Short Circuits

The term “short” has become a meaningless phrase in common culture to mean “anything wrong with an electrical device.”

A short circuit is a particular fault that can mean one of two things in technical lingo:

1. Any two circuits that are connecting in an undesigned manner

This issue would be the case if a control wire had two conductors connected together due to abrasion, like a Y and G circuit “shorted” in a thermostat wire between the furnace and the thermostat. This case would result in the condenser running whenever the blower is energized. Whether this is even a “real” short or not is disputed, but we commonly refer to it that way in the trade.

2. A no-load path between two points of differing charges.

This case would be a traditional “short to ground” low voltage, hot to common connection, or a connection between legs of power without first going through a load of appropriate resistance.

 

Both of these conditions will result in something occurring that should not be occurring. Either something is being energized when it shouldn't be, or fuses and breakers trip or blow, resulting in damaged components.

This is different than an open circuit, which is no path at all. So, if a load has power applied and NOTHING is happening, it is open. If power is applied and breakers or fuses trip or blow or something comes on at the wrong time or order, that is a short.

Many techs advocate using an ohmmeter to find a short circuit. We like that method, but we often find that using the 24v as is and simply using a process of elimination to find the cause is easier for most techs, as shown above.

—Bryan

Related Tech Tips

Ohm My
  One of the most common mistakes I hear techs make is confusing zero ohms with infinite ohms. The fuse above is showing near-zero ohms, which indicates a good electrical path with very little resistance. If there is a perfect path, it would have zero ohms (which isn't actually possible unless you happen to be […]
Read more
Low Airflow - Beyond the Obvious
One of our techs called me the other day and gave me a story of woe. He had been working on a system, and he had the following readings: Low superheat Low suction pressure Low head pressure He reassured me that the system airflow was correct and wondered what could have been wrong. I asked […]
Read more
Hot Pull Down and What it Means
If you work in refrigeration, you may have heard the term “hot pull down.” This phrase describes a condition where the load on the evaporator is above design due to the box temperature or the temperature of the product in the box being higher than it would normally be. My grandpa called me a few […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from