Lab 3: Ground, Ground Loops, and Ground Fault Interruption

Background:  
A 3-prong power outlet has slots for Hot, Neutral and Ground (Black, White, Green).
Hot and Neutral carry current to and from the load, be the load a light bulb, power supply or motor.

Ground should carry no current.
Ground is there for safety reasons, for shielding, and for a stable voltage reference.

In this lab you will look at ground vs neutral, induced voltages in loops of ground wire, and how a circuit can detect a leakage current fault on the ground wire, and open-circuit the hot path as a result. (GFI)

Requirements:
Start with the Lab 3 box, which can be plugged in from two power cords, and which has front panel terminals for each power cord's neutral and ground. Also in the box is a pathway from one hot (left, yellow plug) to ground through a pushbutton switch and a variable resistor.

Currently (get it?) there is only one Lab 3 box, but your team should be able to finish its measurements in a reasonable amount of time.

(1A) Measure with the DMM voltmeter, on the AC setting, the voltage between neutral and ground. Measure gnd vs neut at one outlet, with gnd from one outlet and neut from the other. Record your answers (perhaps in Word, in your Saved Student Work folder). Compare to the voltage between ground and ground. Why the the neutral-to-ground voltage greater?

(1B) Now plug in the 2.5A the drill press and turn it on, spinning. Does the neutral-ground AC voltage difference increase, decrease or remain unchanged? Record the values in mV. Ask JDD to turn on the drill press while holding the chuck immobile. What is neutral-to-ground AC volts now?

(2) Example of Faraday's Law:
Place the voltmeter (AC) between the two ground outlets on the box. You will have formed a ground loop with a voltmeter in series. The ground loop extends from one connection of the DMM to the Lab 3 Box, through one outlet, to the other outlet, back to the box and back to the other DMM connection. See diagram...

What is the voltage across ground? (we hope it is less than a mV!) Record and label your answer. You may want to connect a 1µF "orange drop" capacitor across the ground binding posts to see if it can fitler any "non-60Hz" noise that may be contributing to the AC voltage...

Now turn on the mixer and move it from a distance to inside the "ground loop". Record what is the AC voltage away, near, and inside the loop. Also try stradling the mixer over one of the ground loop wire "borders". Where is the induced voltage greatest?

B field is a vector. How does your induced voltage depend on the orientation of the mixer?

Connect a coil of wire across the ground connections and move it around a stationary mixer to see how the coil might be used as a detector of (changing) magnetic field.

(3) Plug the left side of the Lab 3 box into the GFI outlet.
On the box is a switch that will route HOT through a 75K Ohm potentiometer in series with a 4.7K fixed resistor. The other end of the resistor connects to a banana outlet on the front of the box.

Place your DMM, in AC current mode, in series with the Hot-out to ground,
to measure the current in the ground fault. You will need to put one lead in the lower red banana socket so the current is routed through the proper path in the DMM. Remember to take the current lead out and put it back in the voltage socket at the end of the experiment, or we may blow a fuse in the current detection path.

On the front panel of the Lab 3 box hold down the pushbutton on the left, turn the potentiometer knob CCW and ncrease the ground fault current until you hear a pop and the GFI disengages the hot wire. How much current was flowing to ground when the GFI activated? Write down the answer in mA.

Press the RESET button on the GFI to repeat.

What happens if you turn on the 2A drill press while the ground fault current is increasing?

What happens if you plug the GFI outlet into a 2-prong "cheater"? Can you see any ground fault current?

Reading: Read chapter 14, by Olson, of the Webster book (handed out, and on 095 bookshelf...), and website lecture notes on Elec Safety.

Likely FTQs:
1. Defibrillator design: Hoping you remember the formulas for energy stored in a capacitor and time constants of a CLR series circuit, you will be given parameters of defibrillation duration, torso resistance, and total energy delivered, and asked to calculate L, C, R, and V0 (voltage on charged capacitor) for defibrillator design.
2. Explain how a standard GFI circuit works. You'll want to say what a ground fault is, how a GFI can detect it, what the GFI circuit does to stop the ground fault current, and when you press the manual reset button.
3. Calculate the RMS magnitude of 60 Hz magnetic field in a ground loop, given the area of the loop and the RMS AC voltage recorded from the ground loop. For example, from a search coil to measure eye movments. Use Faraday's Law. The dot product B(t)•a is a term to consider. Assume the B field is parallel to the area vector. Name the unit of magnetic field strength.