It’s a good idea to define the terms we use. Image credit: Joshua Hoehne on Unsplash
This article was written by Bryan Orr to give a full understanding of the role fuses, overloads and circuit breakers play in the protection of HVAC/R equipment.
If you skim-read or jump to conclusions, you will be tempted to argue. Be patient; if you want to understand, you will need to read all the way through and possibly even watch the videos at the end. This topic is WIDELY misunderstood, so the odds are that you will likely think I’m crazy when you first read it. Do your own detailed research once you get to the end if you still dispute what is contained here.
There are a few topics in HVAC/R that get widely confused and result in a lot of misinformation because of the similarity of the concepts. If you have two terms with SIMILAR meanings but get used interchangeably, you can reach completely logical-sounding (though totally incorrect) conclusions.
For example, a tech could say a particular circuit is reading “no ohms to ground,” and by that, he could mean zero ohms, or he could mean the meter is reading OL, which means infinite ohms.
In the same way, I often hear people say that something is “shorted,’’ and what they really mean is that it’s not working or something inexplicable is happening. So, let’s define some terms, starting with one of the most often confused.
Here is the dictionary definition:
Short Circuit: In a device, an electrical circuit of lower resistance than that of a normal circuit, typically resulting from the unintended contact of components and consequent accidental diversion of the current.
When a professional uses the term short or short circuit, they can mean an electrical path with lower resistance than designed, or they can also mean any unintended path.
For example, if two conductors in a cable are compromised and touching one another, a tech will often say that they are shorted even if there is not a low resistance overall in the circuit.
Because of this, the term “short” has become broad and must be used carefully.
Overload: To place too much a load on.
Pretty simple; when you put too much load in the bed of your truck, it bottoms out. When you place too much load on an electrical circuit or device, it fails. In the case of a conductor, this load is in the form of amperage — more amperage than designed — and the conductor will fail due to overheating.
In the case of a motor, this same thing holds true, but the actual load (opposing force) on the motor results in increased amperage load, which causes increased amperage and overheating.
That is why a compressor with failing bearings will draw higher amperage; the motor slips due to the additional mechanical load, which drops the impedance (resistance) in the motor windings, resulting in higher amperage.
Ground Fault: The momentary, usually accidental, connection of a current carrying conductor to ground or other point of differing potential.
A ground fault occurs when an electrical conductor or device that is electrically charged comes in direct contact to ground or a grounded assembly or substance; this will usually result in large current spikes until either a protection device opens the circuit or the circuit itself fails open (breaks) due to heat.
I say “usually” because there are cases when a ground fault may exist with no spike in the amperage, such as when you are using an ungrounded, two-prong appliance like a hairdryer or an old drill (or a drill that you cut off the ground plug in order to use on a two-prong cord). If the internal windings on the device short to the casing, there will be no path from the casing to the ground unless something else makes a path, like, say, YOUR BODY. Then, when your hand touches the drill casing and connects to ground, some current will leak to ground through the very high resistance load that is your flesh and organs. The circuit will not “overload” because it will not be drawing abnormally high amps, but you may still die from the incident. That is why ground fault circuit interrupters (GFCIs) are used in some high-risk applications; they break the circuit when a ground fault exists, even if that ground fault does not result in an overcurrent condition.
Overcurrent Protection: A form of protection in an electrical circuit that prevents excessive current usually at a predetermined value — Usually refers to a type of protection designed to deal with instantaneous spikes in current.
Over-current protection can be used as a broad term that can include circuit breakers, fuses, etc., basically anything that prevents a current from rising above a predetermined value. It CAN be a pretty broad term in some circles; HOWEVER, in the electrical community, when overcurrent protection is used, it is generally referring to short circuit or ground fault conditions.
Any condition that results in quick, massive spikes in current is addressed by overcurrent protection. I suggest reading THIS from Siemens to clarify some of the things I’ve just said.
Overload protection: Overload protection is a protection against a running overcurrent that would cause overheating of the protected equipment.
Overload protection deals with higher current resulting from too much current being pulled by a load. When the compressor goes out on overload after one second because it is locked, that is an example of overload.
When a condenser fan goes off after running with a blade with too steep of a pitch, that is an example of overload. An overload condition in a motor is dealt with by the overload inside the motor, not by the overcurrent protection/circuit breaker/fuses. In the case of motor loads specifically, if the overload were to fail, the overcurrent protection would usually break the circuit eventually, but that is not its primary design function in most cases. Again, I recommend reading THIS from Siemens to clear things up.
