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More Tech Jargon Explained: "Short" and "Ground"

Son of Samurai

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There are some electrical terms that are often used in vague and incorrect ways by the general public. This can make things confusing for us techs, especially those new to the craft, because these terms have precise meanings when used by those in the trade. A couple of these words are short and ground.

Short is often used by the non-technical to refer to any "bad" circuit. The term "short circuit" is a popular one to throw around in this sense. In reality, a short is just one of multiple different ways a circuit can fail. The definition of a short circuit is as follows: a circuit that allows current to travel along an unintended path with no or very little resistance. For example, if you were to have the terrible idea of putting one end of a jumper wire on line and the other on neutral in an AC circuit, this would create a short -- and probably some sparks and a tripped breaker as well!

The important thing is that a short is a specific kind of failure, not a catch-all for any electrical failure. Short does not even refer to instances where a load makes contact with ground -- for example, when a dryer heating element breaks, and one half of it sags and touches the chassis. This does create an unintended path to ground, but it is not a short, since the portion of the heating element that current is flowing through still has some resistance. That's why this failure does not always trip a breaker.

Speaking of ground, let's clear up some confusion there. It's not entirely your fault if you get confused by this one, because it's a single word that's used to describe two very different things, depending on if you're talking about AC or DC.

In a DC circuit, ground is the intended return path for current -- analogous to the function of neutral in an AC circuit. DC ground is the reference point you need to use for any DC voltage measurement, and it is absolutely not the same thing as AC ground.

AC ground, on the other hand, is never an intended return path for current. It exists only as a safety measure to allow AC voltage to safely discharge in case of a fault, such as the broken heating element example from earlier. If wired correctly, the chassis of any given appliance should be connected to ground.

As you can see, those are pretty different things, but with the same word used to describe both of them. The important thing to take away is that AC ground and DC ground are by no means interchangeable. if you measure DC voltage with respect to AC ground, you will read 0 VDC no matter what, because AC ground is not the return path for DC current.

If you would like to shore up your electrical knowledge so that you can get right to the troubleshooting without getting tripped up by terminology, you should click here to check out our Core appliance repair training course over at Master Samurai Tech.



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Great article, thank you!

I always thought that the breaker should trip at the slightest dis-balance, loss of AC current to ground. Doesn't that happen when part of the heating element touches the ground? I understand that partial resistance on the heating element still present, and assuming that the voltage fully drops across that partial element without any leakage to ground. Is that why the breaker doesn't trip sometimes? 

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EthanRanft

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19 minutes ago, Slav said:

Great article, thank you!

I always thought that the breaker should trip at the slightest dis-balance, loss of AC current to ground. Doesn't that happen when part of the heating element touches the ground? I understand that partial resistance on the heating element still present, and assuming that the voltage fully drops across that partial element without any leakage to ground. Is that why the breaker doesn't trip sometimes? 

You’re thinking of a GFCI which does what you’re describing. But you’ll never find that on a dryer circuit. A typical house breaker only trips when current exceeds its rated amperage. A grounded dryer heating element would have to be so short that it had less than 4 ohms resistance to ground if it was going to trip the 30A breaker

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Ah yes, GFCI, that's exactly what I was thinking of.

So according to the Ohm's Law, a typical 30A breaker at 240V can handle as low as 8 ohms of resistance. Anything less than that, should trip that breaker.

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EthanRanft

Posted (edited)

8 minutes ago, Slav said:

So according to the Ohm's Law, a typical 30A breaker at 240V can handle as low as 8 ohms of resistance. Anything less than that, should trip that breaker.

The reason why it would actually be 4 ohms is because in the case of a grounded element, it’s power supply is only 120V at that time.

At least that’s the way I figure it, but I can always be wrong 

Edited by EthanRanft

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10 minutes ago, EthanRanft said:

The reason why it would actually be 4 ohms is because in the case of a grounded element, it’s power supply is only 120V at that time.

Yep, after posting it I though that an electric dryer breaker is always a double 30Amps breaker for each 120V leg, so you are probably right, it could handle as low as 4 ohms of resistance.

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EthanRanft

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My reasoning is that when you have a grounded heating element, the power supply is L1 or L2 to ground. Just like a typical 120V Line to Neutral power supply, as opposed to L1 to L2 240V power supply

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