GFCI quiz

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junkhound

junkhound

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EE, power electronics specialty
For that list of stuff that trips GFCI........

Why do old phonographs and radios, especially those with tubes, nearly always trip GFCI outlets. ?

Also some modern high power battery charges with switching power supplies.

:roll:
 
junkhound said:
For that list of stuff that trips GFCI........

Why do old phonographs and radios, especially those with tubes, nearly always trip GFCI outlets. ?

Also some modern high power battery charges with switching power supplies.

:roll:
Click to expand...
Many old phonographs and radios connected one side of the AC line to the metal chassis, either directly or through a high value capacitor. The result is that the entire chassis is hot and safety depends on insulated control shafts and an intact insulating cabinet. This creates lots of possibilities for leakage, both resistive and capacitive.
 
GoldDigger said:
Many old phonographs and radios connected one side of the AC line to the metal chassis, either directly or through a high value capacitor.
Click to expand...
They were also among the first electrical devices to come with polarized plugs for that exact reason.

I remember my parents taking a portable tube TV on vacation, in a small rental apartment at Va. Beach, and not being able to plug in the TV. Even several extension cords we tried wouldn't accept the plug.
 
I used to get a lot of 120v car chargers tripping the gfci in the garage, i think the manufacturers got that one worked out. Now i find that the car charges are melting 15 amp gfcis that are on a 20 amp circuit from the inside out.

For the GFCI list though, id say Christmas light extension cords that are just laying in the pine straw. That'll trip a gfci

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To add to the list, engine block heaters. there has been two or three threads just in the last month about these tripping GFCI.

I do not have an answer for the battery chargers.
 
181207-2037 EST

GoldDigger provided the radio answer. One side of the AC line was connected to the radio chassis thru a 0.01 ufd or so capacitor. By appropriately plugging in the AC plug the radio chassis was put at near earth potential. This provided shielding of the audio circuits from stray AC capacitively coupled noise and thus hum.

.
 
181207-2111 EST

Adding to my previous post.

The capacitive reactance of 0.01 ufd at 60 Hz is about 265 k, and across 120 V this a steady state current of about 0.5 mA. Not enough to trip a GFCI. But this capacitor is likely fully discharged when the radio is turned on. Thus, the capacitor looks like a short circuit at the time of power application. Will a typical GFCI trip if its hot output goes to the non-chassis side of the capacitor, and some how the chassis has enough capacitive or conductive path to ground. I don't know, haven't run the experiment.

A separate question is if a capacitor is connected between hot and neutral on the output side of a GFCI does the GFCI current transformer balance the two currents, in and out, to prevent tripping. I would hope so, haven't run the experiment.

Some specific controlled experiments are needed.

.
 
gar said:
181207-2111 EST



A separate question is if a capacitor is connected between hot and neutral on the output side of a GFCI does the GFCI current transformer balance the two currents, in and out, to prevent tripping. I would hope so, haven't run the experiment.

Some specific controlled experiments are needed.

.
Click to expand...

It's supposed to be raining here tomorrow. Ill see if i can't run that "controlled experiment" in the afternoon

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controlled experiment...

When a 10 YO kid, knew if we tried to play the record player while sitting on the well grounded air duct to the gravity coal furnace, if we would get a good shock picking up the metal tone arm, we knew to reverse the plug.

Years later, looked inside that cheap little tube (all American 5 set of tubes) radio/phonograph and there was a 0.1 uF cap to chassis from one side of the line cord, if no cap - LOTS of hum on the phonograph. Also 4+ mA 60 Hz leakage with the cap.

The EMI filter on my 120 Vac input, 120Adc 14 Vdc output battery charger uses a SMPS to keep the weight down, the EMI filter has 0.2 uF caps to chassis and green wire. Cannot use on GFCI circuit.
 
GoldDigger said:
Many old phonographs and radios connected one side of the AC line to the metal chassis ...
Click to expand...
LarryFine said:
They were also among the first electrical devices to come with polarized plugs ...
Click to expand...
Installing a polarized plug will sometimes improve matters. If the device is old enough to contain vacuum tubes, it's probably time to replace the entire cord. Cleaning it and installing new non-conductive feet, and/or a non-conductive false bottom, might also help.

I once had an old favorite 6-tube AM radio, probably built c.1960. Hot chassis, no power transformer, plastic case, chassis mounted to the case by sheet-metal screws through the bottom.
It was on a freestanding steel shelf in an unfinished basement, and initially unknown to me, sitting on a small piece of metal junk. It made contact between the shelf and one of those sheet-metal screws, and energized the entire shelf. (no GFCIs at the time) Nobody got shocked because the basement walls leaked. It rained, there was water on the floor, and the points where the shelf legs contacted the wet concrete were steaming.
 
Older Phonograph shock experiment

Older Phonograph shock experiment

This thread takes me back to the 60's and a portable phonograph experience I have never forgot.
As others have said the chassis could become hot if the plug was not polarized.

I had bought my first electric guitar ( I was going to be a Beatle!) I could not afford the amplifier so I added a guitar jack to a portable phonograph to use its amp.
I came home wet after a storm and started to play. Touching both the guitar strings and the chassis I got lit up.
I was shocked to discover I did not have any talent as a musician :weeping:

I think that the leakage standards- to ground, have changed as the years progressed and thus older EMI filtering utilized larger caps than what would be allowed today.
I recall experiencing several pieces of equipment that would trip GFIs when GFIs were still new.
 
My GFCI trips because of Blue Tailed Skinks. [ https://www.virginiaherpetologicalsociety.com/reptiles/lizards/lizards_of_virginia.htm ]

They squeeze into the outside receptacle boxes through the mortar lines in the bricks on one particular receptacle. They seem to live through the experience.

I was working on a Westinghouse TV years ago, swung the "rabbit ears" around and they struck the water pipe above the workbench. Sparks flew, WTF? It has a polarized cord, and I knew my receptacle was wired right. Sure enough, the factory wired the "cheater cord socket" so the chassis was always hot, i.e. narrow blade of the power cord was connected to chassis. Set had been in service more than 10 years at that point, guess no owners got shocked along the way. I corrected the error and returned the set working.
 
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