device yokes measure ~5 volts to ground?

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SPARKLEY

Member
Location
Denver, CO
Occupation
Electrician
Hello experts. In a house with its original general lighting circuitry in 2 wire romex, I changed the first plug of each circuit to GFCI in order to convert the plugs to 3wire as is often done. Can anyone tell me why the plug and switch device yokes of these 2wire circuits now measure between 4 and 7 volts to ground? I measured from the device yokes to a remote ground path such as the ground of newer (grounded) circuits.
 

templdl

Senior Member
Location
Wisconsin
Hello experts. In a house with its original general lighting circuitry in 2 wire romex, I changed the first plug of each circuit to GFCI in order to convert the plugs to 3wire as is often done. Can anyone tell me why the plug and switch device yokes of these 2wire circuits now measure between 4 and 7 volts to ground? I measured from the device yokes to a remote ground path such as the ground of newer (grounded) circuits.

May this be another illustration of the misunderstanding of the type of phantyo voltage readings that are measured with the extremely high importance of the modern digital voltmeters. Get out your old trusty analog meter, the one with the needle that swings back and forth setting the dial to a range that could measure in that voltage range and what the results are. These older meter are of such low impedance that it with almost all certainty drain the voltage down to zero.
Digital meters are so sensitive that they can measure voltage between your two hands.
Rule the sensitivity of the voltage you are measuring first before you conclude that there is a relevant voltage issue present.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
151020-0057 EDT

I agree tht if you are using a high impedance meter that capacitive coupling within the GFCI casing is the liklely cause of your reading.

120/5 = 24. 24*10 megohms = 240 megohms. A capacitor of around 10 pfd has a capacitive reactance around 240 megohms at 60 Hz.

From a DC perspective the GFCI yoke is connected to nothing in the GFCI. But there could be easily 10 pfd of capacitance between the hot circuitry in the GFCI to thew yoke.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
151020-1949 EDT

I measured the capacitance from yoke to either hot or neutral of a Leviton GFCI and read about 15 pfd with my Tektronix 130 L-C Meter. Measuring frequency is about 100 kHz. This is in free space with no connected leads.

.
 

kwired

Electron manager
Location
NE Nebraska
Same reason some electrical inspectors check switch yokes and metallic body fixtures for missing EGC with a non contact voltage tester - capacitive coupling, you don't bond it - it will develop enough coupled voltage to set of these detectors if in nearby vicinity of an energized conductor.
 

SPARKLEY

Member
Location
Denver, CO
Occupation
Electrician
static discharge?

static discharge?

Thank you, that is quality information. The customer had complained of getting shocked when touching a particular switchplate, but after interviewing her I’m pretty sure it was momentary static discharge and the switchplate location was coincidental.

Has anyone seen/heard of this capacitive coupling voltage increasing the potential for static discharge?
 

iwire

Moderator
Staff member
Location
Massachusetts
Has anyone seen/heard of this capacitive coupling voltage increasing the potential for static discharge?

The switch is not building up a static charge, it is grounded. The person touching the switch has a built up static charge that is being disapated when they touch the grounded plate screw or metal plate.
 

SPARKLEY

Member
Location
Denver, CO
Occupation
Electrician
The switch is not building up a static charge, it is grounded. The person touching the switch has a built up static charge that is being disapated when they touch the grounded plate screw or metal plate.

Actually these are 2wire ungrounded circuits which is why I did the GFIs. Ungrounded metal boxes, plastic plates with metal screws to be precise.

So has anyone seen/heard of capacitive coupling voltage (the 4 to 7 volts on the yokes) increasing the potential for static discharge?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Actually these are 2wire ungrounded circuits which is why I did the GFIs. Ungrounded metal boxes, plastic plates with metal screws to be precise.

So has anyone seen/heard of capacitive coupling voltage (the 4 to 7 volts on the yokes) increasing the potential for static discharge?
Well, the capacitive coupling voltage is about 5 volts and the static discharge voltage is typically in the ten of thousands of volts up to megavolts.
Do you think that the 5 volts is going to influence that?
Now if the insulation on the yokes is good for a megavolt or so (!?!), then the capacitance will be the only path by which the static can discharge from your body. The presence or absence of AC will have no effect on that either.
 
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