Tag: electrical


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

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

Any two circuits that are connecting in an undesigned manner. This 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 would result in the condenser running whenever the blower is energized.

A short can also be described as a no load path between two points of differing charges. This 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 being energized when it shouldn’t be or fuses and breakers tripping or blowing or 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 an 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

— Bryan



Relays can be used for many different control applications including controlling fans, blowers, other relays or contactors, valves, dampers, pumps and much more. A 90-340 is a very common, versatile relay that many techs have on their truck so we will use it as the example.


A relay is just a remotely controlled switch that opens and closes using an electromagnet. The electromagnetic portion that provides the opening and/or closing force of the switch is called the coil. Relay coils can come in many different voltages depending on the application, but in residential and light commercial HVAC 24-volt coils are the most common.

The portion of the relay that opens and closes can be called the switch, contacts or points. These contacts can either be closed meaning there is an electrical path or open meaning there is no electrical path. Often this open or closed circuit will be described as “making” a circuit, meaning the switch is closed or “breaking” a circuit meaning the switch is open.


It is important when connecting a relay to distinguish which two relay points connect the coil. In the case of the 90-340, it is the bottom two terminals of the relay. Even though the coil is unmarked on most 90-340 relays, you can find it easily by locating the terminals with the small strands of wire connected. These two points connect together through the electromagnetic coil. When 24 volts of potential is applied across the coil the switch portion of the relay will switch from open to closed and closed to open depending on the terminal. Keep in mind that in a normal 24v circuit one side of the coil is connected to a 24v switch leg such as the thermostat “G” circuit for blower control, and the OTHER side of the coil is connected back to common.

The other six terminals are switch/contact terminals and the relay has a diagram embossed right on the top for easy reference. The way the circuit is drawn shows the de-energized state of the relay, meaning the state of the switches when no power is applied to the coil. When power is applied to the coil the points that were previously open (broken) now become closed (made) and the ones that were closed become open. When two points are closed when no power is applied to a relay coil we call them “normally closed” when they are open when no power is applied they are called “normally open”.


So based on this embossed diagram on the relay 1 to 3 and 4 to 6 are open (normally open) with no power to the coil and closed when power is applied. 1 to 2 and 4 to 5 are closed (normally closed) with no power and they open when the coil is energized. There is never a path between 2 & 3 or 5 & 6 because between them, at least one of them is always open. There is also no path or circuit between the top three terminals and the bottom three terminals or between the switch and coil portions of the 90-340 relay.

The data tag on a 90-340 shows both the coil voltage as well as the LRA (locked rotor amps) and FLA (full load amps) that the contacts can handle at various voltages for inductive (magnetic) loads like motors. It also lists the amp rating if the relay is controlling a RES (resistive) load like a heater or an incandescent light.


This relay can control a 39.6 LRA and 6.9 FLA Motor or a 15 amp heater at 240 volts based on the data tag.

— Bryan

First off, the correct acronym for a GFI (Ground Fault Interrupter) is a GFCI (Ground Fault Circuit Interrupter) and the purpose is to act as a safety device to protect from electrical shock.

GFCIs can be built into outlets, circuit breakers and even extension cords and are generally used for safety in wet environments like bathrooms, kitchens and outside.

A GFCI measures the difference in current between the line (hot) and the neutral. When even a small difference exists between neutral and hot the GFCI trips. This happens because a difference between neutral and hot means that some of the current is “leaking” to ground instead of being carried properly on neutral.

An example would be an electric drill plugged into an outlet outside and the cord plug falls into a mud puddle. If there is no GFCI some of the current will go out of the plug to ground through the puddle, causing hot to carry more current than neutral and making the puddle a potential shock hazard. If the circuit were protected with a GFCI it would trip immediately when the imbalance was detected.

Another nice thing about a GFCI is that it can help protect a circuit that does not have an equipment ground such as tools and appliances with two prong cords or two conductor outlets.

— Bryan

carrier

We keep 2 pole 40 amp 24v coil contactors on all of our vans. They are versatile, reliable and you can replace most residential A/C contactors with them.

There are a few things to watch for though, especially when you have a crankcase heater. Many brands power the crankcase heater constantly and shut it on and off with a thermostat, often mounted on the discharge line (here’s looking at you Trane).  When you replace a single pole with a two pole contactor in this type you need to make sure you connect BOTH sides of the crankcase circuit across the L1 and L2 line side of the  contactor to ensure the heater can function when the compressor is off.

Even more confusing that that…. Look at the diagram at the top and focus on the top left part of the diagram where the crankcase heater is located…

How does that work do you think?….. I will wait while you think it through…. Don’t cheat… Look at it.

This is a common Carrier Heat Pump crankcase heater configuration.

You notice that one side of the heater is going to L1 line side Terminal 1 and the other side is going to L1 load side terminal 2.

So the crankcase heater ONLY functions when the compressor contactor is OPEN and even then it does so by back feeding through the compressor common and back through the run winding of the compressor to the constant powered L2 side of the contactor.

This means if you replace this contactor wire for wire with a 2 pole contactor the crankcase heater will never work. You must put the compressor run wire (yellow) to the bottom of the contactor (L2 line side) instead of the top like it was if you want the crankcase heater to function in this situation…

All of this to remind you, DON’T BE A PARTS CHANGER! Know what you are replacing, why you are replacing it and what each wire and component actually does.

— Bryan

This tip was created by Jason Pinzak and originally posted on the HVAC Technician’s Facebook group. It is reposted here with permission from Jason. Thanks!


Contactors are useful in commercial and industrial applications, particularly for controlling large lighting loads and motors. One of their hallmarks is reliability. However, like any other device, they are not infallible. In most cases, the contactor does not simply wear out from normal use. Usually, the reason for contactor failure is misapplication. That’s why you need to understand the basics of contactors.

When someone uses a lighting contactor in a motor application, that’s a misapplication. The same is true when someone uses a “normal operation” motor contactor for motor jogging duty. Contactors have specific designs for specific purposes.


When selecting contactors, you’ll use one of two common standards: NEMA or IEC. Both match a contactor with the job it has to do, but they do so in different ways.
The NEMA selection process always results in a choice of a contactor you can use over a broad range of operating conditions. For example, you could use a NEMA Size 5 contactor to run a 50-hp motor operating at 230V or a 200-hp motor at 460V.


Using IEC standards, however, you can size contactors very close to their ultimate capabilities. In many cases, this precision allows you to predict how long they’ll last. For example, an IEC-rated contactor may run a motor that draws 40A at full load. In that duty, it should last for more than two million operations. But, if you used it for consistent jogging and plugging, you’d have to replace it after just a few thousand operations.

Since a contactor should last for years, don’t automatically replace one that fails with an identical unit. Instead, take a few moments to see if there is an obvious problem. A contactor really has only two basic parts: the contacts and the coil. The coil energizes the contactor, moving the contacts into position. The contacts transmit the current from the source to the load. Heat can destroy either of them, so take a good look at both.

Contacts will overheat if they transmit too much current, if they do not close quickly and firmly, or if they open too frequently. Any of these situations will cause significant deterioration of the contact surface and the shape of that surface. Erratic operation and failure will be quick. To check the contacts, just look at them. Some minor pitting (see photos) as well as a black oxide coating is normal, but severe pitting or any melting or deforming of the contact surface is a sure sign of misapplication. Replace contacts with such symptoms.

Coils can overheat if operating voltages are too low or too high; if the contacts fail to open or close because of dirt or misalignment; or if they have suffered physical damage or experienced an electrical short. Coil insulation degrades quickly when it gets too hot. When it degrades, it will short out (and blow a fuse) or just open and stop operating.

To check a coil, measure the ohms across the contactor coil. Infinite resistance means the coil is open. A shorted coil will still often register significant resistance and can be confused with a good coil . If you happen to have a matching contactor nearby, compare the two coils. The shorted coil will usually have significantly lower resistance than the good one but a compromised coil can alos have a higher resistance. If the difference is significant, replace it. Replacing the contacts or coil often means replacing the whole contactor. But no matter what you replace, compare the NEMA or IEC rating with the job the contactor will be doing. If you match it to the application, it should last a long time.

— Jason Pinzak

P.S. – Here is another good article on the difference between IEC and NEMA rated contactors

We do this exercise when I teach electrical basics where we sit down and connect a 10 watt bulb to a power supply and through a switch. A SUPER SIMPLE circuit, the kind you might have learned about in high school science class.

But then I grab another 10 watt bulb and tell them to connect it in line with the other 10 Watt bulb (series circuit) and BEFORE they can turn the switch on I ask them a series of questions.

  • Will the two lights be twice as bright as the one? the same? or half as bright?
  • Will the circuit draw twice the amps as before? The Same? or half the amps?

Before we move on, I want you to make your choice.

So everyone makes their choice.. we turn on the switch…

AND EVERYONE IN THE CLASS IS USUALLY WRONG

The bulbs combined are half as bright, using half the amps and thus half the watts. On my quizzes this is an area where experienced techs and electricians will even get frustrated “If you have X2 10 Watt bulbs that is 20 watts” they will say.

The science is actually really simple. In a light bulb, they may be stamped with a rating wattage but that wattage is just a rated wattage when the full rated voltage is applied. The constant in a light bulb is the resistance in Ohms, not the wattage. When you double the resistance of a circuit by adding in another 10 watt bulb in series you are cutting the amperage in half and therefore also cutting the wattage of the circuit in half.

an electrical circuit is a path between two points that have a difference in electrical potential (Voltage) the amperage (and by extension the wattage) is a function of the total resistance of that circuit between those points. If the resistance goes up, the amperage goes down and vice versa. It doesn’t matter if that resistance is added by a bulb, resistor, thermistor, pitted contactor points, motors etc…

Now when we mix in inductive reactance in motors and other inductive loads that resistance is bit less cut and dry to understand… but we will save that for another tip.

— Bryan

Grounding is an area of many myths and legends in both the electrical and HVAC fields. This is a short article and we will briefly cover only a few common myths. For a more detailed explanations I advise subscribing to Mike Holt’s YouTube Channel HERE

Myth – Current Goes to Ground

Actually current (electrons) move according to a difference in charges / potential (Voltage). When a potential difference exists and a sufficient path exists there will be current. In a designed electrical system current is always returning to the source, the opposite side of a generator, transformer, battery, Inverter, alternator etc… current only goes to ground when an undesigned condition is present and ground (earth) is generally a VERY POOR conductor.

Myth – To Be Safe, Add More Ground Rods

Ground is generally an exceptionally poor conductor. The purpose of ground rods is to carry large spikes in current that comes down your electrical distribution lines away from the building. Adding more ground rods can actually EXPOSE the building to current from near ground strikes.

Myth – Connecting Neutral and Ground Together In Multiple Places Is a Good Idea

Neutral and equipment ground should be connected in only one location at the main distribution panel to prevent ground from carrying neutral current. If equipment ground is carrying any current there is a problem.

Myth – Electricity (only) Takes The Path of Least Resistance 

If you have ever wired a parallel circuit you know that electrons travel down ALL available paths between to points of differing electrical charge.

Myth – Common, Ground and Neutral are the Same

Not even close. Common and neutral are terms used to describe the one side of a transformer. They are not grounded unless you ground them and when you do you are designating which side of the transformer will have an electrical potential that is equal with EQUIPMENT GROUND. The earth itself simply acts a really poor and erratic conductor between points of electrical potential that we designate and should not be confused with equipment ground.

Myth – Ground Rods Keep Us From Getting Shocked

Nope. Proper bonding connection between appliances, switches and outlets and equipment ground connected back to neutral at the main distribution panel in conjunction with properly sized circuit breakers and GFCI equipment keeps us safe. Grounds rods have little to nothing to do with protecting you from a ground fault.

Here is a great video on the topic and  you can find an article defining grounding and bonding terms HERE

 

— Bryan

Testo 760 Category IV Multimeter

I was standing at booth at the HVAC Excellence Educators conference and an instructor walks up, grabs a meter and asks me “what’s the difference between a category 3 and a category 4 meter”?

Well, I really wasn’t sure other than that the category 4 is rated for more demanding conditions. So I did some research and dug into IEC 61010-1 and found that category 3 is rated for most uses OTHER than outdoor utility connections and category 4 meters are rated for all uses.

Courtesy of Fluke


There are also some voltage considerations and limitations to the different categories but the primary difference is not the regular duty but the high voltage transients. High voltage transients are often called “surges” or spikes and are most likely when working on outdoor transformers and distribution panels.

Rubber meets the road is that for HVAC use a category 3 meter is likely going to do the job but if you ever work in main panels, or outdoor transformers go for a cat 4 meter.

— Bryan

PS – Fluke has a great info sheet on this HERE 

You can see more about the Testo 760 shown HERE

The definition of a transformer is a device that changes the voltages in an alternating current circuit.

You may have heard of an autotransformer or a buck and boost transformer and these terms are usually being used for the same type of device just highlighting different aspects. A transformer does not need to be a buck and boost to be autotransformer and it does not need to be an autotransformer to be buck and boost but often the two elements go together.

Autotransformer

The word auto in auto transformer really just means one or single not really automatic or automated in the way we usually think of it. It is an autotransformer because it only has one inductive (magnetic) winding shared by both the primary and secondary.

Buck and Boost

Buck just means that it decreases the voltage and boost means it increases it. A buck and boost transformer means that it can both increase or decrease the voltage.

What is their application? 

Buck and Boost autotransformers are often used to make small changes in voltage, say from 208v to 240v (boost) or from 240v to 208 (buck). They are usually efficient and inexpensive when only small changes are needed, whereas a traditional two coil transformer is more practical for larger changes.

Most of these transformers will have multiple tap points for different output and input voltages and can often be connected in different configurations to perform a wide range of functions like in the case of the Emerson Sola HD.

One major consideration with an autotransformer is that there is no isolation between the primary and secondary so a failure of the isolation of the windings of an autotransformer can result in the input voltage being applied to the output and component damage. There is also greater likelihood of harmonic and ground fault issues because of this “mixing” of primary and secondary.

— Bryan

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