ECE 220 Network Analysis I

Lesson 14. Electrical Safety

Sample Problem Answers

You should see a picture here.

You are standing barefoot on a wet concrete floor. Which points are safe to touch (i.e. non-lethal) and why?

A. Dangerous. 120 V between you and ground.

B. Dangerous. 1KΩ resistor reduces current to a maximum of I = V/R = 120 ma, which is still a very dangerous current.

C. Safe. 1MΩ resistor limits current to a maximum of 120 μA, which is not detectable.

D. Dangerous. Voltage divider reduces voltage to 120 × (80 + 10)/(10 + 80 + 10) = 108 V, which is very dangerous.

E. Relatively safe. Voltage divider reduces voltage to 120 × 10/(10 + 80 + 10) = 12 V, which can shock, but which is not lethal.

F. Safe. The isolation transformer cannot provide a complete circuit to ground.

G. Safe. Same as F.

H. Safe. Ground potential.

I. Dangerous. The impedance of the 10µF capacitor is 1/jωC = 1/(j2π60 × 10 × 10-6) = -j265Ω. This has the potential to reduce the current to I = V/Z = 120/265 = 453 ma. This is a dangerous current.

Note! It is never safe or smart to touch any part of a live AC circuit.