GFCI on 2 wire systems

[Bob Elliott]

I thought I was an electrician .May explain my Einstein hair.

[Michael Larson]

Originally Posted by belliott

I thought I was an electrician .May explain my Einstein hair.

I used to work with a maintence electrician that would wet two fingers and swipe at wire exposed wires to make sure they were dead.

[Bob Elliott]

Originally Posted by mlarson

I used to work with a maintence electrician that would wet two fingers and swipe at wire exposed wires to make sure they were dead.

No different from all the Gas company guys that used matches to check for leaks (OK I was guilty of that one too).

What drives me nuts is how many of the Electricians told me to work live or I was a sissy.

Other than getting scared, I ended up with many bloody knuckles from the kickback.

[James E. Braun, CMI]

I have had 440 three phase throw me back when I was an industrial electrician. The flash and the loud pop is unreal. I accidentally bumped some dirt and it fell in the cabinet that I was testing a circuit in. I learned to knock the dust off the panel first before I opened it.
Also once my supervisor was hauled away to the hospital because some water was lying in an electrical panel when he opened the door. I heard the loud pop and I was a good 75 yards away.

[Chuck Evans]

Originally Posted by mlarson

They prevent a fault current in excess of 5 mA from going through a person.

That is sufficient to prevent electrocution in most of the population.

I was referring to all of the other possibilities for electrical incidents e.g., miswired (line/load), malfunctioning, people getting shocked at switches, panels, dryer outlets, other devices, etc. People can come up with myriad ways in which to shock themselves. I just don't like to use absolute terms like "prevent" for these types of things, it can create a false sense of security for the unwitting.

[Michael Larson]

I simply stated the facts.

[rbrady]

Originally Posted by cevans

I wouldn't say they prevent, but rather that they reduce the potential for someone to get shocked. People can always find ways to electrocute themselves.

Many older homes' electrical systems are not grounded, however, short of fully rewiring the house, GFCI can be an effective way of providing improved protection against shocks and maintain "code" compliance with three prong outlets (use the "no equipment ground" stickers).

Since you picked up on Michaels comment but not mine, I will repeat:
They DO NOT reduce the potential for electrical shocks.

Also, the only "improved protection" they provide is to reduce the risk of death by electrocution.
You can also maintain "code compliance" by using 2-prong receptacles.

Personally I only recommend GFCI's in an ungrounded system at the same places I would recommend them with a grounded system. I do add a comment that "addition of a grounding conductor will reduce the risk of less dangerous electrical shocks".

[Michael Larson]

Originally Posted by rbrady

Personally I only recommend GFCI's in an ungrounded system at the same places I would recommend them with a grounded system. I do add a comment that "addition of a grounding conductor will reduce the risk of less dangerous electrical shocks".

Agreed

[George P. Wells]

Originally Posted by rbrady

From what I have heard, at least 220 will knock you off, unlike 110 which just makes your muscles hold tighter.

That is false.

[George P. Wells]

Originally Posted by thowell

Looking for some help on clarification. My understanding is a GFCI will prevent elec shock in a 2 wire system. I have seen them in bathrooms and kitchens. The part that trips me up is, a 2 wire system is still not grounded. That should be a safety concern, but not unsafe with a GFCI installed. (correct?)
Second part of question, in a home with 2 wire outlets why couldn't a person put GFCIs in all the locations? Like the living room for example. I was ask this question the other day. The occupants would be protected, correct?
Thanks folks.

Tim, without getting into a long detailed explanation (which most would probably find boring anyway), let me dispel one common misunderstanding. Two-wire systems are grounded. It is because they are grounded that GFCIs work on them.

Damage caused by electrical shock is a function of time and current. Humans are particularly vulnerable to electric current. The challenge to manufacturers of GFCI devices is to limit current to a where it cannot seriously injure a person but do not deenergize circuits that should not be deenergized. Some loads (specifically, reactive loads) are more difficult than others.

GFCI receptacles have come a long way since the early GFCI devices still, they should not be used on circuits that have heavy motor loads because they could trip when we don't want them to trip. It would be a good idea, for example, not to put a refrigerator on a GFCI receptacle.

It is worth noting that false tripping problems only arise with reactive loads (motors, capacitors, etc). Resistive loads (heating elements, etc) do not cause false tripping (unless, of course, the appliance has malfunctioned).

[Larry F. Rollins]

Originally Posted by rbrady

Have you ever tripped a GFCI with your body (across two fingers doesn't count)? I don't recommend trying it, but would be interested in hearing if anyone has.

From what I have heard, at least 220 will knock you off, unlike 110 which just makes your muscles hold tighter.

While in electricians school(many moons ago) one instructor took an entire class to prove that a AAA battery could potentially kill a person. The technique involved two pieces of wire, two needles and a soldering iron. {you figure it out} He showed where if a needle was stuck in each arm(into a vein) and the battery applied, enough current would flow to stop the heart. As I remember, and it has been many years, .1 milliamp will stop the heart.
Both 220 and 110 will "tend" to throw you off since they are AC voltages. It is DC voltages that will lock you on. Granted 220vac will do a much better job of throwing you off than 110vac will. The rule of thumb is Low voltage AC and Hi voltage DC are the most dangerous.
My largest shock was aboard a submarine where I took a "shot" from the back of my upper arm to my sweaty hand through a pinhole leak in a pair of heavy rubber gloves. It was 440 vac, 400 cycle. Came to about 10 feet away and couldn't use my arm for awhile. The burns hurt like hell. The only long term affect was a very strong desire to never do it again!

[rbrady]

Originally Posted by gwells

let me dispel one common misunderstanding. Two-wire systems are grounded.

I think most everyone understands that the neutral wire is the 'grounded conductor', and as such is grounded, even in a 2-wire system. It's kind of a given, that when we discuss un-grounded systems, we are refering to the lack of a 'grounding conductor'.

One thing that I think most people don't realize is that grounding the system acutally increases the risk of shock. If both conductors were isolated from ground, you would have to make contact with both sides (positive & negative) to get a shock - earth ground would not be the negative side. Grounding is used to stabilize the voltage.

[Jim Port]

Originally Posted by gwells

GFCI receptacles have come a long way since the early GFCI devices still, they should not be used on circuits that have heavy motor loads because they could trip when we don't want them to trip. It would be a good idea, for example, not to put a refrigerator on a GFCI receptacle.

It is worth noting that false tripping problems only arise with reactive loads (motors, capacitors, etc). Resistive loads (heating elements, etc) do not cause false tripping (unless, of course, the appliance has malfunctioned).

If motor loads are causing a GFI to trip the appliance is leaking current in excess of the allowable UL amount. For example a sump pump motor under the UL standard is only allowed to leak a small fraction of the 4-6mA that a Class A GFI would trip at. I cannot find the exact leakage allowed.

As a side note refrigerators in a commercial kitchen are REQUIRED to be GFI protected.

I would ask which is better, a refrigerator with a tripped GFIand spoiled food or a faulty refrigerator with a fault and a dead person?

[John Shishilla]

Those of you that try to predict when an electric current will, hurt, make you hold on, make you let go kill or seriously injure you are doing a disservice to those that are looking for answers.

It takes VERY little current to kill a person under the right circumstances A/C or D/C. If you must have a specific answer it is less than a milli amp. It can not be predicted with any degree of accuracy, to try is reckless.

I will step off the soap box now.

[Michael Larson]

Originally Posted by jshishilla

Those of you that try to predict when an electric current will, hurt, make you hold on, make you let go kill or seriously injure you are doing a disservice to those that are looking for answers.

It takes VERY little current to kill a person under the right circumstances A/C or D/C. If you must have a specific answer it is less than a milli amp. It can not be predicted with any degree of accuracy, to try is reckless.

I will step off the soap box now.

As I said earlier 5 mA is non lethal in a normal person.

Hospital requirements are much lower for patient care areas. The limits are on the order of 100 microAmps allowable leakage current and 10 microAmps in certain areas without gong into more detail than is needed here.