Why would lamp A be shorted here in this circuit when testing the circuit with a power supply?

Question

I'll really appreciate a response to this question, thanks for considering.
Suppose I have this circuit (image from a CIE past paper question):

Actual text (image here) in the problem:

It is said that it would be advisable to test the circuit using an ohm-meter with negligible current rather than with a power supply (the ohm-meter would be connected across the terminals x and y), because using a power supply instead would cause shorted lamp A to cause damage to the lamps or supply. To provide some context: lamp C is shorted according to all the data the question provides, the circuit is being used to detect the faulty lamp (lamp C was found to be shorted after answering following questions).
I require clarification on this and I'll appreciate it. I don't know where to start explaining this because I don't understand how exactly "shorted lamp A would cause damage to the supply/lamps" to quote the solution to  this question, where I don't understand how/when lamp A even gets shorted. I really need clarification on the concept, thanks. I did try to research about this but found nothing useful.

JalfredP · Accepted Answer

I don't get the problem in full, but I assume the answer to your question is that if you close the circuit and if A is shorted, then you have no resistance (or a very little resistance), as $S_1$ is shorted and $A$ is also shorted, which would cause very high currents if connected to a high voltage source. Potentially dangerous.
I assume you know the following, but let's be clear. Assume the path from $y$ to $x$ passing through $S_1$ and $A$ has a total resistance of $r_0$, then if you connect it to a power source with voltage $V$ of course you get a current $I=V/r_0$. Now, if $A$ is shorted then $r_0approx 0$ and so $I$ can be very high.
Basically, the question is equivalent to "why should you never in general connect a potentially shorted circuit to a power source?" and the answer is "because it can lead to very high currents".

Why would lamp A be shorted here in this circuit when testing the circuit with a power supply?

One Answer

Add your own answers!

Ask a Question