modified GFCI

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JR.
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Re: modified GFCI

Post by JR. »

Perhaps it's just a problem with their artist drawing the schematic, GFCI transformers literally run both the line and neutral wires together in the transformer magnetic circuit. ASSuming the current in the line wire is exactly equal and opposite the current flowing in the neutral wire there is zero net magnetic field. Leakage current that does note flow within the proper wiring creates an error magnetic flux that the secondary winding converts to a current/voltage for action.

It is remarkable how sensitive these can be, tripping for a 5mA difference in a 10-15A outlet.

I looked at non-transformer approaches to accomplish this and it is difficult to realize this kind of accuracy/sensitivity.

JR
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mediatechnology
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Re: modified GFCI

Post by mediatechnology »

I think figure 24 intends to show it differentially, though the way its drawn it looks like neither IH or IR pass through the core.

Image

The LM1851 GFI is drawn better.

Image
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JR.
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Re: modified GFCI

Post by JR. »

Yup, that second drawing is more like it...

On another forum I am involved in trying to come up with a cheap fool=proof outlet tester to detect dangerous wiring.

A bad wiring practice when installing grounded outlets into older 2 wire homes is to "bootleg" the outlet ground over to the neutral wire, that should be 0V and provide an effect EGC (equipment ground connector), but in practice an open neutral then energizes that bootleg ground. Even worse if ground is accidentally bootlegged to line, the equipment chassis is energized. (I am aware of deaths caused by this).

The standard 3 lamp tester does not identify reverse polarity or bootleg connections, so one of these killer outlets would measure good using a standard 3 lamp tester. :oops: :oops: :oops:

I'll share what we come up with...

JR
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olafmatt
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Re: modified GFCI

Post by olafmatt »

JR. wrote:Even worse if ground is accidentally bootlegged to line, the equipment chassis is energized.
Oh yeah, I've seen that. It was actually an outlet a computer was plugged into. Every time the user started the dial-up modem connection (right, this was 15 years ago) the mains voltage got connected to the telephone wiring. The guy from the telephone company who was called in to repair the roasted electronics in their switching system couldn't find an explanation for what was going on. After it happened the second time (of course frying the telephone electronic and the modem card again) they let me have a look. Luckily the paint on the computer case was thick enough, so that I could only feel slight vibrations when touching the computer.

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JR.
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Re: modified GFCI

Post by JR. »

I am aware of musicians killed by such a miswired outlet, when accidentally touching two guitars plugged into amps in one good and one bad outlet.

Miswired safety grounds can be a killer in multiple outlet/ground situations.

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mediatechnology
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Re: modified GFCI

Post by mediatechnology »

Slightly OT but one of the things I found in our 1954-era two-wire home was grounded boxes and switch frames that were near water.
Those switches and boxes had solid copper which extended to cold water - also copper.
I also found the gas heater had a ground lead running from the attic through the chase-way down to the basement.
Apparently this was a 1954 NEC requirement.

Fortunately we were able (due to it being pier and beam) to get ground wires from outlets through the bottom stud footer and sub-floor down to the basement.
All of our outlets are fed from the bottom up.
It took a long flex drill and some time but we did it.

Bootlegged grounds from tying third pin to the neutral are very noisy too in addition to being dangerous.
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JR.
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Re: modified GFCI

Post by JR. »

Ignoring safety issues but plumbing grounds can be noisy, especially if one or more outlets have neutral-ground swapped. Another occasional wiring error. I have even heard about really bad venue noise issues when ground current noise was coming from a difference service drop and flowing in plumbing or conduit under a recording space.

JR
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ricardo
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Re: modified GFCI

Post by ricardo »

JR. wrote:The standard 3 lamp tester does not identify reverse polarity or bootleg connections, so one of these killer outlets would measure good using a standard 3 lamp tester. :oops: :oops: :oops:

I'll share what we come up with...
You've really frightened me now :o

In Oz, it's illegal to do your own electrical work. My day job is lifeguard (professional beach bum :D ) I queried the earth continuity on the socket we normally use to vacuum the pool cos it measured more than 1R to a socket on the other side of the room. I still don't have a sensible answer. Electricians don't pay much attention to beach bums. :(

So yes please. If you come up with a simple inexpensive 3 lamp tester which flags reverse polarity & bootleg connections.
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JR.
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Re: modified GFCI

Post by JR. »

If you are handy and I know you are, you can determine which lead is hot with a modern high impedance VOM. On VAC scale if I grip the black lead in one hand, then probe the outlet pins with the red lead I measure 40VAC on the hot lead. << 5v on the neutral. In 240V land probably even easier to see. Sometimes with a high impedance VOM you can measure ghost voltages on a floating ground, and GFCI does not tip until 5 mA of leakage so legal products can dump low single digit mAs into an unterminated ground wire.

Hopefully around your pool they use GFCI (or maybe you call them RCD). But even then I have heard of cases where GFCI devices are installed wrong, and/or are faulty so test them before trusting your life to them.

One very promising KISS test device is using neon lamps. The current draw is low enough that you can grab one lead of the neon lamp and probe with the other (like the VOM). If it touches more than 60V or so, it will glow.

JR
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ricardo
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Re: modified GFCI

Post by ricardo »

ROTFL :D

JR, I've done all that in my time including the neon lamp.

I did manage to get them to put all the sockets we might use to vacuum the pool on RCDs. The pool is very old and has 3 mains distribution boards. It's only this year that we figured out how all the mains is routed. :o
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