three wire
safety ground operation. Internally, there is no difference at all in the
safety considerations of the transformers for these devices. Since the early
90's, all this two-wire equipment has had polarized plugs. There is a reason
for this. One is that it is desirable that the on-off switch must break the
hot side of the circuit. The other is to take advantage of the fact that one
of the two prongs on the power cord is actually at ground potential. When a
transformer is wound, one terminus of the winding is at the iron core. The
other is on the outside. When the inner winding termination is connected to
neutral, this provides protection against a transformer short. Even if the
inside of winding shorts to the frame, which is extremely unlikely, it will
not elevate the potential of the amplifier chassis above neutral, which is
actually at ground potential." (http://www.gatago.com/rec/audio/opinion/3731827.html)

Finally, look at your lamps, etc that are still two prong. I don't think there is any more of a chance of audio equipment shorting than these other household items. Either way, you still get 120V.

I do not want to take risks so feel free to tell me if I am missing something.

Thanks,
Pete

I've used this method also until I found that the Comcast cable was causing the hum. A cable isolator solved the problem.