Question

You have been sent to a new home. The homeowner reports that sometimes the electric furnace trips the 240-V, 60-A circuit breaker connected to it. Upon examination, you find that the furnace contains three 5000-W heating elements designed to turn on in stages. For example, when the thermostat calls for heat, the first 5000-W element turns on. After some period of time, the second element will turn on, and then, after another time delay, the third element will turn on. What do you think the problem is, and what would be your recommendation for correcting it? Explain your answer.

Answer 1

Answer:

Explanation:

We should know that, Power is given as

P = IV

Where

P is power in watts

I is current in amps

And V is potential difference in Volts

The circuit breaker is design to be

240V, 60A.

So current above 60A will trip the circuit breaker off. Circuit breaker trips OFF when much current is flowing through the breaker.

Now,

The furnace that do trip the circuit breaker off has three heating element each of 5000W

And this element do not trip ON at once it is stage by stage.

When the first element is ON, the current it will draw from the MAINS is

We know that the first heating element has a power rating of 3000W.

P = IV

I' = P/V = 3000 / 240

I' = 20.833Amps

The circuit breaker is still OK with this because it has a capacity of 60A

After a while, the second, the second heating element is trip ON

And this will also draws a current of

I'' = P/V = 3000/240

I'' = 20.833amps

So, the total current drawn now is

I = I' + I" = 20.833 + 20.833

I = 41.667 amps

Again, the circuit breaker is still okay with this because it has a 60A capacity

After a while again, the third element is trip ON and this will also draw current of

I"' = P/V = 3000/240

I"' = 20.833 amps

Now, the total current drawn is

I = I' + I" + I"'

I = 20.833+20.833+20.833

I = 62.5 Amps

Wow, this is above the circuit rating of the breaker, then it will trip off.

Recommendation

I will recommend they get a new circuit breaker of higher capacity maybe 240V, 80-100A circuit breaker, since changing the furnace might cause a fortune.