Circuit breakers are resettable automatic switches that protect against a danger
ID: 2038747 • Letter: C
Question
Circuit breakers are resettable automatic switches that protect against a dangerously large total current by “opening” to stop the current at a specified safe value. A 1650-W toaster, a 1090-W iron, and a 1250-W microwave oven are turned on in a kitchen. As the drawing shows, they are all connected through a 20-A circuit breaker (which has negligible resistance) to an AC voltage of 120 V.
a.Find the resistance of the three devices. Hint! Treat as you would for a DC circuit.
b.Is the circuit below parallel or series?
c.Find the equivalent resistance and obtain the total current delivered by the source and determine whether the breaker will “open” to prevent an accident.
Circuit breaker 20 A 120 10 10Explanation / Answer
Given,
Pt = 1650 W ; Pi = 1090 W ; Pm = 1250 W ; I = 20 A ; V = 120 V
a)We know that, P = V^2/R
Rt = 120^2/1650 = 8.73 Ohm
Ri = 120^2/1090 = 13.21 Ohm
Rm = 120^2/1250 = 11.52 Ohm
Hence, Rt = 8.73 Ohm ; Ri = 13.21 Ohm ; Rm = 11.52 Ohm
b)The circuit is Parallel
c)Ptotal = Pt + Pi + Pm
Ptotal = 1650 + 1090 + 1250 = 3990 W
R = 120^2/3990 = 3.61 Ohm
you can also calculate using individual resistances
1/Req = 1/8.71 + 1/11.52 + 1/13.21 => Req = 3.61 Ohm
I = V/R = 120/3.61 = 33.24 A
Hence, Req = 3.61 Ohm ; I = 33.24 A ;
The breaker will OPEN.