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Welcome back, Troubleshooters! Last week on Troubleshooting Thursdays, we learned how to find opens using an ohmmeter. Continuing on our topic of opens and shorts, today we ask (and answer) the burning question: What are short circuits?

Essentially, a short circuit occurs when a portion of a circuit accidentally becomes connected to another portion of the circuit, causing improper operation of the circuit.

The most common kind of short circuits happen when a part of the circuit, usually the conductive portion of a wire, comes into contact with a grounded object such as an enclosure. This is called a “short to ground,” and it can cause large amounts of current to flow in the circuit.

The following video demonstrates a short to ground.


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Below is a schematic of the simplified lighting circuit used in the video.

When S1 is closed, current flows in the circuit as shown:

The amount of current flowing is limited by the resistance of the light bulb.

Sometimes a part of the circuit, usually a wire, becomes damaged and can become connected to ground:

If this were to happen in our scenario, an excessive current would flow through this low-resistance connection:

Sometimes a part of the circuit, usually a wire, becomes damaged and can become connected to ground:

The excess current causes the fuse to open:
Sometimes a part of the circuit, usually a wire, becomes damaged and can become connected to ground:When the fuse opens, it stops the current flow and prevents further damage:

short circuits

Protection Against Short Circuits

All circuits are protected by overcurrent devices such as fuses or circuit breakers. These are designed to open the circuit when a predetermined amount of current flows through them.

Typically, a short to ground will cause excessive current to flow, causing the circuit’s protective device to open. This interrupts the current flow and isolates the circuit, as we saw in the above demonstration.

It’s important to note that when you first analyze a circuit, it will appear that the type of fault is an open rather than a short. If your tests show that a fuse or breaker is open, you should ask yourself why. Even though fuses and breakers can fail on their own, it is unlikely, and there is usually another cause.

If you find a blown fuse or open breaker, chances are there is a short to ground in the circuit. You should investigate the cause before replacing or resetting them.

In the coming weeks on TsT, we will look at two ways to find shorts with an ohmmeter, and the best practices for testing.

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